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NORTHEAST REGIONAL RESEARCH PUBLICATION

PROJECT NE-39 S

HZ
,£2.2

Efficient Use of Nitrogen

on Crop Land in the Northeast

Allen V. Barker, Editor

BULLETIN 792 • THE CONNECTICUT AGRICULTURAL
EXPERIMENT STATION NEW HAVEN • DECEMBER 1980

NORTHEAST REGIONAL RESEARCH PUBLICATION

Project NE-39: Origin, Transformation, and Management of Nitrogen in Soils,
Waters, and Plants.

Cooperative regional research is designed to achieve replication without du-
plication of effort. It is particularly appropriate for examining problems such
as the efficient use of fertilizer, since soils, crops and climate vary widely
throughout the Northeast. Thus, principles developed at one location must be
tested at other locations and modified as necessary. The present project has
benefited from contributions from scientists at all the Northeast Experiment
Stations, as well as from states outside the region including Michigan, North
Dakota and California, and from USDA/SEA/AR at Beltsville, Maryland. The
states in the Northeast region are Connecticut, Delaware, Maine, Maryland,
Massachusetts, New Hampshire, New Jersey, New York, Pennsylvania, Rhode
Island, Vermont and West Virginia. The members of the present research com-
mittee who assisted in the preparation of this report are shown below.

Charles R. Frink, Administrative Advisor

CONTRIBUTORS

J. L. Starr, Connecticut (New Haven)

R. V. Rourke, Maine

R. F. Stafford, Maine

J. H. Axley, Maryland

D. N. Maynard, Massachusetts

J. M. Tiedje, Michigan

N. H. Peck, New York (Geneva)

R. H. Fox, Pennsylvania

J. J. Meisinger, USDA/SEA/AR (Beltsville)

Efficient Use of Nitrogen

on Crop Land in the Northeast

Allen V. Barker, Editor

Principles of nitrogen (N) fertilization result from
the cumulative knowledge of plant responses to soil
fertility and to the environment. Research and experi-
ence in fertilizer management, however, continuous-
ly modify these principles by adding new ones and
strengthening or eliminating old ones.

Nitrogen (N) management for production of crops
involves two phases of plant nutrition:

1. The control of available N in the root zone
during the growing season.

2. The uptake of N and response by plants to
N during their growth, development and ma-
turation.

Uptake curves show the amount of N absorbed by
plants as a function of time during the growing sea-
son. The time of greatest uptake of N may not be the
same as that of the greatest plant response in growth
to the available N in the root zone. To obtain the
desired final yield and quality of a crop, the levels
of available N should be high during the time that
plant response to N is maximum.

Nitrogen management systems are being developed
to control the available N from soils and fertilizers

during the growth, development and maturation stages
to obtain reliable, economical and high quality pro-
duce and to minimize losses of N to the environment.
Ideally, N management practices should control the
soil-plant N system from planting to harvest. The
management practices should insure that N is ade-
quate for the development of uniform, vigorous seed-
lings and, during periods of rapid uptake, for the
development of sturdy vegetative frames. As the plants
approach harvest, available N in the soil should be
essentially depleted. Analyses of plant samples at sev-
eral stages of growth monitor uptake and assimilation
of plant nutrients and are helpful in the development
of systems of N management in crop production. Soil
analyses provide an index of the availability of N dur-
ing the growing season.

Plant and soil analyses, however, are not the only
factors involved in developing a fertilization system
for optimum production. The genetic yield potential
of the crop variety, physical conditions of the soil,
control of pests, and other cultural and environmental
factors must be adequate to obtain a positive response
to a fertilization system.

Crop Yield Goals in Nitrogen Management-

When fertilizers are recommended for specific lev-
els of production, the management program is directed
toward a crop yield goal. For instance, if the produc-
tion goal for corn is 100 bu/acre (6 tons/ha), the N
management program should be developed to supply
N to the crop in the quantities needed and at the
proper time to meet this goal. Applications of recom-

mended amounts of fertilizers do not guarantee reach-
ing the production goals, for all other limiting factors
must be corrected.

If all limiting factors are removed, exceptional yields
may be obtained. Corn grain yields have approached
350 bu/acre (22 tons/ha) in areas with optimum cli-
mates for the crop, but in the Northeast, the maximum

Connecticut Agricultural Experiment Station

Bulletin 792

com grain yields with optimum soil regimes for water
and fertilizer have been between 200 and 240 bu/
acre (12 to 15 tons/ha). These yields are only occa-
sionally reached. For example, yields in research plots
in Maryland, New York, and Pennsylvania have rarely
exceeded the 200 bu/acre level. Among the nearly
2,000 entries in corn growing contests in Pennsylvania
from 1968 to 1978, the 200 bu/acre level has been
broken only twenty times, and the maximum yield re-
ported was 239 bu/acre (15 tons/ha). A much more
common maximum yield, and therefore yield goal, is
about 180 bu/acre (11 to 12 tons/ha) in Pennsylvania
and Maryland and 150 bu/acre ( 10 tons/ha ) in New
York and New England. It would be wasteful to at-
tempt to fertilize corn for crop yields exceeding these
yield goals, using present day production practices
and genetic plant material.

Crop yield goals for other common field crops in
the Northeast are listed below:

Yield Goal
Crop Common Units Metric Units

Corn silage 20 tons/acre 45 tons/ha

Soybeans 40 bu/acre 2.7 tons/ha

Millet 3 tons/acre 6.8 tons/ha

Sudangrass 3 tons/acre 6.8 tons/ha

Sorghum 5 tons/acre 1 1 .3 tons/ha

Alfalfa 5.5 tons/acre 12.5 tons/ha

Clover 3 tons/acre 6.8 tons/ha

Crop

Wheat
Barley
Oats
Rye

Yield Gool

Common Units Metric Units

60 bu/acre 4.1 tons/ha

75 bu/acre 5.1 tons/ha

80 bu/acre 2.9 tons/ha

30 bu/acre 2.0 tons/ha

Moisture and temperature are the principal limita-
tions to yields on fertile, well-managed soils of the
Northeast. A recent study for the feasibility of irri-
gation of corn in Pennsylvania (Kibler et al. 1977)
concluded that supplemental irrigation would increase
yields in one year out of two. In the sandy soils of
the Maryland peninsula, water deficits limit maxi-
mum yields to 100 bu/acre (6.3 tons/ha). Cool tem-
peratures, short growing season, and poorly drained
soils limit maximum yields in many areas of the re-
gion to about 80 bu/acre (5 tons/ha).

In most areas of the region, the maximum yield
goals are rarely met, and most farmers, due to limi-
tations in soil, climate and management, attain yields
lower than the maximum. Thus, when the average
good farmer uses yield goals in N management, crops
will be produced more efficiently and fertilization
will cause less pollution if yield goals are realistic
for the soil and climatic zone.

Fertilization to Meet Crop Production Goals

Nitrogen Fert-ilizat-ion of Corn in the

Amount'

Ideally, the amount of fertilizer N applied to a corn
crop is based on knowledge of plant needs, the amount
of N that can be expected to be supplied by the soil,
and a knowledge of fertilizer N efficiency under lo-
cal conditions. Stanford ( 1973 ) expressed this in a
formula N, = (Np - NJ/E, where Nf = N fertilizer
to apply, Np = plant N uptake, Ns = N supplied by
the soil, and E is efficiency of fertilizer N (i.e., the
fraction of N fertilizer that becomes part of Np).

Determination of Np is easy. Stanford (1973) found
from the literature that N was not limiting corn yields
when about 1.2% N was in the above-ground plant at
harvest. Experiments in Pennsylvania have shown that
at optimum N fertilization rates, 18.5 kg N were in
the above-ground dry matter for every ton of grain
(15.5% H2O) produced. Although the value for E can
vary considerably, depending on growing conditions
and fertilizer N management; it usually ranges be-
tween 0.5 and 0.7 (Stanford 1973). Fox & Piekielek
in Pennsylvania recently found in experiments with
corn over a 3-year period that, where a significant re-
sponse to N was observed, the average value for E
was 0.64. Progress is being made in developing soil
tests to estimate N, (see the section on Predicting
Release of Soil Nitrogen), but there are not enough

Northeast

research data to assure that the test will work under
all environmental and soil conditions in the Northeast.

Until a N availability test has been developed that
is applicable to all conditions, N recommendations will
have to be made on the basis of N fertilizer response
experiments over a range of conditions. Eighteen field
experiments were conducted in Pennsylvania by Fox
& Piekielek from 1976 through 1978 to determine
optimum N fertilizer rates for corn. The results in-
dicate that recommendations should be: 18 kg N/ha
per ton of expected grain yield (1 lb. of N/acre per
bu/acre yield) for continuous corn; starter N only
for the first year after alfalfa; with 9 kg N/ha per
ton of production for the second year after alfalfa;
and 15 kg N/ha per ton of production for the third
year after alfalfa. In all but one experiment, an in-
crease of one-third for silage corn and a reduction
of 2.5 kg N/ha per metric ton/ha of cattle manure
and 10 kg N/ha per metric ton/ha of chicken manure
applied the previous year was sufficient for maxi-
mum yield (Table 1).

The yield in this experiment was higher than the
9.4 tons/ha predicted, and the slight underfertilization
that would have resulted if the recommended rate
had been used would have lowered yields by only a
few percent. The average recommendation for the 18
experiments would have been 27 kg N/ha high. A

Efficient Use of Nitrogen
Table 1 . N fertilizer response and recommendations for corn grain in Pennsylvania

rain Yield

Pert. N

Empirical

Error in

(15.5% H.O)

Needed for

Recommend

Starter

Optimum

Recommend

Soil

Year

(F)

(E)

(E-F)

1976

ton /ha

kq/ha

Murrill 1

6.3

9.6

162

169

Hagerstown 1

1976

10.3

10.8

Berks

1976

8.7

Hagerstown II

1976

6.0

8.7

152

109

Pope 1

1976

8.0

9.8

101

152

Pope II

1976

6.4

7.9

169

Murrill 1

1977

5.5

11.1

140

169

Hagerstown 1

1977

10.2

11.4

Hagerstown III

1977

9.7

Hagerstown IV

1977

7.8

9.3

169

102

Pope 1

1977

9.8

11.1

169

Pope II

1977

3.7

11.0

202

226

Hagerstown 1

1978

6.3

10.5

160

141

- 19

Murrill II

1978

6.8

9.7

129

169

Hagerstown III

1978

10.3

10.6

Hagerstown IV

1978

3.5

10.3

165

169

Penn

1978

6.0

9.0

112

141

Chester

1978

6.9

9.6

162

169

Average

Expected yield potential was 9.4 tons/ha (150 bu/acre) for all sites except for Penn, where it was 7.8 ton/ha.
Data of R.H. Fox and W.P. Piekielek. The Pennsylvania State University.

94 kg N/ha excess would have occurred had the cur-
rent recommendations of the Pennsylvania Soil Test-
ing Program been followed. The N recommendations
for corn in New York by Cornell University ( 1979 )
(also based on response experiments) are similar to
those proposed above.

Further research is needed to determine the amount
of residual N from legumes other than alfalfa and to
ascertain if the proposed recommendations will be
accurate under a wider range of soils, climate, and
management conditions. Additional field experiments
for this research can also be used to test the adequacy
of proposed indices for soil N availability (N^) so
that fertilizer N recommendations in the future can
incorporate the N-supplying capability of a given soil
rather than depending on empirically-derived average
N response curves.

Sources

Little difference in effectiveness among N fertilizer
sources exists when they are incorporated in the soil
either before planting or as a sidedress at least 15
cm from the corn plants. Therefore, the most eco-
nomical and convenient source may be used. The high-
er residual acidity from (NH4)2S04 should be recog-
nized and the increased cost of liming considered
when comparing it to other sources. If the N fertilizer
is not incorporated, however, as could be the case
with no-till corn, a N source that does not contain
urea will minimize volatilization losses. In an experi-
ment on a Murrill silt loam comparing N sources on
no-till corn in Pennsylvania, Fox and Piekielek found

no differences among sources in two years where a
rain fell within 2 days after the N application. In one
year with three and one-half rain-free days after N
application (1977), significantly lower yields (Table
2) were obtained and less plant N uptake was ob-
served in the plots receiving urea or urea-containing
N solution than in those that were fertilized with
NH4NO3 or (NH4)2S04 at a rate of 118 kg N/ha.
Urea also produced significantly lower yields and less
plant N uptake was observed at the 218 kg N/ha
rate. Bandel and co-workers at the Universitv of Mary-
land (Bandel 1974, 1975; Bandel et al. 1976) have

Table 2. Effect of N rote and source on groin yield, Murrill
sil, Pennsylvania, 1977.

N-rate

(kg/ha)

N- source

118

218 Average

Starter

5.7 g

NH4NO3

8.7 ef

10.7 bed

1 1.1 abc 10.2

Urea

8.5 f

9.2 e

10.3 d 9.3

N-solution

8.1 f

9.2 e

11.3ab 9.5

(NHJ.SO,

8.5 ef

10.6cd

11.5a 10.2

Values not followed by the some letter are significantly different
at the 5% level according to Duncan's LSD test. Data of R.H.
Fox and W.P. Piekielek, The Pennsylvania State University.

also shown that with no-till corn, urea-containing fer-
tilizers produce lower yields than NH4NO:j (Table
3). Since urea and urea-containing N solutions are
becoming the predominant N sources in the North-
east, more research is needed to learn how to reduce
N volatilization losses from these sources.

Connecticut Agricultural Experiment Station

Bulletin 792

Table 3. Average grain yield response of no-till and conven-
tional till corn following application of 134 kg/ho
N from three sources, Mattapex sil. Poplar Hill,
Maryland. (Bandel et al. 1974, 1975, and 1976).

Check

NH.NOs

Urea

30% N
solution

ton/ha

7.1
9.3

No-tillage
Conventionol

3.5
4.3

8.3
9.7

8.0
9.5

LSD .05 = 0.9.

