Fertilizers and covercrops for California orchards

i of Agricultural Sciences
UNIVERSITY OF CALIFORNIA

FERTILIZERS

and COVERCROPS for

CALIFORNIA ORCHARDS 1

1

E. L PROEBSTING

-4

n t Station
sion Service

CIRCULAR 466

•7 I **>, *

.:..• ,.. .; ' : ,

An Orchard Fertilizer Program . . .

must take into account the condition of the soil, the
species of fruit, and any nutrient deficiency symptoms
shown by the trees. Since all these factors vary with
individual orchards, no quick and easy method is avail-
able for determining whether fertilizer is needed.

This circular suggests ways for finding out whether
an orchard would profit by fertilization, describes
nutrient deficiency symptoms, and indicates applica-
tion methods.

Covercrops are discussed both as sources of organic
material and as aids for maintaining good soil struc-
ture.

THE AUTHOR:

E. L. Proebsting is Professor of Pomology and Pomologist in the Experiment Station,.
Davis.

JANUARY 1958

Photograph at left shows liquid fertilizer being applied in
an almond orchard.

Roots, soil, and water
affect the fertilizer program

The success of a fertilizer program for
the orchard depends upon a rather com-
plex root-soil-water relationship. A ma-
ture tree has a system of permanent roots
extending throughout the available soil,
plus many small, temporary feeder roots.
The latter grow, die, and are replaced by
new roots one or more times each year.
During growth, the surfaces of these
feeder roots absorb water and mineral
elements necessary for the normal nutri-
tion of the tree.

Many factors influencing the absorp-
tion of nutrients are concerned with the
soil — its fertility, depth, texture, mois-
ture, temperature, drainage, and aera-
tion.

Trees may secure as much from a good
soil only 4 feet deep as from a poor one
twice that depth, but they will rarely
perform satisfactorily on a shallow soil
even if it is a good one, properly irri-
gated and fertilized. In very coarse or
very heavy soils, root branching may be
unsatisfactory, and roots may fail to ex-
tract nutrients efficiently from a given
volume of soil.

Roots will not grow in dry soil nor
will most species grow in saturated soil.
Such conditions reduce the active root
surface and probably the efficiency per

unit of root surface, thus limiting nutri-
ent absorption.

This absorption is also dependent on
the correct soil temperature as deter-
mined for various species. For trees, the
lower limit is probably near 45° F, the
maximum rate of activity, near 70° F.
Above 90° F there is little activity, and
at slightly higher temperatures the roots
will die.

In order to grow and function, tree
roots need oxygen. Saturated or tight
soil through which the air can move only
slowly will not provide a good environ-
ment.

Other factors affecting nutrient ab-
sorption are the species of tree and its
rootstock. Different species have differ-
ent habits of root growth. Some branch
profusely, some very sparsely, under the
same conditions. Different roots also
have different abilities to extract nutri-
ents from a given soil. It has been found,
for example, that the same variety of
apple growing on two selections of root-
stocks in a potassium-deficient soil have
shown deficiency symptoms on one stock
and not on the other.

In order that any added fertilizers may
be absorbed, they must be brought into
areas where the roots can come in con-

[31

tact with them. The depth to which they
must penetrate will depend on conditions
in the orchard. If there is sod, for ex-
ample, the roots may grow to within an
inch of the surface, whereas in some
clean-cultivated orchards there may be
but few roots in the top foot of soil. In
the latter case, the fertilizer must be a
kind that will penetrate with rain or irri-
gation water, or else must be placed in
the root zone mechanically in order to be
absorbed by the tree.

Not all fertilizers, though soluble, will
move downward with water. Most soils
have the ability to fix some of the com-
mon fertilizers. Potassium (potash, or
K) is fixed — that is, taken out of solution
and held — by most California soils. Even
on sandy soils with a fixing power less
than that of the heavy soils, large surface

applications are necessary for penetra-
tion into the root zone. Phosphorus
(phosphate, or P) also may be retained
in large amounts by the surface soil.
Some of the nitrogenous fertilizers, in-
cluding all ammonia compounds, are
fixed temporarily, but are changed by
soil bacteria to nitrate, a form in which
nitrogen is free to move to the root zone.
In light or shallow soils subjected to
heavy rainfall or excessive irrigation,
loss of nitrogen may be serious because
the nitrate may be washed through the
root zone and lost in the drainage water.
Loss may also occur in soil that remains
saturated for a considerable period, a
condition suitable for the destruction of
nitrogen compounds. This so-called de-
nitrification process results in release of
gaseous nitrogen to the atmosphere.

How to determine whether
your orchard needs fertilizer

To insure maximum production, the
grower must know the condition of his
orchard's soil. Many California orchards
are on soils capable of supplying all the
required nutrients. In such cases, addi-
tion of fertilizer is not profitable. How-
ever, some soils may have had low initial
reserves of one or more nutrients, or may
now be depleted of their original supply.
These soils will require fertilization.

Many factors other than the actual
supply or nutrients in a soil affect their
absorption by a particular kind of tree.
Because of this, no single, quick, and
easy method is available for determining
whether a soil requires' fertilizer. Some
progress has been made with several
methods, four of which are discussed be-
low: soil analysis, plant analysis, defi-
ciency symptoms, and orchard plot trials.

