76 SOILS AND FERTILIZERS 



non-volatile elements of the soil. Plants cannot make 

 use of it in other forms than the phosphates. Phos- 

 phorus is usually present in the soil as calcium phos- 

 phate, magnesium phosphate, or aluminum phosphate, 

 and may also be combined with the humus, forming 

 humic phosphates. The form of the phosphates, as 

 available or unavailable, is an important factor in soil 

 fertility. Soils are quite liable to be deficient in phos- 

 phates, inasmuch as they are so largely drawn upon 

 by many crops, particularly grain crops, where the 

 phosphates accumulate in the seed, and are sold from 

 the farm. The phosphorus content of soils is usually 

 reported as phosphorus pentoxide (P 2 O 5 ), anhydrous 

 phosphoric acid, commonly called phosphoric acid. 



86. Nitrogen. This element is present in soils in 

 various forms. As a mineral constituent it is combined 

 with oxygen and the base-forming elements as potas- 

 sium, sodium, and calcium, forming nitrates and nitrites, 

 which, on account of their solubility, are never found 

 in average soils in large amounts. Nitrogen is mainly 

 in organic combination, being associated with carbon, 

 hydrogen, and oxygen as one of the elements form- 

 ing the organic matter of soils. Nitrogen may also 

 be present in small amounts in the form of ammonia, or 

 of ammonium salts, derived from rain water and from 

 the decay of vegetable and animal matter. While free 

 nitrogen is in the air in large amounts, it can be ap- 

 propriated as food in this form by only a limited num- 