Whenever N fertilizers are used on no-till corn,
acidity produced from the nitrification of NH4+ is
concentrated in the surface of the soil. Acidifica-
tion is particularly severe with (NH4)2S04, which
produces up to twice as many H+ ions when nitrified
as other common N sources, urea, NH3, urea-NH.jNO;j
solution, or NH4NO3. In the three-year-experiment
comparing N-sources on no-till com, the soil pH values
in the surface 2.5 cm of the plots receiving 212 kg
N/ha as NH4NO3, N-solution, or (NH4)2S04 were
significantly lower than in the check plots (Table 4).
With (NH4)2S04, the most acidifying source, the

Table 4. Effect of N rate ond source on the soil pH of to
2.5 cm layer after three years of application to no-
till corn, Murrill sil, Pennsylvania.

Nitrogen

Urea-

rate

NH..NO3

kg/ha

Urea

NHiNOa

solution

(NHi),SOi

6.96 a

6.66 ab

6.80 ab

6.68 ab

101

6.72 ab

6.84 ab

6.92 ab

6.23 c

202

6.65 ab

6.09 c

6.60 b

5.11 d

Values followed by the same letter are not significantly different
at the 5% level, according to Duncan's LSD test. The pH of
check plots with no N fertilization was 6.96 a. Data of R.H.
Fox & W.P. Piekieiek, The Pennsylvania State University.

soil pH in the surface 2.5 cm was 5.11, almost two
imits lower than the check plots. This low pH could
reduce the effectiveness of herbicides such as triazine,
and if continued, eventually could reduce germina-
tion and growth because of Al and Mn toxicity.

The use of urea or diammonium phosphate (DAP)
in starter fertilizers for corn can reduce germination
and yield (Bouldin et al. 1968; Creamer 1978). Crea-
mer (1978) found that urea was considerably more
toxic than DAP and that to avoid potential germina-
tion problems, urea should not be used in starter fer-
tilizer. It appeared that NH3 was the phytotoxic agent
with both urea and DAP. The higher pH in the zone
of urea hydrolysis (pH 9.1 in Hagerstown silt loam,
original pH 6.8) drove the equilibrium, NH4+ +
OH^-^NHa + H2O, to the right, so that approxi-
mately 40% of the total ammoniacal N (TAN) was
in the form of NH3. The pH around a band of DAP
was only 7.3. At this pH only 1.4% of the TAN would
be NH3, explaining the less severe toxicity of DAP.
Peck ( N.Y. State Agricultural Experiment Station, Ge-
neva) also observed less severe toxicity to corn with
banded DAP than with urea. These results indicate
that DAP could be used safely in corn starter fer-
tilizers if it is used at moderate rates (25 kg N/ha
or less) and if the band is kept at least 5 cm to the
side and 5 cm below the seed. Less care is needed
when monoammonium phosphate or NH4NO3 is used
as the N source in the starter fertilizer, for in soils
with less than 7.0 pH, almost no NH3 is formed with
these fertilizers.

Timing

Lathwell and co-workers at Cornell ( 1970 ) demon-
strated that side-dressed applications of N when the
corn is 30 to 45 cm high are more efficient than
either fall or spring preplant applications. They found
that, on the average, fall-applied N was only 37% as
effective as side-dressed N. Similar results have been
reported for the Midwest (Olson et al. 1964) and
Southeast (Pearson et al. 1961). Consequently, fall
applications of N are not recommended for the North-
east. Although side-dress applications are the most
efficient, many farmers still apply N as a preplant
broadcast treatment because (1) there is a risk of
wet soils during the time side-dressed N should be
applied (2) the use of chemical herbicides has made
it unnecessary for the farmer to cultivate for weed
control, therefore a side-dressed application means
an extra trip through the field.

Nitrogen Fertilization of Small Grains in the Northeast

Wheat, barley, oats and rye were bred for increased
yields on relatively infertile soils before the present
day use of commercial fertilizers. The result was that
the use of small amounts of N (30 to 60 kg N/acre)
would likely lodge these grains, especially oats, barley
and wheat, in that order. In the past 50 years, im-
provement in these varieties has been made by de-

creasing stem length and increasing disease resistance.
Now some varieties will yield two to three times more
if fertilized with as much as 100 to 120 kg /ha of fer-
tilizer N.

However, each variety of oats, barley or wheat has
its own N requirement. New N availability tests now
being developed in the Northeast will play a major

Efficient Use of Nitrogen

role in small grain fertilization, because all varieties,
old and new, can be lodged with excess fertilizer
N. The rate of application is critical since maximum
yields are obtained if N is just below the amount that
causes lodging.

If lodging is expected, no N fertilizer or manure is
used; however, in the absence of soil test recommen-
dations, 70 kg of PuOr, and 70 kg of K-O are often
used per hectare.

If no lodging is expected, 15 kg N/ha are drilled
with the seed, along with the P20r, and the K2O. The
crop is then top-dressed, after growth has begun,
with 20 to 60 kg of N/ha, depending on grain variety
and soil. The county agent or state crops specialist
can provide the proper fertilizer N recommendation.

In general, rye yields poorly in the Northeast and
it is seldom growoi other than for seed or for erosion
control; its N requirement is often 20 to 30 kg/ha.

Nitrogen Fertilization of Vegetables Grown for Processing in the Northeast

Quality vs. Yield

The quality of vegetables used for processing is
defined mainly by the stage of growth, development,
or maturation of the edible portions at harvest. The
optimum processing quality determines the harvest
date, which in turn affects the yield. As most vege-
tables approach optimum harvest quality, yields rap-
idly increase.

Processing quality may also be defined by the con-
centrations of total N (including nitrate-N and other
N compounds) accumulated by the vegetable and by
other chemical and physical properties of the pro-
cessed portions of the vegetable.

Uniformity

Vegetables grown for processing are harvested with
once-over, plant-destructive mechanical harvesters. At
harvest, the plants are at rapidly changing stages of
growth, development and maturation. Only the plants
or portions of the plants at an acceptable stage of
growth and quality for processing are usable. Most
vegetables are harvested when the most mature plants
have reached processing maturity. The other plants
are treated as weeds. Establishment of uniform, vig-
orous, potentially productive seedlings is the initial
phase and the foundation leading to uniformity among
plants harvested for processing. Any variation among
seedlings may result in unevenness among plants at
harvest time. To extend the harvest season, the first
plantings of vegetables grown for processing are often
seeded in cold, wet soils which retard seed germina-
tion and seedling growth. Late plantings are often
made under stressful conditions of excessively high
temperature or lack of adequate water in soil sur-
rounding the seeds and in the root zone of the seed-
lings. Reliable yields of high quality produce are
needed throughout the harvest season to ensure a
continuous flow of raw product for processing.

Nitrogen fertilization affects the reliability of both
quality and yield in vegetables grown for processing.
Of the factors involved in crop production (genetic
potential of plants, productivity of the soil, weather,
and production management), control of available N
in the rhizosphere is one of the most variable and
difficult factors to manage. All other growth factors

must be adequate to obtain a positive crop response
to a N fertilization program.

A good N management system for crop production
is based on preplant predictions of residual available
soil N plus fertilizer N requirements, but continual
adjustments in N fertilization must be made for un-
predictable weather events which affect availability
of soil and fertilizer N and plant responses during
the growing season. Ideally, soil-fertilization manage-
ment should control the soil-plant system from plant-
ing to harvest, insuring abundant N for development
of uniform vigorous seedlings, supplementing soil N
during periods of rapid uptake for development of
sturdy vegetative structures, and allowing depletion
of available N in the rooting zone as the plants ap-
j)roach harvest.

The practical application of N fertilization is con-
sidered in four steps:

1. Amount of N needed for profitable production
of reliable yields of high quahty produce.

2. Time the N is needed by the plants for optimum
response.

3. The effect of N on the quality of the portion
of the vegetables to be processed.

4. Management of soil N plus fertilizer N for op-
timum quality and yield.

Table Beets and Nitrogen

Since fertiHzation of table beets has been studied
throughout the growing season, table beets will be
used as a case history to illustrate a plant response
to soil N plus fertilizer N. The responses of table
beet plants to N fertilizers have been reported by
Peck et al. (1974).

The practical application of these results to fertili-
zation will be considered under the four steps: up-
take, time, quality and availability.

1. Uptake: Amount of N needed for profitable pro-
duction of reliable yields of high quality table
beet roots for processing.
Table beet plants grown without fertilizer N re-
moved 110 kg N/ha from the soil (Table 5). The
soil was a Honeoye fine sandy loam (Glossoboric
Hapludalf, fine-loamy, mixed, mesic), a productive

Connecticut Agricultural Experiment Station

Bulletin 792

Table 5. Table beet responses to N

Yield

Dry weight

Total N

Fertilizer
ammonium nitrate

roots
(fresh wt.)

Tops

Roots

Tops

Roots

Total

Preplant* Sidedressed
May 4 July 1

kg /ho

17000
21000
30000
28000

kg /ha

kg N/ha

O O o o
o o o o

3500
4600
3700
5000

3400
3200
5500
4900

70
150

90
180

100

140

230
190
320

* Table beet seeds planted May 22. Roots harvested August 12.

Data of N.H. Peck, New York State Agricultural Experiment Station, Geneva.

soil derived from calcareous glacial drift, which had
a cover crop of oats removed the previous year. As-
suming that 110 kg N/ha came from the soil, an ap-
plication of an additional 110 kg N/ha preplant was
70% taken up by the tops and enlarged portion of roots.
Fertilizer N side-dressed early at 110 kg N/ha was
nearly all taken up by the tops and roots.

Fertilizer N applied preplant increased yields more
than side-dressed N. Fertilizer N applied preplant in-
creased the total yield, especially the yield of the
large diameter roots. These large roots are of less
value for processing than small roots. A large amount
of N was returned to the soil in the tops since only
the roots are removed from the field by mechanical
harvesters.

At the early harvest date, August 12, the table beet
plants grown with only soil N plus preplant fertilizer
N were approaching N deficiency. This made har-
vesting by "pulling the roots by the tops" difficult,
for the petioles tended to break from the root. Thus,
additional side-dressed N was needed to maintain
healthy petioles and leaf blades for efficient mechani-
cal harvesting.

2. Timing: Providing N at the time needed by the
plants for optimum response.

Uptake of N preceded gain in dry weight by the
table beet plants (Figure 1). Since the concentration
of total N was high in the seedlings and the con-
centration decreased throughout the growing season,
abundant available N was needed for the seedlings
early in the season, followed by a large supply of
available N during the rapid uptake and a depletion
of available N in the soil as the plants approached
harvest.

3. Quality: Effect of N on the quality of table
roots to be processed.

Root size: Fertilizer N applied preplant increased
the yield of large diameter roots, which are of less
value for processing than are small roots.

Nitrate: An application of only 110 kg of fertilizer
N/ha before planting caused rapid vegetative growth
of the seedlings and a high concentration of plant

200

MAY JUNE

Figure 1 . Growth and N uptake of table beet plants. Planted
May 22 and harvested August 12. (N.H. Peck, New York)

nitrate (Figure 2). As the plants developed and as
the portion of the roots used for processing enlarged,
the concentration of nitrate gradually decreased re-
sulting in high yields of roots low in nitrate at the
early harvest date of August 12 (Figure 3).

Glutamine and sugar: Glutamine accumulation due
to fertilizer N treatment occurred in the roots of the
beet plants. In the petioles and blades, the glutamine
concentration remained constant throughout the grow-
ing season at 0.5 to 1.0% on a dry basis.

In the roots, glutamine represented a third (20-
40%) of the total N; and the buildup of this compound
was directly related to the rate of fertilizer N appli-
cation. If the plants eventually received some fertilizer
N, the roots continuously increased in glutamine con-
centration to a steady-state concentration of 4 to 6%.
Without glutamine, the maximum amount of accumu-
lated glutamine was 2%.

I-

ill

Efficient Use of Nitrogen
7.0

30 7 15 22 29 5 12
JUNE JULY JULY JULY JULY AUG AUG

Figure 2. Concentration of nitrate-N in table beet petioles.
A = No N fertilizer, B = N fertilizer May 4, C = N
fertilizer July I, D = N fertilizer Moy 4 and July 1. (N.H.
Peck, New York)

When fertilizer N was applied before planting, the
steady-state concentration of glutamine was reached
early, and there was little response to additional side-
dressing applications of fertilizer N. When fertilizer
N was not applied before planting, there was marked
response to side-dressing applications of fertilizer,
leading again to a higher concentration of glutamine.

As the roots from plants grown on the various fer-
tilizer treatments matured, the total sugar (sucrose)
concentration increased, but at a rate inversely pro-
portional to the total amount of fertilizer N applied.

0.6

0.5

q:
ui
a

30 7
JUNE JULY

Figure 3. Concentration of nitrate-N in table beet roots. See
Figure 2 for explanation of symbols. (N.H. Peck, New York)

JUNE

JULY

AUG

Figure 4. Concentration of glutamine in table beet roots. See
Figure 2 for explanation of symbols. (N.H. Peck, New York)

Plants that received no fertilizer N showed a linear
increase in sugar from 25% dry basis in late June to
57% on August 12.

At the other extreme, plants that received 330 kg
or more fertilizer N had only 47 to 50% total sugar at
harvest on August 12. An inverse relation was found
between the amount of glutamine and sugar present
in the roots at harvest on August 12. One example of
this relation is shown in Figure 5, where the concen-

10.0
9.0

8.0

iij

S 7.0

(3 6.0

cs 5.0

I-

u 4.0
o

a: 3.0

a.
2.0

H57
56

SUGAR ^o

(S>

1(0

FERTILIZER N, kg/ha

440

49
48

Figure 5. Concentration of glutamine and sugar in table beet
roots OS percent of dry weight. Fertilizer was added before
planting. (N.H. Peck, New York)

Connecticut Agricultural Experiment Station

Bulletin 792

trations of sugar and glutamine in the roots are shown
only for those plants receiving fertilizer N before
planting at the rate of 0, 110 and 440 kg N/ha. The
regression of sugar on glutamine, taking results from
all fertilizer N treatments, is shown in Figure 6. The
fact that a 2% reduction in sugar leads to a 1% in-
crease in glutamine suggests a stoichometric relation,
with important organoleptic significance to process-
ing quality.