Soil analysis

A complete soil analysis is of little or
no value in determining fertilizer re-
quirements because it includes all nutri-
ents present in a given soil without indi-
cating how much of each is actually
available to the plant. However, various
laboratory tests have been developed for
determining available nutrients. These,
too, are not completely satisfactory be-
cause they may show the approximate
total supply of an element that a plant
can use, but not the rate at which it may
be available for a particular crop. If the
supply is found to be large, it may be
assumed adequate; if exceptionally low,
presumably nutrients should be added.
Also, no field is uniform in composition,
and soil varies in character at different
depths. To be of any value, a soil sample

[4]

must be taken near the roots, and must
be representative of the area. If the
change in soil character over the area is
great, samples from each type of soil
must be taken.

Several types of kits for determining
available soil nutrients are on the mar-
ket. Tests with these kits are not reliable
for deciduous fruit trees although good
results have been obtained with some
field crops. No soil test used so far,
whether field or laboratory, has proved
satisfactory for either nitrogen or phos-
phate determination. The Neubauer test
for potassium is slow and expensive, and
while useful, is not reliable in the range
near a slight deficiency.

Plant analysis

Both laboratory and field methods
have been developed for analysis of cer-
tain tree parts. As with soil analyses,
results vary, depending on modifying
factors affecting the tissue tested. For
example, leaf composition changes
throughout the season. The nitrogen con-
tent of apricot leaves in one orchard was
found to drop from 3.75 per cent in
April to 2 per cent in August. The char-
acter and rate of change differ for the
different elements and will be modified
by size of crop, seasonal conditions, and
cultural practices, such as pruning, as
well as by the available nutrients. The
success of this method depends on ex-
perience and a knowledge of the fruit
concerned.

Deficiency symptoms

The mineral elements known to be
necessary for plant growth fall into three
groups:

1 . Commercial fertilizers — nitrogen,
potassium, and phosphorus. These ele-
ments are used in large amounts by
plants, and are often deficient in many
soils throughout the world.

2. Elements usually present in suffi-
cient amounts for plant growth — cal-
cium, magnesium, sulfur — but possibly

required in additional amounts to pro-
vide good soil structure.

3. Minor, or microelements — manga-
nese, iron, boron, zinc, copper, molybde-
num. Plants require minute amounts of
these for successful growth.

In addition to chemical tests for avail-
ability of these elements, the trees should
be observed. Often they will show "de-
ficiency" symptoms that indicate the
need for a particular element. These
symptoms are not completely reliable by
themselves, but are valuable when con-
sidered in conjunction with soil or plant
analyses. However, where deficiency of
an element is suspected, application
should be on a trial basis at first (see
p. 9) to determine whether large-scale
application would be profitable.

Of the elements absorbed by roots and
known to be essential for growth of fruit
trees, all except phosphorus, calcium,
sulfur, and molybdenum have been re-
ported inadequate in deciduous orchards
somewhere in California.

Nitrogen is the most important ele-
ment as a fertilizer for trees. To produce
maximum crops, trees need additions of
this material more than of any other.
Every major fruit district in California,
and all species, have shown nitrogen de-
ficiency in at least some orchards. Many
orchards, however, are plentifully sup-
plied from reserves in the soil. In bear-
ing trees, an acute nitrogen shortage is
indicated by pale, yellowish-green leaves,
smaller than normal; short vegetative
shoots, usually small in diameter; pro-
fuse bloom, but very heavy drop, result-
ing in light set and poor crop; small fruit
maturing early, followed by early leaf
fall. These symptoms appear in the peach
sooner than in most other species. If
nitrogen is supplied to a tree in this con-
dition, the first response will be an im-
proved leaf color and better growth.
Fruit production may or may not be
affected the first season. If the per cent
of set is increased, the yield may be

[51

Deficiency symptoms in peach. Top to
bottom: magnesium deficiency in leaves;
nitrogen deficiency in leaves; copper de-
ficiency in leaves and shoots.

better, and the size improved. The ac-
celerated growth and the larger leaf area
will provide a larger and better-nour-
ished fruiting area, thus permitting the
setting of more and stronger fruit buds.
This development, in turn, should in-
crease production the second season.
Sometimes this cycle is repeated with
annual increases for four or five years.

Phosphorus deficiency symptoms,
less clearly defined in fruit trees, have
been seen almost exclusively in pot-
culture experiments. Under these arti-
ficial restrictions, the condition devel-
oped is one of stunted growth and dark-
green or somewhat bronzed leaves, which
may be thickened. Trees grow and pro-
duce well on a phosphate-deficient soil in
which most annuals fail to make normal
development. In a soil having the lowest
phosphate-supplying power of any so far
investigated in California, the common
species of fruit trees failed to respond to
added phosphate, although annuals in-
creased growth five to 20 times that of
their unfertilized checks.

