THE PEINCIPLES OF MANUKING 179 



for a variety of other reasons which have been previously 

 mentioned. It is unnecessary, therefore, to deal with the 

 subject again under the head of manures. 



It has also been mentioned that certain manures may 

 affect the physical and biological properties of the soil 

 as well as the chemical composition. These effects must 

 be taken into consideration in connection with the sub- 

 stances which produce them, but, as a rule, they are of 

 secondary, if sometimes of considerable importance. 

 According to the definition given above, manures are to 

 be regarded chiefly from the chemical point of view, i.e., 

 as substances containing plant foods to be added or 

 restored to the soil. 



The Constituents of Manures. It is obvious that a 

 mixed product like farmyard manure, consisting largely 

 of vegetable matter, must contain all the constituents 

 of plants, and, if applied in sufficient quantity, should 

 be able to satisfy all their requirements. But it is not 

 generally necessary to supply all the constituents of the 

 crops in the form of manure. Of the elements that are 

 essential for the growth of plants, carbon, hydrogen and 

 oxygen can be dismissed at once. The first is obtained 

 from the air, and so far as the nutrition of the crops is 

 concerned, it apparently makes no difference whether the 

 soil contains much or little or, indeed, any carbon com- 

 pounds at all. The presence of carbonaceous matter in 

 the soil may make a considerable difference in other 

 ways. Hydrogen and oxygen are derived mainly from 

 water, and need not therefore be further discussed in 

 this connection. 



Sulphur, iron and magnesia are usually present in 

 the soil in quantities far in excess of those required 

 by plants. The sulphates brought down to the soil in 

 the rain water contain more than enough sulphur to 

 make good all the loss of that constituent by removal of 



N2 