4. Availability: Management of soil N and fertilizer
for optimum quality and yield.

Research with New York soils which are in con-
tinuous vegetable crop production (with no legume
in the rotation) indicates that such soils will supply
about 110 kg N/ha to table beet plants during the
growing season, depending on soil aeration, soil tem-
perature, soil water available for soil organisms and
plants, and losses of N due to leaching, runoff and
volatilization (Figure 7). A warm, moist soil favors
release of soil N to plants.

Fertilizer N needs may be estimated prior to plant-
ing. Some of the required N fertilizer may be applied
preplant, but N fertilizer application rates and timing
may be adjusted during the course of the growing
season, depending on the weather, potential crop yield,
and length of time to harvest.

Assuming that the soil supplies 110 kg N/ha and
assuming that 20 to 40 kg N/ha are supplied in the
starter band fertilizer, approximately 110 kg addi-
tional available N/ha from fertilizer are needed dur-
ing the growing season, especially during the early
"middle period" of the growing season, when uptake
of N by the roots is rapid.

60
59

"I 1 r

Y=58.9-2.06X
r" -0.925

45'

1.0 2.0 3.0 4.0 5.0

PER CENT GLUTAMINE

6.0

Figure 6. Sugar and glutamine correlation In table beet roots.
(N.H. Peck, New York)

FERTILIZER N

SOILN

22
MAY JUNE

12
AUGUST

Figure 7. Available soil N plus fertilizer N for table beet plants planted May 22 and harvested August 12. The shaded areas in-
dicate the variability in soil and fertilizer N caused by variable weather. (N.H. Peck, New York)

Efficient Use of Nitrogen

About two-thirds of the fertiHzer N applied broad-
cast-preplant and worked into the soil is available to
the plants during the growing season. Thus, 150 kg
fertilizer N/ha are needed preplant to supply 100 kg
available N/ha to the plants during the growing
season.

Most efficient plant use of N fertilizer occurs when
the N fertilizer is applied as side-dressed applications
during the growing season just prior to the plant de-
mand for rapid uptake and response. Early applica-
tions of side-dressed N are necessary within the limit-
ed rooting zone of seedlings to develop large plants
and rooting systems which extend through the sur-
face soil. Plants need a large, efficient root system
to remove soil N during the growing season. As har-
vest approaches, the soil may be nearly depleted of
available N. But side-dressed N applied below the
surface and within the root zone is nearly all avail-
able to the plants. Sources of N fertilizer for side-
dressing include solutions, urea and ammonium ni-
trate. Additional side-dressed N may be needed later

in the growing season to maintain top growth for
easy mechanical harvesting, if the beets are held for
late harvest.

Overall fertilization system

Fertilizer N may be added to supplement soil N
when the crop response is likely to be most intense.
Seedlings fertilized with N at or near planting time
had a high concentration of nitrate and developed
large plants which used both the fertilizer and soil
N during the growing season. As harvest approached,
nitrate concentration in the roots declined to a low
level. Further, the roots had a low glutamine con-
centration, and a high sugar concentration, and yields
were abundant. Since excess nitrate and nitrite in
food are potential health hazards and since excess
glutamine may cause bitter flavor in canned beets,
their levels should be low at harvest time. In this
experiment, proper N management resulted in quality
processing produce and the soil was nearly depleted
of available N at harvest.

Predicting the Release of Soil Nitrogen

Review

Nationally, nitrogen fertilizer consumption has in-
creased steadily over the past 15 years (Figure 8). In

10 -

2 6

m
g

i 5

Z 4

(r.

UJ
N

fe 3

2 -

. ,/V / wheat"

7.0
6.5

6.0

5.5 o
5.0 I

4.0 a

3.5

2.00 S

1.75 ^
1.50

I960 1965 1970 1975 1980

Figure 8. Nitrogen fertilizer use and maize and wheat yields
for the United States since 1960. (J.J. Meisinger, USDA,
Beltsville)

the Northeast, consumption has also been rising, al-
though at a slower rate (Figure 9). Grain yields of
com have grown during this period (Figures 8, 10,
11 ), but yield increases have slowed in the past 5 years.
Fertilizer N is an important resource in crop pro-
duction; however, crops also obtain significant quan-
tities of N from other sources. The data in Figures
10 and 11 indicate that Maryland agriculture is more
dependent on fertilizer N sources than is Pennsyl-
vania agriculture, since the quantity of N applied
per unit of yield is greater in Maryland. This situa-
tion reflects the greater livestock-based agriculture in

I960 1965 1970 1975

Figure 9. Nitrogen fertilizer use in the northeastern United
States. (J.J. Meisinger, USDA, Beltsville)

Connecticut Agricultural Experiment Station

Bulletin 792

1965

1970

Figure 10. Maize grain yields and N fertilizer use for Mary-
land, 1960-1977. (J.J. Meisinger, USDA, Beltsville)

Pennsylvania and the consequently greater use of le-
gume and manure N in place of fertilizer N. Native
soil N can supply significant quantities of N to a crop,
but soils differ in their N-supplying capacity. The
ultimate aim of any system used to assess soil N is
to predict the N supply accurately so that the crop
requirement can be met efficiently through some com-
bination of native soil N and fertilizer N.

Predicting available soil N in the Northeast is dif-
ficult because the soil is dominated by the N min-
eralized from organic sources during the growing
season. This situation arises from the fact that about
30 cm of rainfall leach through northeastern soils
annually (Frere 1976) and any residual mineral N
is usually moved into the soil profile. This soil N
status contrasts sharply with that of the western states,
where residual mineral N dominates the available soil
N supply. Because of dependence on recently min-
eralized N, the northeastern states face a more dif-
ficult problem in predicting the availability of soil N
than the western states, where many laboratories are
using NO:i-N as a soil test (Carter et al. 1974; Dahnke
& Vasey 1973).

Procedures to predict the release of soil organic N
can be broadly grouped into crop vegetative tests,
microbial tests, total analysis of soil components, and
chemical extraction tests (Allison 1956; Dahnke &
Vasey 1973). Vegetative tests include greenhouse and
field studies. These procedures might involve measur-

=! 80

1965

1970

1975

Figure 1 1 . Maize grain yields and N fertilizer use for Pennsyl-
vania, 1960-1977. (J.J. Meisinger, USDA, Beltsville)

ing yield, dry matter, or total N uptake of a crop re-
ceiving no fertilizer N or a known quantity of N.
These procedures have generally been accepted as
the standard by which other methods are evaluated,
since they integrate the factors of crop growth and
soil N released under natural conditions. They rep-
resent the oldest group of tests, and form an indis-
pensible part of modem soil N evaluation by serving
as the means of calibrating other more empirical pro-
cedures. Grove (1979) suggested that the soil N sup-
ply can be estimated from corn grain yields on non-
fertilized areas. The resultant grain yield is converted
to a N uptake value, which is subtracted from the
N uptake required for the grower's yield goal. The
difference is the N fertilizer need.

Microbiological tests usually involve incubating a
sample of soil under ideal temperature and water
conditions and periodically measuring the N mineral-
ized. Variations on this procedure involve differing
lengths of incubation (1 week to several months), aera-
tion conditions, and types and amounts of inert bulk-
ing materials added ( Bremner 1965a; Dahnke & Vasey
1973). Sample pretreatment has a marked effect on
microbial tests ( Bremner 1965a; Harmsen & VanSchre-
ven 1955; Stanford 1968), especially those involving
short-term incubations. Microbial tests have been used
for many years (Fraps 1921; Stanford & Hanway 1955;
Waksman 1923) and are considered by most workers
to be the most satisfactory method of assessing N
status of the soil apart from field or vegetative tests.
However, to obtain meaningful comparisons among

Efficient Use of Nitrogen

soils, care must be taken to measure all of the inor-
ganic N, to standardize the pretreatment of the sam-
ple prior to incubation, and to standardize the tem-
perature and water during incubation. Several authors
(Bremner 1965a; Dahnke & Vasey 1973; Harmsen &
VanSchreven 1955) have reviewed these points, un-
derscoring that soils are a complex biological entity.
Nevertheless, microbial procedures have proven useful
and have been shown to be a good indicator of soil
N-supplying power ( Bremner 1965a; Carter et al. 1974;
Eagle and Matthew 1958; Gasser & Kalembasa 1976;
Jenkinson 1968; Keeney & Bremner 1966b; Stanford &
Legg 1968). Others have shown that laboratory min-
eralization data may be adjusted to field conditions
by correcting for the field temperature and water
regime (Smith et al. 1977; Stanford et al. 1977). This
less empirical approach, however, is time consuming
and is principally a research approach.

Procedures based on total soil analysis usually es-
timate soil organic matter, organic N, or organic car-
bon. These nonbiological procedures have proven most
useful in separating soils \\ith large differences in
total N contents and in classifying soil within a series.
This procedure has greatest acceptance in states with
a large range of soils and crop management systems.
Typically the soil organic N data are utilized with
soil drainage class, soil texture, and previous man-
agement history to anive at an estimate of soil N
release. The major criticism of these procedures is
that they measure the total N pool, which is composed
mainly of slowly available material. The active frac-
tion of the soil organic N makes up a small percentage
of the total N but dominates the N release ( Bremner
1965b; Jansson 1958). Thus, a procedure which mea-
sures the total soil N \\'ould be relatively insensitive
to changes in the size of the small, active N pool.
Predicting soil N release solely on the basis of a total
analysis has, therefore, met with only limited success.

By far the most intensively investigated area for
predicting N release has been the chemical extraction
tests. These strictly empirical procedures provide sim-
ple, quick methods of selectively removing a portion
of the active soil N. These tests are appealing for rou-
tine soil analysis because they require simple analyti-
cal procedures, are quick, and can be used on large
numbers of samples. The range of chemical extractants
includes water, weak to strong salt solutions, mild
acids and bases, and strong acids and bases with or
without an oxidant. The extracted material has like-
wise been analyzed in a number of ways including
carbon content, distillable ammonia, absorption of ul-
traviolet radiation, and total N. The results of these
empirical methods are usually influenced by factors
such as extraction time, sample drying procedure, and
soil to extractant ratio.

Livens ( 1959a, b ) was one of the early investigators
who showed the close relation between N extracted
with boiling water and aerobic N mineralization. He
also showed that the NaOH-distillable N fraction of
the extract was more closely related to mineralization

than the total N extracted. Large effects due to ex-
traction time, extractant to soil ratio and quantity of
soil were noted. The hot water procedure was later
modified (Keeney & Bremner 1966a, b) to improve
filtering and was again shown to be highly related
to N released during aerobic and anaerobic incuba-
tion and to crop N uptake under greenhouse condi-
tions. Keeney & Bremner ( 1966a, b ) considered the
hot water method to be as good as incubation meth-
ods; furthermore, they found that it was not subject
to sample drying effects which influence the micro-
biological method. A boiling extract with a mild salt
solution ( 0.01 M CaCL ) has also been investigated
(Fox & Piekielek 1978a; Stanford 1968; Stanford &
Smith 1976) and has been shown to be highly related
to biological mineralization and to crop N uptake.

Methods involving extraction with mild salt solu-
tions at room temperature have centered on sodium
bicarbonate (MacLean 1964) and have also been
shown to be related to N mineralization and uptake
(Fox & Piekielek 1978a; Jenkinson 1968; Smith 1966).
Recent work (Fox & Piekielek 1978a, b) has shown
that absorption of ultraviolet radiation by extracts
with sodium bicarbonate bears a close relation to to-
tal N content. The quicker determination by ultra-
violet absorption could be substituted for the more
tedious total N analysis. Strong salt solutions, such
as molar potassium salts at room temperature, have
given mixed results when compared to crop N uptake
(Fox & Piekielek 1978a; Lathwell et al. 1972).

Interest in basic and acidic extracting solutions de-
veloped as a natural extension of the procedures of
fractionating soil organic matter with differential sol-
ubilities in acids or bases. The various reagents com-
monly involved are H2SO4 (Purvis & Leo 1961; Rich-
ard et al. 1960), Ca(bH)o (Prasad 1965), Ba(OH).
(Jenkinson 1968), and NaOH (Cornfield 1960). When
strong acids or bases are used, much of the soil N
is attacked and the N removal bears a close relation
to the quantity of total soil N ( Bremner 1965a ) . Strong
acidic and basic extractants suffer the same disadvan-
tages as those discussed above for the total N pro-
cedures. Milder acidic and basic extractants have been
evaluated in several studies, performing well in some
but poorly in others. The remaining group of chemical
extractants has employed oxidizing agents (perman-
ganate or chromate) in acidic or basic media to se-
lectively oxidize a portion of the organic N (Nommik
1976; Stanford 1978a, b; Truog 1954). The alkaline
permanganate procedure, including several modifica-
tions, has been widely tested (Keeney & Bremner
1966b; Prasad 1965; Richard et al. I960; Stanford &
Legg 1968) and the recent review of this procedure
(Stanford 1978b) indicates that it has not given con-
sistently good results. More recently, oxidation pro-
cedures have been investigated under acid conditions
(Nommik 1976; Stanford 1978a) with encouraging re-
sults for their general applicability in a practical soil
testing operation.

Connecticut Agricultural Experiment Station

Bulletin 792

Research in the Northeast

In the Northeast, soils and farm management
schemes are diverse, ranging from dairy farms using
heavy, annual farm manure applications and alfalfa
in the rotation every few years to farms that have
grown corn every year on the same soil for at least
10 years with no manure applications. In view of this
diversity, the N supplying capability of a soil must
be known if accurate recommendations for fertiliza-
tion are to be made. Though some allowance is made
in N fertilizer recommendations for legumes in a ro-
tation and for manure applications, enough research
data are not available to reliably predict the effect
of legumes and manures on the N availability in a soil.