Potassium deficiency has usually ap-
peared as local spots varying in size from
a few trees to several acres, and includ-
ing most of our fruit species. The Sacra-
mento Valley and coastal valleys have
shown the most trouble. Treatment has
not always been successful, particularly
where symptoms are severe. Deficiency
may result in leaf scorch and die-back,
sometimes with burning and shriveling
of the fruit. Leaf scorch, observed in
several fruit species, usually appears on
the leaf margin, but may also involve
most of the leaf blade. It seems likely
that the reduced leaf area limits the food
supply available to the roots. This, in
turn, reduces the absorbing surface and
the efficiency of the roots, resulting in
further deficiency. The most severe and
widespread potassium deficiency has
been found with prunes. This has usually
been associated with overbearing, and
the condition has been markedly im-

[6]

Peach leaves showing typical leaf scorch
of potassium deficiency.

Manganese deficiency in prune leaves.
Note yellowing in areas between veins.

Boron deficiency symptoms in prune

shoots, as indicated by dying

back of terminals.

Peach leaves showing yellowing as a
result of iron deficiency.

tk.tL&,.JWMJ

Zinc deficiency (little-leaf) of apricot. Treated branch at right.

proved by limiting the crop to about 4
dry tons per acre in the interior valleys,
and less in the coastal areas. Peaches and
almonds have shown much less damage
in situations where prunes have been un-
profitable. Early loss of leaves and dying
back of the tips, followed by new growth
from the last live bud, tend to give a zig-
zag growth, short and brushy. Trees
which show no deficiency symptoms
rarely have responded to added potas-
sium.

Calcium deficiency has not been
noted in California orchards. However,
calcium in the form of gypsum or lime
has proved beneficial as a soil amend-
ment in certain areas.

Magnesium deficiency is seldom
found in California. When it does occur,
it is mostly in coastal areas. The basal
leaves of affected trees develop brownish
blotches and drop off. The tips may con-
tinue growth while more leaves drop and
a few remain at the ends of bare shoots.

Fruit-bud production may be greatly re-
duced.

Sulfur deficiency has not been re-
ported for fruit trees in California, but
the material has been used extensively
as a corrective for alkali soils.

Manganese deficiency symptoms of
a severe nature have been found in sev-
eral species (notably walnuts) in Ven-
tura, Santa Barbara, and San Luis
Obispo counties, and in small areas else-
where. In mild cases, yellowing occurs
in the areas between the veins of leaves.
In severe cases, these areas die, and
many leaves fall prematurely. Some trees
may be practically defoliated by late
summer. Milder cases on peaches and
apricots, and less often on other species,
occur in both the coastal and the interior
valleys.

Iron deficiency, or so-called "lime-
induced chlorosis." is common on highly
calcareous soils. A deficient area along
the southern end of San Francisco Bav

[8]

has been known for many years, and
other such areas have been noted over
the state. The lack of iron causes yellow-
ing of leaves (except the network of
veins) and, in some cases, complete loss
of green color. The soils on which trees
develop these characteristics are not usu-
ally low in iron, but the excess lime
renders the iron unavailable.

Boron deficiency was first noted in
California in the olive, with the follow-
ing symptoms: death of terminal buds;
scorch of leaf tips; greatly reduced set
of fruit; and deformed fruit known as
"monkey-face." The apple and pear in
the Sierra foothill area may show "blast"
of blossoms, dying back of shoots, and
the development of hard, brown, corky
areas in the flesh of the fruit. The latter
symptom seems much less common in
coastal counties. In the European plum,
brown, dry, pithy areas may develop in
the fruit flesh. There may also be dying
back of terminals. The prune in Sonoma
County has shown a witch's broom effect
called "brushy branch." The walnut
shows poorly developed leaves, often
misshapen, usually accompanied by die-
back and chlorosis. So far, no evidence of
boron deficiency has been found on the
Japanese plum, even when growing
among European plums with marked
deficiency symptoms. In the coast coun-
ties, deficiency and excess situations
occur within a few miles of each other.

Zinc deficiency is responsible for a
trouble long known as "little-leaf," "ro-
sette," or "corral sickness." (Corral sick-
ness has also been us©d to designate cop-
per deficiency.) Extensive zinc-deficient
areas have been found in the San Joaquin
and Sacramento valleys and in smaller
spots elsewhere in the state. The most
common symptom is a tuft of small, often
deformed, yellowish leaves at the ends
of shoots. Symptoms vary somewhat with
the species. Fruit abnormalities are com-
mon, and crops are usually very small.

Copper deficiency is rare, but has
been found associated with zinc defi-

ciency in some corral spots and old
Indian camps, as well as in small areas
of pear and apple orchards in the central
coast district. Symptoms resemble those
of zinc deficiency, but leaf scorch and
roughened bark may also occur. Almonds
in San Luis Obispo County show severe
gumming on the trunk.

Molybdenum, although essential for
plant growth, is needed only in minute
amounts. On the basis of present infor-
mation, deficiencies seem highly improb-
able, and have not been observed in
California orchards.

Orchard plot trials

The need for fertilizer is indicated by
the condition of the trees, as determined
by observation of symptoms, and by soil
and plant analyses. If these factors point
to the need of a particular element, the
grower should try it on a limited scale.
Suitable fertilization practices can be de-
veloped if the plot test shows a profitable
response.

The grower must have clear objectives
before laying out a test plot. He may
wish to know whether any fertilizer will
pay, or what element is needed, or how
much of a needed material should be
used.

Plots should be chosen carefully to
represent the average of the block be-
cause individual trees vary in their re-
sponses to the same treatment. Each plot
should contain at least 10 trees.

