240

QUANTITATIVE AGRICULTURAL ANALYSIS

thoroughly. Dilute to the mark and determine the per cent of ammonia
nitrogen in 100 cc by placing the solution in a Kjeldahl flask, adding 15 cc of
10-per cent potassium hydroxide and distilling the ammonia into 25 cc of
fifth-normal hydrochloric acid, using a tin condenser.

Place 200 cc (or enough to cover half the sand) of the ammonium sul-
phate culture solution, prepared as above, on clean white sand in a percolator
(Fig. 54). Cover the percolator with a watch glass. Keep in a dark place
for three or four weeks, then drain out all liquid and rinse
the sand with about 200 cc of distilled water (ammonia-
free), using about 50 cc portions at a time. Make the solu-
tion up to 500 cc in a volumetric flask, mix well and deter-
mine the amount of ammonia nitrogen in an aliquot part
by the distillation method, as above described. The differ-
ence between the amount present at the beginning and at
the end will represent the amount converted to nitrate*
Calculate the per cent of nitrogen which has been changed
from ammonium sulphate to a nitrate. This gives an
estimate of the activity of nitrifying bacteria in the soil.

Denitrification.—Certain bacteria (B. denitrifi-
cans alpha, also beta) have the power, under ap-
propriate conditions, to reduce ammonia> nitrites
and nitrates to the form of elementary nitrogen.
This is usually brought about in a water-logged
soil or in the presence of an excess of nitrogenous
organic matter. The amount of released element-
ary nitrogen may be measured and the bacterial
activity noted.

FIG. 54.
Percolator.

Demonstration of Denitrification.—In a 250-cc wide mouth flask or bottle
place 20 gm of horse manure and add 100 cc of water containing 1 gm of
potassium nitrate. Fill the bottle with water and close the mouth with a
rubber stopper connected with a delivery tube. The tube is inserted into a
500-cc graduated cylinder wHich has been previously filled with a 5-per cent
sodium hydroxide solution and inverted into a 1000-cc beaker containing
about 300 cc of water. The method of assembling the apparatus is shown in
Fig. 55. After standing 24 hours at about 35° a mixture of carbon dioxide
and nitrogen will begin to be produced. The former will be absorbed by the
sodium hydroxide while the latter will be caught in the cylinder and can be
measured. Calculate the per cent of denitrification of the nitrate addecL
Phosphorus.—Phosphorus is present in all soils, usually in
small amounts, varying from 300 to 5000 Ib. per acre of 2,000,000
Ib. of soil (0.015 to 0.25 per cent). The plant demand for
phosphorus is large, as crops remove from 5 to 30 .Ib. per acre