A number of chemical tests have been developed in
which an extracted fraction of soil organic N is mod-
erately well correlated with the N supplying capacity
of soils in greenhouse experiments (Keeney & Brem-
ner 1966b; Lathwell et al. 1972; MacLean 1964; Purvis
& Leo 1961) or with the N mineralization potential
of a soil (Stanford & Demar 1969) but the relia-
bility of these tests has not been checked under field
conditions.

Experiments were conducted in Pennsylvania to de-
termine if one or more of the chemical N availability
indices were correlated with the capability of several
soils to supply N to field grown corn ( Fox & Piekielek
1978a). Rates of fertilizer N up to 179 kg/ha plus
starter N were also appHed to determine the N re-
sponse by corn in these soils. Since the analyses for
all the tested chemical N availability indices were
time consuming, an attempt was made to develop a
more rapid method of analysis without sacrificing the
ability of the index to predict the N supplying capa-
bility of a soil (Fox & Piekielek 1978b).

The results of correlating eight chemical availability
indices with soil N supplied to corn in 12 experiments
on eight sites over a 2-year period demonstrated that

four indices were significantly correlated with the N
supplying capability of the soils (Table 6). The best
correlated test was the amount of N extracted by re-
fluxing with 0.01 M CaCl. for one hour (r = 0.86,
modified Keeney & Bremner 1966b) followed by
0.01 M NaHCOs extractable N (r = 0.77, MacLean
1964), NH4-N extracted by 0.01 M CaCL in a 16-
hour autoclave treatment (r = 0.70, Stanford & De-
mar 1969) and Kjeldahl total soil N (r = 0.68). The
soils in these experiments were well drained Ultisols
(Hagerstown silt loams) and Inceptisols (Pope silt
loam and Berks shaley silt loam ) in southeastern, cen-
tral, and northeastern Pennsylvania. They had received
a wide variety of previous management, including 10
years of corn with no manure, 8 years of alfalfa grass
meadow, or continuous corn with annual applications
of chicken manure. This diversity of management
was reflected in the wide range of N, 27 to 170 kg/
ha/season, supplied by these soils to a growing corn
crop. In the two corn crops planted the first year after
alfalfa, there was enough residual N from the alfalfa
to obtain maximum grain yields (10.3 and 9.7 metric
tons/ha) with only starter N.

The analysis for the N availability indices which
correlated significantly with the N supplying capa-
bility of the soil required either a Kjeldahl total N
analysis or an overnight autoclaving. These analyses
were thought to be too time consuming and expensive
to include in a routine soil testing program where
hundreds of samples per week are analyzed. In seek-
ing ways to simplify and shorten the analysis of the
soil extracts used for these indices, the ultraviolet
(UV) absorption of the 0.01 M NaHCO^ extract at
260 nm was found to be as well correlated with the
field measured soil N supplying capability as the best
of the previously tested indices (Figure 12) (Fox &
Piekielek 1978b).

Toble 6. Linear correlation coefficients between several N availability indices and soil N supplying capability.

Year

O.M.

Total
N

Index for Assessing N Availability^

Average Keeney
NO3-N Bremner MacLean
0-60 cm N N

Stanford
NH.,-N

Purvis

Leo
NHd-N

Lathwell
N

0.76

0.94°°

0.77°°

0.72

0.92°°

0.70°

0.51

1976
1977
combined

0.77
0.03
0.38

0.65

0.81°

0.68°

0.49
0.10
0.16

0.88°
0.90°
0.86°°

0.56

' See bibliogrophy for references to named procedures.

° Significant at the 5% probability level.

°° Significant at the 1% probability level.

t Linear correlation coefficient for index with total plant N in check treatments minus 0.75 X starter N added.

Adapted from Fox Cx Piekielek (1978a).

Efficient Use of Nitrogen

170

<
O

160-
150-
140-
130-

120-
10-

OI976
•1977

Y«-74 + 773X
r= 0.865

.14 .16 .18 .20 22 .24

ABSORBANCE

.26

.28

.30

.32

Figure 12. Nitrogen supplying capacity of soil vs. ultraviolet absorption of soil extract. (R.H. Fox, Pennsylvania)

The double bonds in organic matter absorb UV light
near 260 nm; the absorption by the extract is a mea-
sure of the organic matter content of the extract ( Rao
1967). Further, the concentration of N in the organic
matter is assumed to be quite constant from soil to
soil (Bremner 1949). In support of this assumption
the coefficient between the UV absorption at 260 nm
and the N concentration of the 0.01 M NaHCO:j ex-
tracts was 0.91. Therefore, it follows that if the N con-
tent of the extract was correlated with the N supply-
ing capability of soil, the UV absorption by the ex-
tract would also be correlated with this capability.

The equation, Nj =

Np - N,
E

, proposed by Stan-

ford (1973) illustrates how this test could be used
to predict N fertilizer requirements of a crop of corn.
Assumptions to be made for a com crop follow:
Np (kg/ha) = 18.5 X (maximum grain yield ex-
pected, metric ton/ha, 15.5% HoO)
N, (kg/ha) = 773 x (UV abs at 260 nm of 0.01

m NaHCOa extract) - 74
E = 0.6

Nf (kg/ha), maximum = 20 X (maximum grain yield
expected, metric ton/ha, 15.5% HoO)
minimum = 15 in starter fertilizer
The Np factor was calculated from the ratio of the
grain yield to total crop N at the lowest N rate that
produced maximum yields in the N response experi-
ments (Fox & Piekielek 1978a). The equation for N^
was derived from the regression between the UV ab-
sorbance of the 0.01 M N9HCO3 soil extract and the
N supplying capability of the soil shown in Figure
12. An E of 0.6 was fairly representative of the fer-
tilizer N efficiency that can be expected at N fer-
tilizer rates adequate for maximum yields with good
crop and fertilizer management.

Using the above equation and the data from the
1977 corn yield response to N fertilization experiments,
the N fertilizer recommendations would have averaged
24 kg/ha high for the six sites with a range of to
56 kg/ha high for the individual sites (Table 7). The
validity of the test for the equation may be ques-
tionable, since 1977 data were used in formulating
the equation for Nj; however, it does serve as an ex-

Connecticut Agricultural Experiment Station

Bulletin 792

Table 7. Comparisons of N fertilizer recommendofions for maximum corn grain yields by N availability index
and crop requirement methods with experimentally determined requirement, 1977 experiments in
Pennsylvania.

N-recommenc

lotion

Error

Field Experiment

method

recommendation

N-opplied

Crop

N-ovoil.

NAI

Experiment

Maximum

max. yield

require.

index

mrnus

minus

site

yield

(NF)

(CR)

(NAD

ton/ha

kg/ho

kg /ho

11.0

140

269

187

129

11.3

269

103

185

9.7

140

118

9.1

224

123

157

11.0

112

269

1 14

157

I 1.0

202

269

220

avg.

10.5

104

240

127

136

Data of R.H. Fox, The Pennsylvania State University.

ample of the improvement that can be made on the
current practice of basing N fertilizer recommenda-
tions solely on crop requirements. If the currently
used N fertilizer recommendations, based on the N
requirement of the crop, had been used for the 1977
experiments, they would have been on the average
136 kg/ha too high with a range of 67 to 185 kg/ha
in excess.

If further field studies show that this test is as
well correlated with the N supplying capability of a
wide range of soils as it was for the soils tested, it

could greatly increase the accuracy of N fertilizer rec-
ommendations made with routine soil testing. The
test is as simple, quick, and inexpensive as any nu-
trient availability test currently used and would cer-
tainly be more accurate than making N recommenda-
tions based solely on crop N requirements ( Schulte
1977). This increased accuracy will not only increase
the efficiency of N fertilizer use by farmers, but will
also minimize possible water and air pollution from
excess N fertilization.

Assessing and Controlling Nitrogen Losses from Soils

Management of N in the environment to conserve
N against losses is of increasing priority in the North-
east. One of the first requirements of management is
to determine the relative quantities of N moving into
subsurface waters and lost through denitrification or

Leaching Losses of Nitrogen

Studies in Connecticuf-

Tobacco plots: Nitrogen movement through a Merri-
mac sandy loam (Entic Haplorthod) and losses to
ground water under shade tobacco were studied by
Starr and DeRoo (cf. Starr 1975) for 3 years at the
Valley Laboratory, Windsor, Connecticut. Nitrogen
losses from conventional organic fertilizers for cigar
tobacco (DeRoo 1958) were compared with losses
observed when nitrogen was supplied from ammonium
sulfate. In the first 2 years with conventional fertili-
zation of 224 kg N/ha, results showed that most of
the movement of N to the ground water occurred
during the fall season, resulting in a NO:.-N content
of about 25 ppm in the groundwater, with little dif-
ference between organic and inorganic N sources
(Figures 13, 14). In the third year, 40% less N fer-
tilizer was applied at five intervals as ammonium sul-
fate during the growing season, and was compared
with the standard fertihzer practice of broadcasting

ammonia volatilization. Within the Northeast Region,
assessments of N losses have been made in laboratory
and field studies under different cropping and fertili-
zation regimes.

224 kg/ha at the time of planting (Figure 15). Un-
der these conditions, the concentration of NO.i-N in
the water table from October to December under the
low N plots was 50% less than that under the plots
with the standard fertilizer treatment.

These studies were expanded to a commercial field
of shade-ground cigar tobacco. This study on a Mel-
rose sandy loam (Entic Haplorthod) showed that the
NO.i-N concentrations in the ground water at a depth
of 1.8 to 2.4 meters were 14 to 27 ppm, and tended
to remain at these concentrations through the year.
This result is indicative of the cumulative effect of
long term, intensive fertilizations with predominantly
organic materials. Decreasing the total amount of ap-
plied fertilizer, which was detrimental to tobacco
quality, or increasing the number and amount of
postplanting application, which was slightly benefi-
cial, did not significantly affect NO:i-N leaching ( De-
Roo 1980).

Efficient Use of Nitrogen

ORGAN C-N

NORGAN C-f

RAIN ^

III

1 1

"'1

1 '

(cm) 5

I
UJ

224 KG/HA

100^

2 KG/HA

30-CM

_QJL

,•-•

6 0- CM

O />^^^

..«^-2B2g^, "^9i»-»<S.-»-

>s:

TThatfflU^fli rf

>-^=^:'°

, ^.^..-te^i^^e -^ — -p-^-g^-^r

i8Q-CM o p^

240-GM

m».lmAni»m.mA,*, dm » ft « ko ^ I W^

6 8 10 12 2

MONTHS

Figure 13. Distribution of rainfall with time and distribution of nitrate-N concentration with soil depth and time beneath shade
tobacco in 1974. (Data of J.L. Storr and H.C. DeRoo, Connecticut)

Turf plots: Nitrate-N movement and losses to ground-
water beneath turf grass grown on a Merrimac sandy
loam were studied for 3 years at the Valley Lab-
oratory (Starr & DeRoo 1979).

Field plots, instrumented with suction lysimeter and
neutron probe access pipes, were utilized to study the
fate of N fertilizer applied to turf grass. Fertilizer N
was applied at an annual rate of 180 kg /ha to each
plot in a split application of 90 kg/ha each in May
and September for three consecutive years. Grass clip-
pings were returned, after subsampling, on two of
the four plots. In the third year, the use of ^'N as
a tracer in conjunction with management of grass
clippings provided the means to quantify the N in
the grass derived from fertilizer, soil, current year's

grass clippings, and the previous two years of grass
clippings.

A summary of N uptake showing the relationship
of the various sources of N found in the grass is given
in Figure 16. In this study, on a low-N soil (i.e., 0.06-
0.08% total N) when clippings were not returned, ap-
proximately half of the plant-N was derived from the
fertilizer and half from the soil. When clippings were
returned, yield of grass increased by about one-third,
with approximately one-third of the total N in the
grass being derived from the cumulative return of
the clippings.

The rate of fertilizer N uptake was initially high,
with 20% of the fertilizer N applied being taken up
within 2-3 weeks following both applications. Subse-

Connecticut Agricultural Experiment Station

Bulletin 792

ORGANIC- N

NORGANIC-N

i| '

-■ T

RAIN 5
(cm)

224KG/HA

12 KG/HA

Jmji h tt-A 7»U- ck-rmnmittrmk g ■>- « i

-W^- ¥ cwl ' f U ^>%Lridt-«- !-•. -I

6 8

12 2 6 8 10 12 2
MONTHS

Figure 14. Distribution of rainfall with time and distribution of nitrate-N concentration with soil depth and time beneath shade
tobacco in 1975. (Data of J.L. Starr and H.C. DeRoo, Connecticut)

quently, the rate of fertilizer N uptake dropped to
nearly zero, resulting in a low overall total fertilizer
efficiency of about 35% from the first application and
20% from the second. Owing to the low leaching loss
observed in these experiments, the low efficiency
must be due to a large degree of immobilization and/
or denitrification.

Studies in Maine

The impact of rates of N fertilizer applied to po-
tatoes on the soil solution levels and on the leaching
losses of NO3-N to groundwater was investigated at
Presque Isle, Maine, from 1973 to 1978. The soil at
the site was classified as a Caribou loam (coarse-
loamy, mixed, frigid Typic Haplorthod). Samples of
soil solution were removed from the B and C horizons

at respective depths of about 45 cm and 120 cm. Ka-
tahdin potatoes were grown in an alternate year ro-
tation with buckwheat. Rates of N applied to potatoes
were 0, 168 and 224 kg/ha. No N was applied to the
buckwheat. Samples of soil solution were removed at
bi-weekly intervals beginning at spring thaw and con-
tinuing until freeze-up. Potato yields were measured
annually. Potato tubers were the only material re-
moved from the plots during the rotation; all other
organic residues were incorporated into the soil.