To insure success, plots must be prop-
erly compared. Detailed records are
rarely necessary, but some measure
should be made. In addition to observ-
able symptoms, a count of the number
of boxes of fruit per tree is usually an
accurate enough index. Observation
alone may not be adequate in evaluating
differences ranging up to 20 per cent.

For help in planning orchard test
plots, consult your University of Cali-
fornia Farm Advisor.

[9]

How to apply the fertilizers
your orchard needs

Nitrogen

The most common problems of nitro-
gen fertilization concern source of the
element, time of application, and amount
to use. The chart below summarizes
the characteristics of the most common
sources of nitrogen. (Organic sources,
other than synthetic urea, are not in-
cluded. Manure and covercrops consti-
tute the primary sources of organic ma-
terials, although sewage sludge, blood
meal, tankage, fish emulsions, bone meal,
and seed meals contribute to the total.
These materials are largely by-products
from other manufacturing processes, and
their nitrogen content is usually rather
low compared with that of the inorganic
sources listed.)

All of the materials in the chart have
been used successfully in orchards as
sources of nitrogen. For most growers,
the price per unit of actual nitrogen will
determine the choice. In special situa-
tions, however, other factors are impor-
tant. For example, it would be unwise
to use sodium nitrate where sodium
toxicity is a danger. A material with an
acid residue is to be preferred in an
alkaline soil and to be avoided in a
highly acid soil.

Experimental plots with different
sources of nitrogen have been compared.
The trials normally extended over a five-
year period. These trials showed that the
tree response was the same for a given
amount of actual nitrogen regardless of
the source.

Nitrogen is necessary at the time
of bloom and of spring growth to insure
an adequate per cent of set and proper
vigor in the new growth. The leaf area

developed on this new growth manu-
factures the food which is necessary for
further vegetative growth of both top
and roots, and for fruit development.
Soon after blossoming, the stimulus is
given to fruit-bud formation for the next
year's crop, and nitrogen is required for
this process. It seems logical to assume
that the need for a supply of nitrogen is

Here Are the Pri

NAME

Compound
formula *

Anhydrous ammonia

NH3

Ammonia solution

NH4OH

Ammonium sulfate

(NH4)2S04 j

Ammonium nitrate

NH4N03

Ammonium phosphate-
sulfate (16-20) mixture

*

Ammonium phosphate
Calcium nitrate

NH4H2P04 T

Ca(N03)2

*-

Urea

NH.CONH, 4

Sodium nitrate

NaN03

Calcium cyanamide

CaCN2

* There is no serious trouble w

ith the physical pro
-0

[10]

most critical at this stage of the growth
cycle. To insure this supply, most grow-
ers apply nitrogen in the dormant season.
If it is in the form of nitrate, the timing
may be as late as a month before bloom.
Time must be allowed for rains to carry
the nitrate into the root zone. Nitrate
may be applied earlier unless the soil is
very light or shallow, in which case
leaching may reduce the effectiveness.
If the nitrogen is in the form of am-
monia, whether combined with other
substances (for example, ammonium
sulfate) or not, it will be "fixed" by the
soil. That is, it will combine with a cer-
tain portion of the soil in a form that
prevents its movement into deeper layers.
At ordinary rates of application, am-

monia will be practically completely re-
moved from solution in 2 inches or less
of soil. Soil bacteria then act on the
ammonia to change it into nitrate, in
which form it is free to move. It is neces-
sary to allow at least a month for this
process if nitrate is to be in the root zone
when it is needed.

There is evidence that nitrate can be
absorbed by roots before top growth
begins if the soil temperature is not too
low. It appears that most of the nitrate
used in the growth cycle is absorbed
fairly early in the season. Late applica-
tions, during the growing season, may
increase the absorption and give a nitro-
gen response, but do not take the place
of available nitrate in the early spring.

pal Commercial Sources of Nitrogen for Orchards

er cent

itogen

Advantages

Disadvantages*

82

High nitrogen percentage ; ease of appli-
cation; no residue; little danger of
leaching

(a) In irrigation water: Uneven distribu-
tion if irrigation system not adapted to
its use. Cannot be used with sprinklers.
(b) Dry injection: Some loss if ground
is trashy or cloddy

sually
>20

31

Easier to handle than anhydrous; no
residue

Same as for anhydrous

Acid residue (for alkaline soils) ; little
danger of loss by leaching ; ease of han-
dling

Acid residue (for very acid soils). Delayed
availability during nitrification

High N percentage ; no residue. Half im-
mediately available, half delayed

,16

Same as ammonium sulfate. Carries
phosphate if needed for covercrop

Same as ammonium sulfate

__v

15.5

142

L

High phosphate content where needed
for covercrops

Low N percentage

Calcium residue (for acid or high sodium
soils). Immediate availability

May be leached

High N percentage. Is not fixed if irri-
gated at once, before conversion to
ammonium carbonate. No residue

May be toxic in high concentrations

^6

1-24

Alkaline residue (for acid soils) ; imme-
mediate availability

Sodium residue undesirable on high so-
dium soils. May be leached

Alkaline residue (for acid soils). Calcium
residue

Danger of burning, especially at high rates
or in growing season

s of any of these materials unless they are stored too long or under poor conditions.

