CHARACTERISTICS OF SOILS 5 



be used by the plants, these food elements must be sepa- 

 rated from the mineral particles and be dissolved in the 

 soil water. Whether these changes are chemical or 

 physical, they take place more readily and more rapidly 

 under high than under low soil temperatures. 



6. Biological activities. The plant secures its nitro- 

 gen supply from the soil also, and appropriates it by way 

 of its roots. The greater part of this nitrogen supply 

 must be in the form of nitric acid, usually after the acid 

 has combined with some mineral element, such as calcium 

 or potassium, thus forming what is called a nitrate. The 

 greater part of the nitrogen supply in the soil is in some 

 form other than nitric acid or nitrate. It is chiefly 

 locked up in the organic matter; or it is found as free 

 nitrogen in the air in the soil. 1 There is some ammonia 

 and some nitrous acid in the soil, and these are nitrogen 

 compounds. 



7. Nitrogen preparation. Before the nitrogen of 

 the organic matter in the soil can be used for food by grow- 

 ing crops, it must enter into new combinations with other 

 elements. The greater part of the nitrogen, by a series 

 of changes, is finally combined with hydrogen and oxygen 

 to form nitric acid (HNO 3 ), which in turn combines with 

 some base to form a salt. It is in this form chiefly that 

 nitrogen is used for food by plants. 2 



1 Air is composed of a mixture of gases, of which mixture 

 oxygen constitutes 23.22 %, nitrogen 75.55 %, and carbon dioxide 

 .045%-.06% by weight. HILGARD, Soils, p. 16. 



2 Recent investigation indicates that plants use, to some extent 

 at least, other forms of nitrogen than nitric and ammonia nitro- 

 gen. See article of H. B. Hutchinson and N. H. Miller of the 

 Rothamstedt Experiment Station, which appears in the Journal 

 of Agricultural Science, Vol. 3, part 2, 1909. See also Bulletin 87, 

 Bureau of Soils, U. S. Dept. Agr. 