Tuber yields obtained over a 6-year period are
shown in Table 8. The addition of 168 kg N/ha re-
sulted in tuber yields that were not significantly dif-
ferent from those at 224 kg/ha. The lack of response
by Katahdin potatoes to the 224 kg/ha rates of N as
compared to 168 kg/ha indicated that the additional

Efficient Use of Nitrogen

RAIN 2 5

(cm) 50

ORGANIC -N

IHUITIIIFT

INORGANIC -N

30- CM DEPTH

I 200

^ 100

200-

100-

200-

_ 224

100-

100

*\/* \

>«*-

.o'^ad °^^.c>o-o.o'°" ^^

_ 19 4;8I?

^.00°

60-CM

'z=::cy:^*^°-''^^-

120-GM

^j»—

— •

,••

' —

^,o

w^^

•
1

" *-iM

•«

80-CM

p,^

» — — *'o

w-^.

m mmmi0» t*-"*^-^^

240- CM

<■ ^^m^imr*»» l^mn^m. •_

8 10 12

MONTHS

Figure 15. Distribution of rainfall with time and distribution of nitrate-N concentration with soil depth and time beneath shade
tobacco in 1976. (Data of J.L. Starr and H.C. DeRoo, Connecticut)

200
1 60

ill
a.

"1 1 1 r

Connecticut Agricultural Experiment Station

B HORIZON

1 1 r

Bulletin 792
C HORIZON

320

360

Figure 17. Six-year average of nitrate-N contents in soil solu-
tions from plots that received no N. Open circles are po-
tatoes, closed circles are buckwheat. (R.V. Rourke, Maine)

Figure 16. Cumulative N uptake derived from fertilizer N
(FN), soil N (SN) ond grass clippings of the current year
(CNl) end of the previous two years (CN2-CN3). Arrows
indicate dotes of fertilizer application. (Data of J.L. Starr
and H.C. DeRoo, Connecticut)

56 kg/ha was not needed. As can be seen in Table
9, N removal in tubers was not significantly different
when the 168 kg/ha and 224 kg/ha rates were com-
pared. The excess N at the highest rate remained in
the soil to leach, denitrify, or be incorporated into
the soil organic matter.

Table 8. Yield of Katahdin potatoes obtained with three levels
of N at Presque Isle, Maine.

Rate of

1973

1974

1975 1976

1977

1978

kg/ho

q/ha

116 a*

205 a

150 a 184 a

125 a

174 a

168

179 b

394 b

239 b 341 b

277 b

318b

224

185 b

396 b

289b 314b

279 b

299 b

• Numbers followed by the same letter within columns are not
significantly different at Boyes .05 level.
Data of R.V. Rourke, University of Maine, Orono.

Nitrate-N levels in soil solution from B or C hori-
zons of the plots fertilized with various rates of N
over a 6-year period are presented graphically for
buckwheat and potatoes in Figures 17 through 19.
The data in Figure 17 show that NO3-N levels were
higher in the B horizon when the area was planted
to potatoes that received no N than when buckwheat
was rotated on the same plots. In both of the rota-
tions, peak values of NO3-N in the B horizon were
reached by late August. Less than 10 ppm NO3-N

existed in the soil solution from the B horizon of
plots seeded to buckwheat by late August. Nitrate-N
concentrations in soil solutions from the B horizon
did not drop to less than 10 ppm on the potato plots
until late October. Peak levels of NO3-N in the soil
solution from the C horizon occurred in mid-May
for both crops where no N was added. Less than 10
ppm of NO3-N were in the soil solution from the C
horizons of the buckwheat plots from mid-August un-
til freeze-up, whereas NO3-N level in soil solution
from the C horizon of the potato plots exceeded 10
ppm from late August to late September and for a
brief period in early November.

A comparison of NO3-N levels in the B and C hori-
zons, where 168 kg N/ha were applied to potatoes,
is presented in Figure 18. Levels of NO3-N in the
solution from the B horizon were above 10 ppm from
mid-May until freeze-up in late November for the

8 HORIZON

C HORIZON

18 26 34 42 50 18 26 34 42 50

Figure 18. Six-year average of nitrate-N contents in soil solu-
tions from plots that received 168 kg N/ho. Open circles are
potatoes, closed circles are buckwheat. (R.V. Rourke, Maine)

Efficient Use of Nitrogen

8 HORIZON

I 10
a.

251—1 — r

- /v^:\ A

■•w-\

_1 I I L_

C HORIZON

n 1-

26 34 42

Figure 19. Six-year average of nitrate-N contents in soil solu-
tions from plots that received 224 kg N/ha. Open circles are
potatoes, closed circles are buckwheat. (R.V. Rourke, Maine)

year in which potatoes were grown. When buckwheat
was grown, soil solution levels of NO3-N in the B
horizon were similar to those with no N treatment.
An early peak of NO3-N in the soil solution in the
C horizon was found in mid-May with potatoes. The
early level of NO3-N in the C horizon of the buck-
wheat plots was half as high. Following the early
peak concentration, the NO3-N in soil solution re-
mained at or below 10 ppm in the C horizon until
mid-November for potatoes and varied from 8 to 11
ppm from mid-June to freeze-up for buckwheat. Little
change in the NO3-N content of the C horizon oc-
curred relative to that of the control after the initial
early season high.

Table 9. Nitrogen removal in tubers ot Katahdin variety,
Presque Isle, Maine.

Rate of
N

1973 1974 1975 1976 1977

1978

kg/ho _

168
224

26.0 a* t 24.5 a 35.1a 24.7 a 33.8 a

59.5 b 63.0 b 93.8 b 74.6 b 94.5 b

60.5 b 79.5 b 90.4 b 80.9 b 94.8 b

* Numbers followed by the same letter within columns ore not

significantly different at Boyes .05 level,
t Data not available.

Data of R.V. Rourke, University of Maine, Orono.

The level of NO3-N in the soil solution from the
B horizon of soils receiving 224 kg N/ha when the
plots were planted to potatoes is presented in Figure
19. Soil solution levels for the potato plots increased
slowly from late April, when NO3-N in the soil solu-
tion averaged 6 ppm, until mid-July, when the con-
centration in the B horizon was 18 ppm. The soil
solution level of NO3-N for potatoes remained between

14 and IS ppm from late June through freeze-up. In
late July and early August, the soil solution level of
NO3-N decreased in the buckwheat plots and in late
August was below 10 ppm, where it remained with
only slight fluctuations until freeze-up. After an early
peak of 19 ppm NO3-N with potatoes or of 21 ppm
with buckwheat, levels of NO3-N in the soil solution
of the C horizon dropped rapidly and reached a low
point by early June. The soil solution level of NO3-N
for the remainder of the year changed only slightly
with both crops and remained between 10 and 14 ppm
until freeze-up. Soil solution levels in the C horizon
of the high N treatment (224 kg/ha), although slight-
ly higher, demonstrated a pattern similar to that of
the 168 kg/ha rate.

The application of N to potatoes increased the soil
solution levels of NO3-N in the B horizon the first
year of application, and the highest levels were as-
sociated with the highest N treatment. A carry-over
of N in the soil solution levels was evident with the
highest N rate the following year, but these values
decreased to levels similar to lower N application
rates by early August. Leaching losses were possible,
as judged from soil solution levels of the C horizon,
from late April to mid-June. There was little evidence
of any further leaching loss during the remainder of
the freeze-free period.

A field study with poultry manure initiated in 1970
measured long-term effects of annual applications of
different rates of manure on NO3-N concentration
and movement in two soils. The soils were a Colton
gravelly loamy sand (Typic Haplorthod sandy-skele-
tal, mixed, frigid ) and a Peru fine sandy loam ( Typic
Fraglorthod, coarse-loamy, mixed, frigid) having a
firm layer at approximately 75 cm. The manure was
applied annually in the fall for 5 years, 1970-74. All
treatments were replicated four times.

The results of spring and fall samplings from in-
place, suction lysimeters in the Peru soil are presented
in Table 10. The data obtained from well heads three
meters deep in a Colton soil are presented in Table
11. Total soil N levels in both soils at different times
and depths are shown in Tables 12 and 13 respec-
tively. Data shown in tables for years 1971-72 were
taken from a Project Completion Beport by Hutchin-
son (Hutchinson, F.E., University of Maine, Orono,
1972 "Effect of Animal Wastes Applied to Soils on
Surface and Ground Water Systems," Project #A-020-
ME).

Within plots, the 5-year mean values of NO3-N con-
centration in the spring soil solution samples ranged
from 1.73 ppm for the control up to 57.1 ppm for the
high manure rate ( Table 10 ) . These mean values show
a definite relationship to manure rate. However, the
values obtained at 6.1 m and 12.2 m downslope were
not related to manure rate. As the NO3-N moved over
the firm layer down a 10% slope from the plots, it

Connecticut Agricultural Experiment Station

Bulletin 792

Tqble 10. The NO3-N concentration (ppm) of soil solution samples collected on top of the firm layer in a
Peru fine sandy loam treated with five rates of poultry monure.

Nitrogen
kg/ha Sampling

1971

1972 1973 1974
Within plots

0.12 1.14 3.29

0.06 1.09

0.69 2.47 5.00

0,03 0.88

11.67 7.42 5.70

0.09 0.01

41.25 17.38 5.10

1.98 0.48

100.00 14.25 44.20

6.40 0.13

6.1 meters downslope

20.2 20.59 6.20

0.14 0.45

12.2 meters downslope

4.1 10.85 3.49

0.12 0.13

1975

1976

Mean
70-75

spring
foil

3.00
0.03

330*

spring
fall

4.00
0.02

660*

spring
fall

1 1.00
3.87

1240"

spring
fall

7.00
1.94

2040*

spring
foil

23.00

spring
foil

10.20t
0.65

spring
fall

0.92
0.07

1.09

0.08

2.31

2.83

51.15

1.10

51.39

2.01

104.04

4.67

7.86
0.25

0.50
0.25

0.44
1.52
1.25
0.37
2.63

0.56

1.73

2.89

17.39

24.42

57.10

0.41

* Five-year overage rate of N applied annually through poultry manure in fall, 1970-1974.
t Values for 6.) and 12.2 m downslope ore means for five nitrogen treatments applied through poultry manure.
Data of R.F. Stafford, University of Maine, Orono.

diffused throughout the area; therefore, values shown
for downslope (Table 10) are means for all manure
rates. There was no indication of any NO3-N move-
ment downslope beyond the 6.1 m distance until
spring 1973, where values greater than 10 ppm were
obtained 12.2 m downslope (Table 10). The NO3-N
concentration of the soil solution for all treatments
decreased greatly from spring to fall throughout the
period of study. Assuming that little NO3-N would
be leached downward through the firm layer, much
of the NO3-N present in the spring must have been
denitrified. This speculation is further supported by
the low soil N values obtained at the 75 to 90 cm
depth, a zone below the firm layer (Table 13). The
low NO3-N values obtained in the spring of the re-
sidual year 1976, show little carryover of NO3-N from
the previous years of manure apphcation ( Table 10 ) .
The NO3-N concentration in ground water samples
taken at the 3-meter depth in a Colton soil reached
a high of 10 ppm in the spring of 1972 at the 710
kg/ha rate of N (Table 11). At the same time, a low
value of 0.21 ppm occurred at the highest rate, show-
ing that all values obtained were not related to ma-
nuring rate and that lateral movement of the soil
water occurred. It is also possible that higher con-
centrations of NO3-N in the ground water could have
occurred at times other than the three sampling times
used. However, it is important to note that the con-

centrations of nitrates decreased to less than 1 ppm
in the fall samples for all manure rates throughout
the period of the study.

Total soil N, before and after five applications of
manure, ranged from a low of 0.150 to 0.316 for the
Colton soils, and from 0.213 to 0.303 for the Wood-
bridge soils (Tables 12, 13).

Table 11. The NO3-N concentration (ppm) in ground water
samples taken at a 3 -meter depth in a Colton
gravelly loamy sand.

Nitro-
gen*
kg/ha

Sampling

1971

1972

1973

1974

1975

1976

spring
fall

1.77
0.27

1.81

7.83
0.64

0.27
0.06

2.36
0.59

1.62

390

spring
fall

1.94
0.07

0.27

2.35
0.12

0.14
0.05

2.43
0.59

1.28

710

spring
fall

0.42
0.07

10.03

2.83
0.16

0.08
0.04

4.48
0.31

1.96

1 160

spring
fall

3.06
0.07

6.77

9.30
0.44

3.78
0.07

3.29
0.42

3.85

1730

spring
fall

2.69
0.06

0.21

1.72
0.29

0.17
0.40

0.17
0.90

0.84

Mean
Mean

spring
fall

1.98
0.11

3.82

4.81
0.35

0.89
0.12

2.55
0.56

2.90

Five-year average rote of N applied annually through poultry

manure in the fall, 1970-1974.

Data of R.F. Stafford, University of Maine, Orono.

Efficient Use of Nitrogen

Toble 12. Total soil nitrogen of samples collected at a depth
of 0-15 cm prior to initial application and after
the third and fifth applications of poultry manure.

Manure

rates

Mt/ha

Nitrogen'
kg/ha

Prior to
initial

After
third

After
fifth

% Total N

Colton gravelly loamy sand

0.188

0.166

0.160

28 390

0.175

0.179

0.224

56 710

0.178

0.188

0.235

84 1160

0.150

0.207

0.265

112 1730

0.155

0.232

0.316

Woodbridge fine sandy

loam

0.230

0.232

0.245

24 330

0.213

0.21 1

0.206

47 660

0.220

0.209

0.234

94 1 240

0.217

0.227

0.249

1 88 2040

0.233

0.268

0.303

Five-year average rate of N applied annually through poultry

manure in the fall, 1970-1974.

Data of R.F. Stafford, University of Maine, Orono.