[ii]

Comparisons of tree behavior in

plots receiving nitrogen at different times
of the year showed that timing is very
important during the first year of appli-
cation. Dormant applications were best,
with spring applications next. After the
experiments were established, however,
there was enough carry-over from one
year to the next so that timing seemed
of secondary importance except in the
case of early shipping fruit. It was found
that the same amount of nitrogen applied
immediately after harvest gave less re-
sponse than at other seasons. With an
application at this time it was possible
to obtain a response in leaf color, time
of leaf fall, condition of fruit buds, and
tree condition without delay in maturity.
Some growers split their nitrogen fer-
tilization, putting on a part in the dor-
mant season and a part in the spring.
The size of the second portion is regu-
lated by the condition of the tree at the
time. If the crop is heavy, a little more
is used, and if light, less. Some material
is saved but the extra labor may offset
the saving. For most conditions where
leaching is not a problem, a single appli-
cation has been satisfactory.

The fixation of ammonia influences
timing of fertilization in irrigation water
during the growing season. Fertilizers
differ in their behavior when applied in
this way. For example, calcium nitrate
will move into the soil with the water,
and may cause greening of the leaves in
a few days. Ammonia will be held back,
and ordinarily will not be available until
it has been nitrified and moved into the
root zone with the next irrigation. A
compound such as ammonium nitrate
will do both. Half of the nitrogen, as
nitrate, will move down immediately;
the other half will be held back for later

The choice of application method

in orchards seems to be mainly a matter
of cost. Trials have shown a slightly
greater uptake of nitrogen when it is

applied in a ring the diameter of the
branch spread than when the same
amount is distributed over the whole
area, but the response did not differ.
There seems to be no difference in re-
sponse between broadcasting and drill-
ing. Material dissolved in irrigation
water is spread about as evenly as the
water. In basins with a good head, dis-
tribution is very even. In furrows there
is more likelihood of uneven distribu-
tion, especially with small heads and
long runs. This is particularly true with
ammonia, which tends to be fixed by the
soil in the furrow at the upper end of
the run. Noncorrosive, nonvolatile ma-
terials can be used in sprinkler systems.

The amount of nitrogen necessary
in a particular orchard can be deter-
mined only by experience, and the rate
of application must be based on tree
condition and response, the kind of fruit,
age of trees, vigor, type of pruning,
water supply, climate, and character of
soil. For example, the peach is likely to
respond to nitrogen under conditions in
which some other species will have an
adequate supply. Trees that bear normal
crops and at the same time make vigor-
ous vegetative growth probably require
little or no treatment. In soil of a high
initial fertility, young trees may grow
vigorously without nitrogen addition,
but may show deficiency after some
years of bearing. Trees which are heavily
pruned usually require lighter applica-
tions of nitrogen than do trees lightly
pruned. Trees suffering from an inade-
quate water supply may have a somewhat
higher need for nitrogen than those with
a normal water supply. The same variety,
in the same kind of soil, may respond
differently in different climates. Apricots,
for example, require less nitrogen in the
Santa Clara Valley than in the interior
valleys. The supply of nitrogen in a light
soil is often limited, and becomes ex-
hausted sooner than that in a heavier
soil. Trees making weak growth because
of lack of nitrogen may need, on an

[12

average, 60 to 100 pounds of actual
nitrogen per acre — equivalent to 300 to
500 pounds of ammonium sulfate or 360
to 600 pounds of calcium nitrate per
acre. Higher rates of application are
rarely profitable. The amounts indicated
above are suggested for those species
with a high nitrogen requirement, such
as peaches and almonds. Under the same
growing conditions, other stone fruits
require less nitrogen for best results.
Apples and pears likewise have consider-
ably lower nitrogen requirements than
peaches.

In many orchards it should be possible
to obtain an unusual spread in time of
maturity by fertilizing part of the area
more heavily than the rest. This practice
will delay maturity on the more heavily
fertilized portion, and smaller picking
crews may be able to handle the fruit.
The rate of nitrogen application should
be coordinated with other orchard prac-
tices.

Excessive use of nitrogen is not com-
mon, and should be avoided. In certain
cases, fruit quality has been impaired
and maturity delayed by heavy applica-
tions. Moderate excess leads to a few
days' delay in maturity, with some fruit
in the lower and interior parts of the
tree failing to attain satisfactory color.
Further excess may give softer fruit of
poorer color and flavor over the whole
tree. Uneven ripening of fruit halves in
stone fruits and a delay in maturing of
wood in the fall have also been noted
with high nitrogen.

Phosphorus

Although tests to date show that Cali-
fornia orchards are not deficient in phos-
phorus, this material can be applied with
profit to encourage growth where cover-
crops are beneficial and need phosphorus
for satisfactory growth (see p. 15).
Superphosphate is the standard source
of phosphorus. Apply when a covercrop
is planted, at about 50 to 100 pounds
per acre.

