CHAPTER XIII
FERTILIZERS
Fertilizers, or manures, are those materials which either
increase the supply of elements in the soil, needed for the growth
of plants, or exert a corrective action in making conditions more
favorable for the plant's best development. Farm manures
are usually mixtures of the excrement and urine of farm animals
with stable litter.
A distinction is sometimes made between materials which
furnish plant food directly, such as nitrates, phosphates and
potassium and ammonium salts, and indirect fertilisers like
calcium carbonate, which neutralize soil acid as well as serve as
plant foods. There are also those which furnish plant food and
aid in loosening hard clay, as is *the case with manures. The
direct fertilizers containing nitrogen, phosphorus and potassium
furnish the elements that are most frequently lacking in soils.
Availability.—The value of a fertilizer is usually determined
by the per cents of the fertilizing elements and by the solubility
of the compounds containing these elements in water or soil
acids, also by absence of injurious salts, such as those containing
boron or aluminium. Solubility is an obvious measure of avail-
ability to plants. The most commonly used, easily available
water-soluble salts containing nitrogen are sodium nitrate,
ammonium sulphate and calcium cyanamid. Materials in
which the nitrogen is available more slowly are manures, legumes
ia green manuring, stubble and dead roots of plants. In these,
nitrogenous organic matter is gradually broken down into
simpler, soluble compounds, by bacterial action. Examples of
nitrogenous materials in which the nitrogen is practically
unavailable are hair, hoof, horn and leather. These are rich in
nitrogen but they are insoluble and decompose very slowly in the
soil. The phosphate fertilizers also present considerable variation
in solubility. This subject is discussed more fully on page 275.
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