Table 13. Total soil nitrogen of samples collected at three
depths, (October 1975)

Manure

rates

Mt/ha

Nitrogen
kg/ha

0-15

Soil depth

(cm)

30-45

% Total N

Colton gravelly loamy sand

0.160*

56 710 0.235

112 1730 0.316

Woodbridge fine sandy loam

0.245

47 660 0.234

188 2040 0.303

0.010
0.074
0.049

0.062
0.062
0.088

75-90

0.021
0.042

0.042

0.033
0.008
0.017

* Five-year overage rote of N applied annually through poultry
manure in the fall, 1970-1974.
Data of R.F. Stafford, University of Maine, Orono.

Losses of Nitrogen to the Atmosphere

Denitrification

Denitrification is the process by which nitrate is
reduced to gases, principally nitrous oxide (N-O)
and nitrogen ( N2 ) , not directly available to plant life.
This process is carried out by a diverse group of bac-
teria which are widely distributed in soils and sedi-
ments. Denitrifying bacteria are numerous in most
soils; a population density of about 1 million per gram
of soil is expected for most agricultural soils ( Gamble
et al. 1977). Thus, the potential for significant ni-
trate removal by denitrification exists in most soils.
The process is inhibited by oxygen, however, causing
denitrification to be relatively insignificant when soils
are weU-drained and well-aerated.

The amount of N lost to denitrification is funda-
mental to goals of efficient fertilizer use and to pre-
dicting and minimizing nitrate pollution of ground
waters. Quantitative answers to this central question
have long been sought, but the methodology has not
allowed more than qualitative answers. The best es-
timates come from lysimeter studies in which ^°N is
added as fertilizer, and unaccounted losses are as-
sumed to be due to denitrification. From a recent
evaluation of all lysimeter studies, Hauck has indi-
cated that about 25% of fertilizer is lost to denitrifi-
cation under normal agricultural conditions (Hauck
1977). This evaluation provides a useful estimate but
is not helpful for specific cases caused by different
soils, seasonal changes and other influential manage-
ment factors.

Studies at Michigan State University (J.M. Tiedje,
Dept. of Crop and Soil Sciences) have focused on

by Denitrificotion and Volatilization

using new methods which measure short-term deni-
trification rates to determine the environmental fac-
tors affecting denitrification. These studies have eval-
uated how these factors affect the proportion of N^O
produced. This gas has received much attention re-
cently, since atmospheric chemists found that it stimu-
lates destruction of the protective ozone layer of the
earth. Because a principal source of N2O is denitri-
fication, some have suggested that expanding fertilizer
use could cause further destruction of ozone, thereby
enhancing the flux of damaging ultraviolet radiation
at the surface of the earth.

Table 14. Denitrification rate of Miami sandy loom as in-
fluenced by oxygen.

Denitrification rate
Soil treatment nmol N^iO/g dry soil • min

Anaerobic aggregates
Aerobic aggregates

0.15
0.00027

Adapted from Smith & Tiedje (1979a).

Effect of aeration status: Denitrifying activity has
been found in all soils examined, even in well-drained
sandy soils (Smith & Tiedje 1979a). This indicates
that the denitrifying enzymes are generally present
and that denitrification can occur immediately once
the O2 is removed. The pronounced effect of the O2
inhibition is shown in Table 14. The rate under air
is about 1/1000 of that under an anaerobic atmo-
sphere. Increased moisture content causes reduction
in available O2 by restricting diffusion of this gas,

Connecticut Agricultural Experiment Station

Bulletin 792

thereby allowing zones of anoxia to grow in aggre-
gates. The effect of various moisture contents on
denitrification rate is shown in Table 15. This in-
creased rate does not occur immediately after wetting,
probably because time is needed for O2 consumption
by respiration, but it usually occurs within 6 to 10
hours. Thus, the increased rate could be observed
naturally after rain or irrigation.

The rates of denitrification observed in dry, aero-
bic soils are low, in the range of 0.1 g N/ha • day.
The rates following wetting typical of a heavy rain
can be a hundred times greater, reaching a maximum
rate of a few kg N/ha • day.

Toble 15. Denitrification rate of Spinks loamy sand (Psom-
mentic Hapludalf) preincubated aerobicolly at
varying water contents.

Gravimetric

water content

(%)

nitrification rate

6
16
27
37

nmol

NoO/g soil • min
0.06 ± .02*
0.05 ±.01
0.10 ±.02
0.1 6 ±.06
0.38 ± .09

* ± Sbt .05.
Adapted from

ith Cr Tiedje

(1979a)

This estimate lies in the range expected for a 25%
loss of fertilizer N. Using the above estimates for
"wet" days, one could envision a reasonable case of
20 days when the soil was rather wet and a denitrifi-
cation rate of 2 kg N/ha • day for those days, which
would equal 40 kg N/ha lost, about 25% of the N
from typical fertilization rates (160 kg N/ha). These
calculations are only estimates extrapolated from lim-
ited laboratory data. No one has yet measured rates
of denitrification in the field, and laboratory measure-
ments under realistic conditions are limited; thus,
these projections should neither be accepted as pre-
cise nor as transferable to other soils and conditions.

Phases of denitrification: As stated previously, de-
nitrifying enzymes exist in the indigenous denitrify-
ing bacteria despite the dry, aerobic state of most soils.
Once soils become wet (anoxic or partially so), ad-
ditional denitrifying enzymes are made by the exist-
ing organisms, thereby increasing the capacity of soils
for denitrification. The increased synthesis occurs
within 1 to 3 hours after anaerobiosis and is usually
complete after 4 to 8 hours (Smith & Tiedje 1979a).
Apparently only the enzymes responsible for the con-
version of NO3 to NoO are synthesized early, caus-
ing an increase in the proportion of NoO produced
(Firestone & Tiedje 1979). Eventually the enzyme re-
ducing N2O to N2 is synthesized in greater quantity,
and NoO is removed. At this stage the soil becomes a
sink for N2O.

The early period before de novo synthesis is called

Phase I, and the period after synthesis is called Phase
II (Smith & Tiedje 1979a). When these rates are mea-
sured under anaerobic conditions with adequate ni-
trate, the rate reflects the quantity of enzyme present.
Since in situ synthesis supposedly occurs only under
conditions conducive to denitrification, the quantity
of enzyme, as determined by a Phase I anaerobic as-
say, can be used to indicate previous soil conditions
suitable for denitrification. Because these measure-
ments are given as rates, readers may consider these
activities as reflecting field rates. This is incorrect, for
these rates are determined under total anaerobiosis
and are therefore much higher than expected for field
rates. They reflect, however, a denitrification poten-
tial of soils. This potential is different from that which
is usually cited in the literature and which is based
on long-term measurements from the Phase II period,
or after growth of denitrifiers. This rate is farther re-
moved from the true in situ state and gives high and
unrealistic values.

Using the Phase I assay, synthesis of denitrifying
enzymes has been shown in response to irrigation of
corn plots (Smith & Tiedje 1979a). The denitrifica-
tion activity prior to a 4-hour irrigation period was
0.11 (-0.3) nanomoles NoO/g soil ■ min, compared to
0.25 (±0.3) after irrigation. The Phase II rate was
0.68 (±0.6). This increase in activity suggests an in
situ synthesis of denitrifying enzymes in response to
O2 stress caused by irrigation.

Effects of roots: Increased carbon (excreted by roots),
lower O-j due to respiration, and more nitrate being
brought to the rhizosphere by mass flow should make
the rhizosphere a zone of more active denitrification.
Using a Phase I assay, it was shown that denitrifica-
tion enzymes increase near the rows in corn fields. De-
nitrification activities measured as picomoles NoO/g
soil • min are given as follows in order of increasing
distance from the corn row: 83 (-7) at cm, 66
(±7) at 15 cm and 64 (=*=12) at 30 cm. This and
other studies always showed more denitrifying en-
zymes near roots; these measurements, however, do
not reveal whether actual denitrification was greater
in the rhizosphere (Smith & Tiedje 1979b).

A new method was developed which gave denitri-
fication estimates under more realistic conditions. Corn
and orchard grass were grown in pots, and then acet-
ylene, which inhibits N2O reduction (Smith et al.
1978), was added to the soil atmosphere and to a
Saran bag placed over the plant top and pot. The
increase in N2O, reflective of total denitrification,
was measured by gas chromatography for an 8 to 10-
hour period. The main point of this assay is that the
soil atmosphere remains aerobic; the rates should ap-
proximate true denitrification rates.

The findings are summarized in Table 16. The ni-
trate content of the soil influenced denitrification dra-
matically. When nitrate content was high, the results
verified the prevailing opinion that the denitrification
was higher in the rhizosphere. However, when nitrate

Efficient Use of Nitrogen

Table 16. Denitrification rate of soil as effected by plant
roots and nitrate concentration.

Plant

and soil

Nitrate*

Denitrification rate
Planted Unplanted

Corn,

Brookston

High
Low

(pmol N»0/g

18.6

1.5

soil • min)
2.6
9.1

Grass,

Miami

High
Low

87.1
1.8

19.4
9.4

* High NOj is greater than 25/ng N/ml, and low NOa is less
than 5i"g N/ml soil solution.
Adapted from Smith et ol. 1978.

content was low, denitrification was reduced, by a
factor of at least five, over that in unplanted soils.
This result suggests that competition for nitrate be-
tween denitrifiers and plant roots results in reduced
denitrification when nitrate is low, as in most non-
fertilized habitats. Thus, the percentage of N lost to
denitrification is probably influenced by the fertilizer
regime.

Factors affecting proportion of N2O produced:

Early estimates of N2O production assumed that a
constant percentage of N^O is produced from deni-
trification in all terrestrial habitats (CAST Report
1976). At the time, this was probably the only reason-
able approach for large-scale approximations, and it
is probably not surprising that many factors have now
been shown to affect this percentage. Perhaps what
is surprising is the magnitude of the effect and the
number of environmental factors; some of these re-
sponses are summarized in Table 17.

Table 17. Influence of various factors on the quantity of NoO
produced from soil denitrification.

Percent of gas

Factor

Concentration

product OS N2O

None

NOa

2 ppm

NO3

20 ppm

N0=

2 ppm

NOi

20 ppm

0.016 atom

6.5

4.9

pH + NO3

6.5-1-10 ppm

pH + NO3

4.9-1-10 ppm

Time

onae

irobic

1 hour

Time

anoe

irobic

12 hours

Time

anoe

irobic

24 hours

Adapted from Firestone et al. 1979.

Thus far, increasing concentrations of nitrate, ni-
trite, oxygen and sulfide have been shown to increase
the proportion of N2O. Furthermore, lower tempera-
ture, increasing time of anaerobiosis and lower pH also
raise the proportion of N2O (Firestone et al. 1979;
Firestone & Tiedje 1979). A number of denitrifier
strains have been shown to be incapable of NoO re-
duction or to lose the capacity for N2O reduction

on cultivation (Gamble et al. 1977; Okereke 1978).
The impact of this phenomenon on N2O production
in nature is not known. The effect of pH is not as
dramatic as previously reported, since interaction of
high nitrate plus low pH cause the large increase in
N2O (Table 17).

The mechanisms causing the increase in NoO pro-
duction have been determined in soil slurries where
rates of diffusion were not limiting; therefore, inter-
pretation was not confounded by physical factors.
These factors may affect the magnitude of response
in NoO emissions from the soil surface, since NoO
diffusion through soil is slow and the path is hetero-
geneous. Thus, the net emission is probably modified
from that at the sites of production. These studies do
point out which habitats and activities of man might
alter the normal state of NgO emission.

Denitrification kinetics in a flowing system were
studied in the laboratory by Starr and Parlange ( 1975,
1976) in Connecticut. They developed a model in
which the rate of denitrification could be described
regardless of the kinetics observed.

To determine the dynamics of denitrification in a
field soil, Volz and Starr (1977) continuously passed
a solution of nitrate and glucose through a labora-
tory column of Cheshire fine sandy loam. Oxygen
was excluded by passing N2 gas through the soil.
The time for solution to pass through the column was
about 10 hours. Concentrations of nitrate, nitrite and
soluble carbon were monitored in the effluent. The
quantity of NO3 in the effluent was reduced steadily,
converting largely to nitrite during the first 60 hours.
During this time the total of nitrate plus nitrite was
reduced slightly. Presumably, if the soil were to be-
come aerobic again, much of the nitrite present would
be oxidized back to nitrate. Such a cycling has been
observed to occur in the field. After 70 hours of an-
aerobiosis, however, the inflowing nitrate was deni-
trified rapidly, and the concentration of both nitrate
and nitrite in the effluent was reduced essentially to
zero after 96 hours. Nitrate reducers and denitrifiers
were enumerated, and specific denitrification rates
per microbe were calculated. These rates decreased
with time from about 1 X 10-* to 2 X 10-" fig N/
ml/hr/microbe.

Ammonia Volafilization

Urea offers more N per unit of weight than other
solid commercial fertilizers. It is now manufactured
in granules similar in size to other fertilizer particles
used in bulk blending. Urea formerly would cake in
storage, and so manufacturers were reluctant to use it.

Coating these granules with urea formaldehyde has
practically prevented caking, and safe storage is al-
most assured. This new product is so acceptable that
fertilizer companies promote it as the N of the future.

Urea, in addition to its obvious advantages, has dis-
advantages as follows: (1) when broadcast-urea is
not incorporated into the soil, substantial losses of
urea N in the form of ammonia may occur, (2) when

Connecticut Agricultural Experiment Station

Bulletin 792

it is incorporated into the soil, ammonia toxicity to
germinating seedlings may occur where it is applied
too close to the seedlings, and (3) if placed too close
to the seedling, it may increase the soil pH suflicient-
ly to seriously reduce the availability of some nutrients.

Ammonia losses to the atmosphere from surface-
applied urea may be large and are governed to a
degree by the acidity of the soil, by the type of soil
( cation exchange ) , and by protective materials which
can be added with the urea.