Potassium

This element should be applied in
orchards as potassium sulfate (sulfate of
potash) rather than the chloride (muri-
ate). California soils are frequently high
in chloride, and the addition of more
should be avoided. It has been found
that a single heavy application of potas-
sium sulfate is more effective than the
same amount used as a mixed fertilizer
applied over a period of years, and will
last for a number of years. The amount
required varies with the soil type. Trees
growing on a few soils with low fixing
capacity have responded to as little as
5 pounds of potassium sulfate per tree.
More commonly, 15 to 25 pounds are
necessary, and on some heavier soils with
high fixing capacity, 50 pounds were re-
quired.

Where symptoms and leaf analyses
indicate potassium deficiency, it is sug-
gested that the grower treat a few trees
with different amounts of potassium sul-
fate to determine the most economical
level. Placing the material in bands just
below the usual depth of cultivation re-
duces the amount required to give re-
sponse.

Boron

Deficiency has usually been corrected
by addition of borax at the rate of 50
to 100 pounds per acre, broadcast evenly
on the soil. Response in the spring usu-
ally follows applications made the pre-
ceding fall. More rapid response results
from spraying borax at 1 pound per 100
gallons during the growing season. Ap-
plications much in excess of the above
rates are likely to produce toxic symp-
toms.

Iron

This was the first minor element de-
ficiency to be identified, and has been
the most difficult to correct. Soil treat-
ment has usually been unsatisfactory.
Organic salts of iron, such as the citrate,
tartrate, or oxalate, placed in holes in

13

the trunk, have given correction for as
many as three years, but have damaged
trunk tissue. Various sprays have been
used, the most promising being various
iron chelates at the rate of 1 pound per
100 gallons.

Magnesium

This element has been supplied as
magnesium sulfate (Epsom salts) or
Dolomitic limestone. The former is used
in neutral or alkaline soils, the latter
under acid conditions. Rates between 10
and 40 pounds per tree have been recom-
mended. On soils low in potassium, use
of large amounts of magnesium may in-
duce potassium deficiency and vice versa.
A spray of 20 pounds Epsom salts per
100 gallons of water has also been used
for more rapid response.

Manganese

On most species, manganese deficiency
can be corrected by spraying with a mix-
ture of 2.5 to 8 pounds manganous sul-
fate, 5 pounds lime, and a spreader, per
100 gallons. Spray in late spring or early
summer. Correction of symptoms should

follow in a few weeks. Annual sprays are
likely to prove necessary. Manganous
sulfate can be added to the soil in holes
or trenches, but more material is re-
quired with this method. Broadcasting
is not satisfactory because the chemical
is fixed by most soils. An experimental
method of injecting dilute solutions into
holes bored in the trunk or main
branches has given good results, but the
holes may also admit destructive fungi.
Acidification of soil with sulfur will
usually correct the deficiency, but may
be too expensive.

Zinc

This element is used to correct little-
leaf. The application method must be
adapted to the species concerned. Treat-
ments have been made by means of
sprays, pieces of zinc or galvanized iron
driven into the trunk, holes bored in the
trunk, and direct application to the soil.

For nearly all fruits (except sweet
cherry and walnut), the most satisfac-
tory method of zinc application is spray-
ing. For severe cases, zinc sulfate sprayed
during the dormant season at the rate of

Fertilizer spreader being refilled in prune orchard.

50 pounds per 100 gallons of water is
recommended. For cases of moderate
severity, half that strength is sufficient;
and for mild cases, as little as 10 pounds
per 100 gallons may be used. Summer
sprays must be much more dilute, not
more than 6 pounds per 100 gallons, and
must contain 3 pounds hydrated lime or
soda ash to prevent burning. A more
satisfactory summer spray is zinc oxide
with a spreader, but this spray will in-
jure fruit. Zinc chelates now being used
experimentally have given good control.

Metallic zinc points or pieces of gal-
vanized iron driven into the tree will
correct little-leaf for a long period of
years in most species. This is the most
satisfactory method for walnuts and
sweet cherries. An area around each
piece of metal will be killed, and if these
areas merge, the trunk or branch will be
girdled. To prevent this, stagger the
points or place them in a spiral. About
four to six pieces per inch of circum-
ference are recommended. Results will
be better if branches are treated rather
than the trunk.

A treatment with dry zinc sulfate in
gelatine capsules (size 000), placed in
holes about 4 inches apart around the
trunk, will correct the symptoms for
three years or more. (The same objec-
tion to boring holes in the tree that ap-
plies to manganese also applies to the
use of zinc or other minor elements —
rot may develop.)

Direct soil application requires large
quantities of zinc sulfate, and the rate
of transmission is too slow for rapid re-
covery. Because the zinc is fixed by the
soil, it must be applied in holes or a
trench in the ground.

Copper

Copper deficiency has not been found
where trees are sprayed with bordeaux
mixture for the control of disease. When
bordeaux is not used, dry copper sulfate
in capsules may be added through holes
in the tree, as with zinc. Because of the
higher toxicity of this material, however,
greater care in application is necessary.
The copper must be kept away from the
bark, cambium, and younger sapwood.
Adding copper sulfate in a trench about
4 to 8 feet from the tree at the rate of 5
to 20 pounds per tree has also been suc-
cessful.

Growing alfalfa in orchards having
either zinc or copper deficiency has
proved beneficial: mild cases have been
entirely corrected, and severe ones
greatly improved. Just how the alfalfa
functions is not understood. Whether or
not the practice is feasible must be de-
cided for each orchard. Alfalfa is not
suitable for such crops as prunes or
almonds because it hampers harvesting
operations. Also, other cultural practices
may require modification if alfalfa sod
is maintained.

