ig8 SYSTEMS OF PERMANENT AGRICULTURE 



between one of the decomposition products of organic matter and 

 the process of making insoluble plant food available, thus: 



Ca 3 (P0 4 ) 2 + 4 HN0 3 = CaH 4 (P0 4 ) 2 + 2 Ca(NO 3 ) 2 . 



In accordance with this equation, 56 parts of nitrogen are equiva- 

 lent to 62 parts of phosphorus in the reaction; whereas, when 

 measured by the requirements of the corn crop, 56 parts of nitro- 

 gen are equivalent to less than 9 parts of phosphorus, or only one 

 seventh of 62. 



Even though the nitric acid maybe at once neutralized by reac- 

 tion with calcium carbonate, it is known that the liberated carbonic 

 acid exerts an influence in the conversion of insoluble phosphates 

 and potassium salts into soluble compounds. 



Of course, the quantity of organic acids and carbonic acid other- 

 wise produced in the decay of organic matter is many times as 

 great as that of nitric acid. (See also page 633.) 



Recent investigations of Hall, Miller, and Gimingham (Pro- 

 ceedings Royal Society, 1908) seem to prove that nitrification 

 proper does not occur in acid soils, and that crops growing on 

 such soils must take up their supplies of nitrogen in the form 

 of ammonium salts, formed in the process of ammonificatioh. 

 It is shown, however, that there may be a small amount of nitri- 

 fication in soils that are, on the whole, acid, but which contain 

 here and there particles of calcium carbonate within whose limited 

 sphere of influence the soil is alkaline and nitrification takes place. 



Under certain abnormal conditions, as under a slime or scum 

 from sewage which prevents access of air, some denitrification 

 may occur. In this process the denitrifying bacteria may even 

 decompose nitrates in order to secure oxygen, and the element 

 nitrogen may be liberated as free gas. Such loss may readily occur 

 in the decay of manure in piles, but in normal soils there is prac- 

 tically no denitrification. 



METHODS OF SUPPLYING ORGANIC MATTER 



There are three general methods of supplying organic matter 

 to the soil in practical agriculture: 