The TVA Fertilizer Summary Data (Hargett 1976)
states that 63% of all the N directly applied to soils
in the Mid- Atlantic states (which include Pennsyl-
vania, New York and Maryland) was applied in N
solutions (41%) or urea (22%). The use of N solution
in this region has increased from 82,900 tons in 1970
to 165,900 tons in 1976, a 100% increase in 6 years.
The greater use of N solutions (which contain urea)
and urea, combined with the high percentage of no-
till corn with surface-applied fertilizers, has undoubt-
edly resulted in greater volatilization losses of N.

Ammonia is lost into the atmosphere from unincor-
porated surface-applied urea prills regardless of the
initial soil pH (Ernst & Massey 1960; Martin &
Chapman 1951; Volk 1961), for the enzymatic hy-
drolysis product of urea, (NH4)2C03, is basic and
raises the pH of the soil around the prill to approxi-
mately 9.0, resulting in the conversion of NH4+ into
NH3, Other conditions that lead to increased volatili-
zation losses from prilled urea are low CEC (Gasser
1964; Lippold et al. 1975; Overrein & Moe 1967;
Volk 1961), higher temperature (Fernando & Rob-
erts 1975; Martin & Chapman 1951; Matocha 1976),
higher initial soil pH (Ernst & Massey I960; Lippold
et al. 1975; Matocha 1976; Olson et al. 1964; Volk
1961), high N applications (Overrein & Moe 1967).
larger prills ( Volk 1961 ) , and rain-free weather after
application (Forster & Lippold 1975). The effect of
soil moisture on NH:j volatilization is not clear. Ernst
& Massey (1960), Nommick (1966), and Kresge and
Satchell (1960) found that higher initial soil water
contents increased volatilization losses from urea; Lip-
pold et al. (1975) reported that increasing the water
content lowered NH3 losses, and Martin & Chap-
man (1951) and Gasser (1964) found no effect of
soil water on NH3 losses. The presence of litter does
not seem to affect losses from solid urea, but it sub-
stantially increased losses from N solutions (Olson
et al. 1964). Crop cover has been reported either not
to affect losses from urea (Nommick 1966) or to de-
crease them (Kresge & Satchell 1960).

In spite of the heavy use of N solutions, especially
in the Mid-Atlantic states, surprisingly little research
has been conducted on NHa volatilization from sur-
face-applied, unincorporated solutions. Olson et al.
(1964) found that with a soil pH of 6.8, three times
as much N was lost from surface-applied urea as from
the same rate of N solution; but with a soil pH of
7.8, the losses from N solution and urea were ap-
proximately equal. Kresge & Satchell (1960) also ob-

served that losses from N solutions were only one-
fourth to one-third as much as from the same rate
of urea applied to soils with a pH of 6.3. As men-
tioned earlier, Olson et al. (1964) also found that
applying N solutions on crop litter increased losses
by about 50% over applications on bare soil.

In N fertilization of no-till corn, nitrification of NH4
fertilizer will create an acidic soil surface (Blevins
et al. 1978), which may inhibit seed germination and
decrease the effectiveness of triazine herbicides ( Hilt-
bold & Buchanan 1977).

A field experiment was initiated at Pennsylvania
State University in 1975 to compare the effect of four
N fertilizer sources [urea, urea-NH4NO:( solution, NIL
NO3 and (NH4)uS04] on the yield and N content of
continuous corn and on the soil pH. The fertilizers
were applied at three rates to five replications on a
Murrill silt loam (Typic Hapludult; fine-loamy mixed
mesic) in the last week of April, about a week before
planting.

Rain fell within 2 days of fertilizer application in
the first 2 years of the experiment, and no differences
occurred among N sources in the effect on corn yield
or N content. In the third year, 1977, there were
three and a half rain-free days after fertilizer appli-
cation, and the grain yields in the urea and N solu-
tion plots were significantly lower than in the ( NH4 ) :•
SO4 and NH4NO3 plots at the 118 kg/ha rate (Table
2). The grain yield in the urea treatment was also
significantly lower than with NH4NO3 or (NH4)-
SO4 at the 218 kg/ha rate. The amount of N taken
up by the above-ground portion of the corn was also
significantly less in the urea and N solution treatments
at the lis kg N/ha rate and for urea at the 218 kg/ha
rate.

Research from Maryland shows that acid (pH 5.5)
soils lose less ammonia than limed (pH 7.5) soils, that
sandy soils with low cation exchange capacities will
lose more NH:i from surface applied urea than will
loamy soils with their higher cation exchange, and
that urea may be protected in part from ammonia loss
by having a coat of Fe2( 804)3 which affords a hy-
droxyl ion sink (Table 18 and 19).

Urea placed in the soil too close to the germinating
plant may result in loss of stand, late germination,
lower overall thriftiness and limited yield (Table
20).

Table 1 8. NH3 lost to the atmosphere from urea applied to
the surface of a Lakeland sand treatment.

Treatment

Amount

NH3-
pH 5.5

Lost
pH 7.5

No urea

Urea only

Urea + Fe, (SO.,);

N/ho

184
184

59.2

76.3
66.8

Data of F.A. Abbruscato and J.H. Axley, University of Mary-
land.

Efficient Use of Nitrogen

Table 19. NHs lost to the atmosphere from urea applied to
the surface of a Penn loam.

Treatment

Amount

Ammonia N Lost
pH 5.5 pH 7.5

kg N/ho

. % .

No urea

Urea only

184

4.6

15.9

Urea + Fe(S0j3

184

0.8

4,9

coating

Data of Abbruscato and Axley, University of Maryland.

Winter wheat increased in yield as the rate of
NH4NO3 fertilizer increased (Table 20). This was
not so for urea. As NH4NO3 has a salt index of 49.3
for its major nutrients and urea has only 26.7, NH4
NO3 should create the greater toxicity, but it does
not. Ammonia toxicity, not increased osmotic pressure
of the soil solution, is the reason for the poorer yields

Nitrate Accumulation in Vegetables

For the most part, vegetables are fast-growing, suc-
culent crops which respond quickly to N fertilization.
The response is generally favorable and results, in
many cases, in improved yields and quality of the
commodity. An important part of the vegetable nu-
trition research in the Northeast has been directed
toward efficient N use for high yields and quality
together with minimal nitrate concentrations in the
harvested commodity.

Nitrate accumulation in plants is a natural occur-
rence caused by nitrate uptake beyond that which is
reduced and assimilated into organic N compounds.
The degree of accumulation is controlled by the ge-
netic potential of the plant and is influenced by the
nitrate content of the soil and the plant environment.
Nitrate concentrations differ among plant parts and
with stages of growth and development.

Distribution of Nitrate in the Plant

Nitrate concentrations vary among plant parts; high-
est concentrations are generally found in stems and
petioles with decreasing concentrations in roots, la-
mina, grain or fruit, and flower parts (in that order).
This order may not apply to all species, but is correct
in most cases. Variation among species has also been
noted, e.g., carrot and sweet potato roots are generally
low in nitrates while radish and beet roots are high
in nitrates (Table 21).

Nitrates accumulate with time when soil nitrate is
not limiting. Workers at Cornell University (Aworh
et al. 1980) found that nitrate concentrations increased
from seeding in spinach fertilized with 340 kg N/ha,
but declined in unfertilized spinach. Minotti (May-
nard et al. 1976) found that nitrate concentrations in
older celery petioles were about 2.8 times higher than
in younger petioles and that older lettuce leaves had

from the soil treated with urea.

Finally, soils with excessive free ammonia from
urea treatments have a higher pH and less available
Ca, Mg, Mn and K than similar soils treated with
NHiNOa. Plants growing on such soil may tempo-
rarily suffer from the lack of one or more of these
nutrients.

Table 20. Yield of winter wheat as influenced by urea and
ammonium nitrate drilled with the seed.

Applied nitrogen

Granular Urea
Wheat

Ammonium Nitrate
Wheat

. kg /ha .

16.8

33.6

67.2

134.4

kg/ha _

1060 cd*

823 be
1381 de
1 1 85 de

377 a

_ kg /ha

1060 cd
1394 def
1394 def
1687 fg
1952 g

Mean separation by Duncan's multiple range test at 0.05

level.

Data of Abbruscato and Axley, University of Maryland.

2.6 times higher nitrate concentrations than younger
lettuce leaves. On the other hand, nitrate is readily
translocated to younger plant parts when soil nitrate
is limiting or moves to nitrate sinks such as develop-
ing vegetative storage organs.

Genetic Control of Nitrate Accumulation

Nitrate accumulation differs among species and
among cultivars within a species. Whole plant varia-
tion may result from differences in nitrate uptake or
reduction, whereas differential translocation may af-
fect nitrate concentrations in a specific plant organ.

The best known example of cultivar variation in
nitrate concentrations is in spinach, where savoyed-
leaved types consistently have higher nitrate concen-
trations than smooth-leaved types ( Barker et al. 1974 ) .

Table 21. Nitrate-N Concentrations in the edible portion
of fresh vegetables.

NO3-N

Plant part

Vegetable

(ppm fresh wt.)

Leaves

cabbage

165

lettuce

170

spinach

524

Petiole

celery

535

rhubarb

Roots

beet

600

carrot

sweet potato

radish

402

Fruit

peas

snap bean

tomato

Stem

asparagus

Bulb

onion

Tuber

potato

Adapted from Maynard et al. 1976.

Connecticut Agricultural Experiment Station

Bulletin 792

Semi-savoyed types are generally intermediate be-
tween these extremes, but individual cultivars may
overlap into one or the other group (Table 22).

Table 22. Nitrate concentration in spinach.

Blades Petioles

Leaf

7.5

15 7.5 15

type

(meq/liter)

NOa-N (% Dry wt.)

savoyed

0.30

0.56 1.15 2.95

semi-savoyed

0.18

0.50 0.84 2.48

smooth

0.08

0.45 0.42 2.56

Means of six cultivars of each leaf type.
Adapted from Barker et al. 1974.

The savoyed-leaf character is not universally asso-
ciated with higher nitrate concentrations. Kowal (J.J.
Kowal, unpublished data, University of Massachusetts)
studied smooth and savoyed-leaved cultivars of cab-
bage, endive and spinach fertilized with 56, 112, 225
or 456 kg N/ha in field experiments. Smooth-leaved
cabbage and endive cultivars generally had higher
nitrate concentrations than did savoyed-leaved cul-
tivars, and the savoyed-leaved spinach cultivar had
a higher nitrate concentration than the smooth-leaved
type (Table 23).

Table 23. Nitrote-N accumulation in smooth and savoyed-
leaf cabbage, endive and spinach cultivars.

Species

plant
part

leaf
form

N Application (kg/ha)
56 112 225

450

NO3-N (% dry wt.)

0.19 0.24 0.28 0.31

0.28 0.31 0.37 0.42

0.27 0.33 0.38 0.44

Cabbage Head Smooth

Harris' Resistant Danish
Market Prize
Market Victor

Cabboge Head Savoy
Chieftain Savoy
Savoy Ace
Savoy King

Cabboge Leoves Smooth

Harris' Resistant Danish
Market Prize
Market Victor

Cobboge Leaves Savoy
Chieftain Savoy
Savoy Ace
Savoy King

Endive Leaves Smooth

Florida Deep Heart

Endive Leaves Savoy

Green Curled

Spinach Leaves Smooth

Hybrid 424

Spinach Leaves Savoy

Long Standing Bloomsdale 0.38 0.39 0.42 0.43

Unpublished data of John Kowal, University of Massachusetts.

0.24

0.27

0.35

0.37

0.24

0.26

0.27

0.32

0.19

0.27

0.30

0.34

0.22

0.27

0.4!

0.49

0.51

0.61

0.75

0.91

0.65

0.80

1.01

1.12

0.26

0.31

0.52

0.66

0.35

0.42

0.52

0.54

0.24

0.33

0.60

0.63

0.60

0.81

0.82

0.97

0.35 0.38 0.53 0.74

0.24 0.27 0.28 0.33

Differential nitrate assimilation in savoyed and
smooth-leaved spinach types seems to account for the
difference in nitrate accumulation in specific cultivars.
Nitrate reductase activity of smooth-leaved 'Hybrid
424' was 2.1 to 3.3 times greater than in savoyed-leaved
'America' spinach (Olday et al. 1976). Minotti noted
marked differences among lettuce cultivars in nitrate
accumulation (Maynard et al. 1976). Regardless of
the nutritional or environmental regime, 'Minetto'
plants consistently have higher nitrate concentrations
than 'Val Rio' lettuce plants.

Environmental Control of
Nitrate Accumulation

Nitrate accumulation: The root and aerial environ-
ment of the plant affect the uptake, reduction and
transport of nitrate in the plant. In addition, environ-
mental variables, particularly temperature and precip-
itation, affect the nitrate content of the soil. In most
cases, however, the predominant influence determin-
ing plant nitrate concentrations is the rate, source,
and timing of N fertilizer applications.

Nitrogen fertilizer rate: Nitrogen concentrations
in vegetables generally increase with rate of N fer-
tilization. In greenhouse experiments, Maynard and
Barker ( 1971 ) showed that nitrate did not accumulate
in lettuce, radish and spinach until nitrate concentra-
tions in solution exceeded 3, 1.5 and 6 mM, respec-
tively. Significant increases in plant nitrate concen-
trations occurred at each successive increment to 48
mM. Similar results are usually obtained from field
experiments, e.g.. Peck and associates ( 1971 ) found
that the nitrate content of whole beet plants doubled
with the addition of 56 kg N/ha. The nitrate-N con-
tents of beet plants at harvest was 4 mg/plant with-
out N fertilizer, 8 mg with 56 kg/ha, and increased
to 67 mg with 450 kg/ha.

