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annually. It occurs in the soil chiefly as apatite (calcium fluoro-
phosphate) and, to some extent, in organic forms.

The most pronounced effect of phosphorus upon the plant is
noted in the greatly increased development of lateral and fibrous
roots. This feature is of much importance in clay soils, especially
as it induces the formation of an extensive system of roots, thus
enabling the plant more successfully to withstand drouth. A

FIG. 55.—Denitrification apparatus.

deficiency of phosphorus is often shown by late maturity of
crops and, in the case of cereals, in the lack of good grain
development.

Before phosphorus in soil can be determined it is necessary to
remove organic matter, oxidizing phosphorus so held to phos-
phoric acid, and to bring the phosphorus of both organic and
inorganic matter into solution. The methods now in use for
this purpose are (a) oxidation of organic matter by heating with
sodium peroxide, with subsequent solution of phosphates by
hydrochloric acid, (6) a procedure similar to (a) but substituting
magnesium nitrate for sodium peroxide, and (c) oxidation of
organic matter and solution of phosphates by heating with

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