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QUANTITATIVE AGRICULTURAL ANALYSIS

Nitrogen. — Nitrogen is one of the most important of the
elements that are concerned in plant growth. Although abun-
dant in the atmosphere in an uncombined form, it is an expensive
element when used in making up a fertilizer. This is because its
inert nature makes difficult the problem of forming nitrogen
compounds which may be used by plants. Nitrogen should
therefore be obtained, so far as possible, through growing inocu-
lated legumes in rotation, rather than through purchase in the
form of fertilizers.

Nitrogen is usually present in a fertilizer in one or more of the
following forms: (1) Ammonium salts, such as ammonium sul-
phate or nitrate; (2) animal or vegetable matter, such as dried
blood, cotton seed meal, stable manure and guano; (3) atmos-
pheric nitrogen fixed by electrical energy, as various nitrates.
Sodium nitrate is found also as a natural product, chiefly in
South America.

Organic fertilizers have some advantages over the others in
that they promote bacterial action. Because of their limited
solubility they do not readily leach out of the soil, the result being
that they are used less rapidly and supply the plant with nitrogen
through a longer period of growing season. Calcium cyanamid
also acts like the organic forms as it slowly breaks down in the
soil, somewhat as follows :

CaNCN + C0a + 2H20 -» CaC08 + CO(NH2)2, (1)

Calcium cyanamid Urea

CO(NH2)2 + 2H20 -> (NH4)aC03 ,

(2)

ammonium carbonate being available to plants.

Nitrogen used in the form of ammonium sulphate has not the
most desirable action, as it finally leaves free acid in the soil,
due to hydrolysis and absorption of the resulting ammonia.
Chili saltpeter (sodium nitrate) has the opposite effect in the
soil as the nitric acid formed by hydrolysis is used, leaving
sodium hydroxide which lessens the acidity of the soil or even
causes a basic condition. This is sometimes desirable, although
excessive basicity may change the texture of the soil because of the
deflocculating effect upon the clay particles, thus resisting the
penetration of rain water and the normal movements of drainage
water. This was illustrated in the experiment on deflocculation,
page 268.