Nitrogen forms and sources: Nitrate is the usual
form of soil N available to plants, assuming that suf-
ficient time is allowed and that suitable temperatures
are present for mineralization of organic N and ni-
trification of ammonium to occur. With low soil tem-
peratures or shortly after ammonium fertilizer ap-
plication, nitrate uptake may be low because of the
failure to convert ammonium to nitrate. The influ-
ence of low soil temperature on nitrification is illus-
trated in results obtained by Minotti with head let-
tuce gro\\'n on organic soil. A spring crop fertilized
with 224 kg N/ha from ammonium sulfate had 0.49%
NO:i-N, whereas a summer crop fertilized in the same
way contained 1.16% NO3-N (Maynard et al. 1976).
With a short time interval between side-dressed fer-
tilizer application and harvest, Barker et al. (1971)
showed that nitrate accumulation was greatest from
potassium nitrate, intermediate from ammonium ni-
trate and least from urea. Controlled-release nitrogen
sources may limit nitrate accumulation in certain in-
stances.

Efficient Use of Nitrogen

Time of N Application: When the effects of pie-
plant and side-dress applications of equivalent
amounts of N are compared on plant nitrate accumu-
lation, results may vary depending on the species.
Spinach accumulated more nitrate from a preplant,
broadcast application than from side-dressings 9 days
before harvest (Barker et al. 1971). On the other
hand, side-dressed N applications resulted in higher
nitrate concentrations in beet roots (Peck et al. 1974).
In both cases, the highest nitrate concentrations re-
sulted from a combination of preplant and broadcast
applications.

Light: Nitrate concentrations are greater when
plants are exposed to low light intensities or to short
photoperiods. The effect of low light is primarily due
to restricted nitrate reductase activity without a con-
comitant restriction in nitrate uptake. Thus, Mi-
notti and Stankey ( 1973 ) found more than a tripling
of nitrate-N concentrations from late afternoon to sun-
rise in young beet plants. In like manner, the nitrate-
N concentrations in beet leaves fertilized with 225 kg
N/ha doubled when the plants received 8 hours of
light as compared to 20 hours. Differences were less
pronounced at lower rates of fertilization (Cantliffe
1972a).

Temperature: Plant science literature concerning
temperature effects on nitrate accumulation is am-
biguous. This may be due to the inability to separate
temperature effects on nitrate uptake and on reduc-
tion or to confounded effects of temperature on soil
N dynamics and plant N metabolism. A study of tem-
perature effects on nitrate accumulation by Cantliffe
( 1972b ) showed that under light conditions conducive
to nitrate accumulation, nitrate accumulated at 5° C
from 112 and at 10° from 56 kg N/ha. Nitrate accu-
mulation did not occur where N was not applied un-
til the temperature reached 15°, presumably because
mineralization and nitrification were impeded at lower
temperatures.

Carbon dioxide: The available evidence suggests
that elevated nitrate concentrations may occur in
plants growing in C02-limiting atmospheres. The ef-
fect appears to be indirect, i.e., caused by a lowered
amount of photosynthetically produced substrate for
the generation of reducing equivalents. In addition,
active nitrate reductase systems in some species may
be dependent on the presence of CO-. (Maynard et
al. 1976).

Water: The accumulation of nitrates is enhanced
during water stress. Assimilation of nitrate may be
restricted by lowered nitrate reductase activity or by
reduced availability of photosynthetic reducing equiv-
alents.

Minotti showed that nitrate accumulation occurred
under high atmospheric moisture as well as with low
soil moisture. He postulates that higher humidity re-
stricts transpirational flow of the nitrate ions to the
leaves, where most of the substrate-inducible nitrate

reductase is found. Consequently, he found that ni-
trate accumulated away from the primary sites of
reduction in the plant (Maynard et al. 1976).

Postharvest- Nitrate Conversions

The measurement of nitrate concentrations in har-
vested vegetables is useful because it provides an
indication of potential adverse effects on health. Ni-
trate, however, is relatively low in toxicity in com-
parison with its possible conversion products— nitrite
and nitrosamines. These toxicants are undetected or
are negligible in freshly harvested vegetables. The
conversion to nitrosamines has not been widely stud-
ied, but postharvest nitrite production has been con-
clusively demonstrated in vegetables (Minotti 1978).

Aworh and associates ( 1978 ) showed that nitrite ac-
cumulated in spinach when it was exposed to simu-
lated transit at 5° C for 14 days followed by post-
transit storage at 10° for 3 days. Pretransit storage
at 21° for 15 hours caused additional nitrite accumu-
lation. Nitrite did not accumulate at lower tempera-
tures or shorter time periods. Nitrite-N concentra-
tions greater than 10 ppm were found only in visibly
decayed samples.

Nitrite concentrations increased with time of storage
at 10° C in spinach grown at or 340 kg N/ha. Ni-
trite concentrations at harvest were the same for both
treatments, but were twice as great in the fertilized
spinach after 15 days at 10°. Nitrite concentrations
were higher following 15 days at 10° in spinach har-
vested at market maturity than spinach harvested 10
days earlier (Aworh et al. 1978).

Storage of spinach in modified atmospheres ( 1.8%
O2, 19.4% CO2) resulted in improved appearance but
increased conversion of nitrate to nitrite over rates in
normal atmospheric storage (Aworh 1976).

Potentially toxic nitrite concentrations may occur
in vegetables stored for excessive periods or at high
temperatures. Microorganisms probably contributed
substantially to nitrite accumulation under abusive
storage (Minotti 1978).

Regulation of Nitrates in Vegetables

Since the magnitude of nitrate accumulation in veg-
etables depends upon an array of internal, environ-
mental and fertilization variables, opportunities for
controlling the nitrate accumulation through these
variables are substantial. Maynard (1978) has sum-
marized for spinach the potential restriction in ni-
trate concentration by choice of cultivar, choice of
nitrogen fertilizer form, use of a nitrification inhibitor,
time of harvest in respect to light conditions, and
preparation for the table (Table 24). The previous
discussion of genetic and environmental effects ori
nitrate accumulation has suggested numerous pos-
sibilities for control of nitrate accumulation. The use
of controlled-release N sources or inhibitors of nitri-
fication offer alternatives for limiting nitrate accumu-
lation (Maynard and Lorenz 1979).

28 Connecticut Agricultural Experiment Station

Table 24. Procedures for reducii>g nitrate concentrations in vegetables.

Bulletin 792

Original

Adjustment

Reduction

Condition

Specific adjustment

NO3

-N (ppm)

(%)

Reference

Cultivar

Use of smooth-leaved (Tufte-
gard) instead of savoyed- leaved

(Bloomsdale)

1673

444DW*

Cantliffe (1972a)

Fertilizer

50% NH.-N and 50% NOa-N

instead of 100% NO3-N

20300

1 3000DW

Mills et al. (1976b)

Nitrapyrin used with 1 : 1

NH.-N;NOs-N fertilizer

13000

9000DW

Mills et al. (1976b)

Light

Harvest after 12 hr light instead

of hr

1164

839DW

Minotti & Stankey (1973)

Preparation

Petiole removal

cv. America

545

394FW

Olday et al. (1976)

cv. Hybrid 424

220

121FW

Olday et al. (1976)

• Dw = Dry weight basis
FW = Fresh weight basis

Detailed studies have been conducted of the use of
nitrapyrin, a nitrification inhibitor, for restricting ni-
trate concentrations in vegetables, particularly in the
nitrate accumulators, radish and spinach.

Mills and associates (1976a) studied the effects of
N form and presence or absence of nitrapyrin on
radish growth and nitrate accumulation. Nitrate ac-
cumulation was restricted with ammonium sulfate as
compared to potassium nitrate at low N application
rates. No differences between N forms in respect to
nitrate accumulation occurred at high N rates. Nitra-
pyrin was effective, ho\\'ever, in suppressing nitrate
accumulation with high rates of ammonium-N appli-
cation. Reductions of at least 70% in nitrate concen-
trations were attributable to nitrapyrin application

with ammonium sulfate. The optimum nutritional
regime for growth and low nitrate concentrations in
radish was 25% nitrate, 75% ammonium and at least
5 ppm nitrapyrin. Results similar to those with radish
were obtained with spinach. Nitrate accumulation was
lower with the ammonium form than with the nitrate
form of N, and further reductions in nitrate accumu-
lation occurred with the addition of nitrapyrin to the
ammonium treatments; but unacceptable growth re-
strictions accompanied the lower nitrate concentra-
tions. Maximum growth coupled with low nitrate ac-
cumulations occurred when 50% of the N was supplied
from nitrate and 50% from ammonium sources with
5 ppm nitrapyrin added (Mills et al. 1976b).

Conclusions

Most cultivated crops remove from the soil more
N than any other plant nutrient, and N is the plant
nutrient to which crops most often produce increased
growth and economic yield in response to field ap-
plications of fertilizers. Crops are often fertilized on
the basis of their estimated N removal with consid-
erations being given to the nitrogen-supplying power
of the soil and to any losses or conversions of N which
may occur. Notwithstanding the difficulty of estimat-
ing N contained in roots and other underground plant
portions, tissue analysis and dry matter production
factored together give fairly accurate estimates of N
removal by a crop. The nitrogen-supplying power of
the soil and the losses of N from the soil are much
more difficult to assess. Some of this difficulty arises
from the many reactions in which N participates in
nature as shown by the Nitrogen Cycle (Figure 20).

In most agricultural soils, nitrate is the primary
source of the inorganic N upon which plants feed.
The nitrate in soils may be derived directly from ad-
ditions to the soil or indirectly from ammoniacal

sources or soil organic matter. Ideally, therefore, ni-
trate levels in the soil should predict the status of
a soil with respect to its ability to supply N to a
crop. However, in the humid Northeast, amounts of
nitrate in the soil can be highly variable, their levels
being controlled by biological and environmental fac-
tors. Plant growth, fallowing, rainfall, soil moisture,
and soil temperature are among factors governing
nitrate concentrations in the soil. Consequently, their
concentrations at any given time may have no bearing
on the fertility of a soil with respect to its N supply.
Soil tests for N availability have been researched
considerably. Many of the tests are highly empirical
and involve extraction procedures or incubation pe-
riods which have been developed for conditions of
importance to a particular investigator. Direct com-
parisons of the various soil tests are difficult because
comparable procedures for evaluation of the tests have
not been used. Soil scientists in the Northeast have
been evaluating and developing laboratory procedures
to predict mineralization rates for soil organic matter

Efficient Use of Nitrogen
AIR

NgO.Ng

NH,

DUST

RUNOFF
EROSION

LEACHING

CLAY AND

ORGANIC

COMPLEXES

Figure 20. The Nitrogen Cycle.

or to predict the contribution of soils to the N de-
mands of a crop.

Some laboratory procedures appear promising, but
calibration of these procedures in the field with crop
demands is lacking. More data must be obtained from
research under farm conditions and at numerous sites
before laboratory evaluations of the N status of soil
can be correlated to field conditions. Heretofore, time
and expense has limited these investigations; how-
ever, increased support of this research may now be
justified by the rising costs of N fertilizers and by
the necessity to improve the efficiency of N use.

The concern for the effects of N on our environ-
ment is another factor pressing for efficient use of
N in agriculture. The recovery by a crop of only half
of the applied N is a frequently cited statistic. There-
fore, some of the fertilizer N is said to be a source
of environmental pollution. Research is underway in
the Northeast that indicates that the fraction of fer-
tilizer N recovered by a crop under good manage-
ment practices may approach 90%.

Ammonia volatilization from urea and other am-
moniacal fertilizers is likely to result from poor man-
agement. Research is needed to develop procedures
to diminish these losses.

The leaching of nitrates into ground water and the
movement of nitrate into drinking and recreational

waters has been a long-term environmental concern.
Agriculture has frequently been accused of being a
source of nitrates found in these waters, although
leaching losses from applied fertilizers have been dif-
ficult to verify. On the other hand, agriculturists have
had difficulty in inventorying and accounting for all
of the N applied to the soil. Investigations on the
movement of N in the soil and on the conversion of
N into soil organic matter and into gaseous denitri-
fication products are needed to provide definite an-
swers to the fate of fertilizer N applied to cropland.

Denitrification losses of N from the soil have been
considered harmless and nonpolluting since such losses
do not enter the ground water. Today, attention must
be given to the effect which NoO produced in the
soil and escaping into the atmosphere has on the
ozone layer of the stratosphere. The contribution of
fertilizer to N for denitrification must be evaluated.
Research is needed on methods of assessing denitri-
fication in the soil and for measuring on-site, field
losses of N through denitrification and the products
and significance of these losses under various man-
agement practices, including the presence or absence
of crops or of crop residues.

Management practices are a key factor in the ef-
ficient use of N. The validity of a field-calibrated
soil test may be lost if the correct procedures for

Connecticut Agricultural Experiment Station

Bulletin 792

applying N to a crop are not understood and fol-
lowed. Hence, efficient procedures for the use and
conservation of N must be integrated into manage-
ment systems involving no-till or preplanted side-
dress applications of N in cultivated soils. The up-
take, transport, and utilization of N by crops must
be known so that fertilization may be correlated with
crop demands in time and quantity.

Excessive uptake and accumulation of nitrate in
plants used for food or feed crops has been a long-
term concern for the health of humans and livestock.
All management systems must consider the potential
and consequences of luxury consumption of nitrates
by crops. Nitrate accumulation in a crop may be con-
sidered as wasteful, but on the other hand, efforts
to restrict nitrate accumulation may lead to dimin-
ished yield or to poor quality of the produce. Efforts
to limit nitrate availability by the use of chemical in-
hibitors and ammonium fertilizers may lead to am-

monium toxicity or may otherwise adversely affect
crop growth and composition.

Postharvest conversions of nitrate in plant products
are not thoroughly investigated. Researchers in the
Northeast must continue to study nitrate accumula-
tion in vegetables and livestock feed and postharvest
conversions of nitrate to nitrite.

Nitrate fertilization is a factor contributing to the
annual increase in crop productivity which has been
observed consistently over the past 30 years. There-
fore, the use of N fertilizers in agriculture will con-
tinue. Presently, crop yields are increased in at least
70% of the cases where some N is applied to the soil.
Research must continue to determine the amount of
N fertilizer which must be applied to reach yield
goals and to minimize losses through leaching, deni-
trification, and luxury consumption. This research will
lead to efficient use of N in Northeast agriculture.

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