Covercrops are good for
some orchards, bad for others

Any crop grown between the trees and
turned under may be considered a cover-
crop, even if it is a weed that volunteers.
Such crops affect the problem of fertili-
zation and the trees' response to ferti-
lizers.

The first objective in planning a
covercrop is the addition of organic mat-
ter, not only as a source of nitrogen that
will be released over a long period in the
soil, but also as a major factor in main-
taining good tilth, or soil structure. With

[15]

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continuous cultivation, organic matter
tends to disappear. It can be restored
either by bringing it in from other
sources, such as manure or bean straw,
or by growing it in place and working
it into the soil. Manure or other suitable
material is not often cheap enough to
warrant the use of adequate amounts. In
many orchards the growing of cover-
crops has tended to replace manuring.

Actual field data regarding the effect
of covercrops on soil structure are scanty.
However, much laboratory work has
been done to show the effects of adding
covercrop material under controlled con-
ditions. The decomposition rate of dif-
ferent materials under varying moisture
and temperature has been studied, to-
gether with the effect of these processes
on the formation of soil granules. Cer-
tain factors important in the orchard are
difficult to study in the laboratory — for
example, the formation of root channels
through plow sole, or the cracking of
certain soils. Since information on many
of these points is still fragmentary,
present opinions may be changed later.
It seems certain that, in many soils,
water penetration is better after a few
years of covercrops. The action of cover-
crops in improving water penetration
may lie in either of two zones. One is
the prevention of "surface sealing" which
occurs in some southern California soils
when they are wet. The other is the im-
provement of the compacted layer below
the depth of cultivation, known as the
plow pan or plow sole. At Davis, for ex-
ample, the latter effect was so great that
the water from a 6-inch irrigation dis-
appeared from the surface of a cover-
cropped basin in less than 24 hours,
whereas across a levee, in an adjacent,
clean-cultivated check, the time required
was more than a week. More economical
use of water and a better supply to the
roots will result in the absence of plow
pan. The use of covercrops is not, how-

ever, a substitute for careful soil han-
dling. Cultivation when the soil is too
wet will puddle many soils so badly that
years of good care may be required to
repair the damage. Good soil structure
can be developed, moreover, and main-
tained without covercrops if sufficient
care is taken to avoid compaction. When-
ever such care is impossible because the
soil is too wet, covercrops may be of
great benefit.

A distinction should be made between
the improved soil-water relations result-
ing from better penetration, and those
from increased water-holding capacity
of the soil. Under cool, humid conditions
the soil's organic matter can be increased
by annual covercrops, and with it the
total nitrogen and perhaps the water-
holding capacity. Under hot, semiarid
conditions, this is not the case: the rate
of destruction of organic matter is so
great that there is little, if any, net accu-
mulation. At Davis, 30 years of annual
covercrops of three types — winter leg-
ume, winter nonlegume, and summer
legume — have failed to change the mois-
ture-holding capacity of the soil measur-
ably. This factor, therefore, can prob-
ably be ignored in California orchards.

Much the same situation exists with
regard to total nitrogen as with moisture-
holding capacity. Leguminous cover-
crops with proper inoculation of nitro-
gen-fixing bacteria have given increases
of total nitrogen in cool, humid sections;
but neither summer nor winter legumes
has done so at Davis. There probably
was some fixation of nitrogen, but either
it has been used, and therefore does not
appear in analyses, or the amount is too
small to be detected. In sandy soils,
where heavy rains might leach nitrate
below the root zone, its absorption by
the covercrop, with later release as the
crop rots after being turned under, may
save important amounts for use by the
trees.

Left: tractor-drawn disk and harrow disking under a mustard covercrop in walnut grove.

[17]

When organic material is incorporated
into the soil, most of it is decomposed
by soil bacteria and fungi. These organ-
isms, like other plants, need mineral
nutrients for their growth and function-
ing. During the first part of the period
of decomposition, the soil microorgan-
isms are increasing in number, and may
use nitrate from the soil solution as well
as nitrogen from the decaying covercrop.
The nitrate concentration is thereby re-
duced in the soil solution, leaving less
for the trees. The extent of this depletion
depends on the supply of the material in
the soil (especially nitrate), the condi-
tion of the covercrop or other organic
material turned under, the moisture sup-
ply, and the temperature. Of these fac-
tors, the most important, usually, is the
character of the organic material. If it
has a high nitrogen content, as in a suc-
culent covercrop that is not mature, de-
composition is rapid. Because of this,
nitrates are released sooner than with a
material lower in nitrogen. The organ-
isms can therefore secure most of the
nitrogen they need from the material
itself, and less from the soil. Strawy ma-
terial, high in carbohydrates and low in
nitrogen, may cause a depressed nitrate
level for months after being turned
under.

Obviously, any tendency toward ni-
trate deficiency in a soil will be much
increased by the incorporation of large
amounts of low-nitrogen organic matter.
Additional amounts of fertilizer will then
be needed to supply both the soil organ-
isms and the tree. Covercrops, further-
more, absorb nitrate while growing, and
during that period may compete seri-
ously with the tree. An attempt should
be made to correlate the timing of the
growth of the covercrop with the ferti-
lizer program and with the needs of the
trees.

Covercrops may play an important
role on slopes that are subject to erosion.
They increase the rate of water penetra-
tion, thus reducing runoff, and their

[

roots tend to hold the soil in place, re-
ducing the amount washed down by the
water that does flow away. A crop to be
used for erosion control must be one
that establishes a root system quickly
throughout the surface soil, unless a
permanent sod is already established.
Various crops of this type have been
tried in most districts, and information
about their use can be obtained from the
local University of California Farm
Advisor.

Annual covercrops may be divided
into four groups: winter legumes, sum-
mer legumes, winter nonlegumes, and
summer nonlegumes. Among winter
legumes, the most widely grown are
bitter clover or annual yellow sweet-
clover (Melilotus indica) , the vetches,
and bur clover. Horse beans, fenugreek,
lupine, and field peas have been success-
ful in more limited areas. The following
crops have had some use as summer
legumes: cowpeas; velvet, mung, tepary.
and mat beans; soybeans; sesbania; and
Hubam clover. The most widely used
winter nonlegumes are: mustards (com-
mon, black, and Trieste) and cereals
(rye, oats, barley), together with vol-
unteer weeds. Where summer nonle-
gumes are desired, orchardgrass, Sudan-
grass, and summer-growing weeds have
proved satisfactory.

In addition to these crops, an increas-
ing number of growers are using perma-
nent sod. This system eliminates the cost
of cultivation, and is the most effective
check on erosion. It permits orchard
operations when the soil is wet that are
not feasible under clean cultivation. On
the other hand, this method requires
more water, increased use of nitrogen
(even with a leguminous sod) , and more
rigorous efforts in pest control. It pro-
vides cover for mice and gophers. It is
not suitable for species whose fruit is
harvested from the ground — for example,
prunes, almonds, walnuts, or figs. Alfalfa
has been widely and successfully used
for permanent sod, and perennial rye-

18]

grass has also proved satisfactory. In
some areas, throughout the year, volun-
teer weeds provide a succession of plants
which, though containing few perennials,
serve adequately.

Despite the advantages to be obtained
from covercrops, they can be harmful
in certain orchard areas. Nonirrigated
orchards in regions of low rainfall need
all the moisture available to take them
through the season. The use of any con-
siderable portion of the supply by cover-
crops may result in failure to mature the
fruit and, during very dry years, in
severe damage to the trees. Any cover-
crops used in such areas must be turned
under early enough so that the late
winter rains will restore the water used

by these plants in the early winter. Under
these conditions, large tonnages of cover-
crops cannot be expected, and conditions
may keep the grower from turning the
crop under in time to prevent some mois-
ture depletion. The increase in rate of
moisture penetration and the decreased
loss by runoff may compensate for the
water used when the practice has been
carried on long enough to be effective.
Since covercropping must be practiced
for several years before water penetra-
tion can be noticeably improved, this is
still a hazardous program in nonirrigated
areas. Furthermore, the growers of stone
fruits havpnfnTTf>"H"^jiiTgTip"r mclde™^ of
pTownnFoFlF^n^riards having covercrops
at blossomTn^tirne!

Co-operative Extension work in Agriculture and Home Economics,. College of Agriculture, University of California, and United States Department of Agriculture
co-operating. Distributed in furtherance of the Acts of Congress of May 8, and June 30, 1914. Ccorge B. Alcorn, Director, California Agricultural Extension Service.

20iw-l,*58(C7034)LL

[19]

Careers fwiW>te

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ogy on ttten( California.
University or person.

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variety o« "OP5- J hefruitand

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Some year we hope

^PPiy ^demand-

A knowledge of fruits and fruit-growing
offers many fine careers. And the best
positions go to those who have mastered
the subject through a balanced program
of training.

At Davis the course in pomology is bal-
anced between practice and theory — the
"how" and the "why" — using the finest
facilities . . . taught by one of the largest
and best-trained horticultural staffs in the
world.

The Department of Pomology main-
tains about 300 acres of orchards, con-
taining nearly all the important varieties
of deciduous tree fruits, nuts, olives, and
berries (strawberries, boysenberries, etc.).
The student has an opportunity to become
acquainted with most of the fruit-grower's
techniques of production and marketing.
He becomes familiar with the best and
most modern orchard equipment.

For study and research, facilities also include a packing house, complete sun-
drying and dehydration equipment, a cold-storage plant, lath-houses and green-
houses, and laboratories equipped with apparatus for fundamental studies.

The staff of the department includes specialists in fruit breeding, pruning, pol-
lination, spraying, irrigation, fertilization and plant nutrition, soil management,
physiological plant diseases, propagation, varieties, harvesting, handling, and
storage of fruits and nuts.

Trained people are in demand for . . .

PRODUCTION

Orchard management
Orchard operation

PROCESSING

Packaging • Canning
Freezing • Drying

DISTRIBUTION

Purchasing • Selling
Marketing • Shipping

SERVICE

Fertilizers • Sprays
Equipment • Nursery

OTHER

Agricultural Extension
U. S. Dept. of Agriculture
State Dept. of Agriculture
Teaching— School and College
Research— Industry, University