COMPOSITION OF SOILS. 39 



cal and chemical. The holding of such bases as potash, 

 lime, etc., is due to the presence in the soil of what are 

 termed simple silicates; these are capable of combining 

 with other silicates to form double silicates. A silicate 

 of alumina, for instance, will combine with a silicate 

 of ammonia to form a double silicate of alumina and 

 ammonia. All soils possess this absorbing power in 

 some degree, though it belongs particularly to soils con- 

 taining clay. 



Soils do not, however, possess an equal absorbing power 

 for acids. Nitric acid is not absorbed, but is freely 

 movable. The only acid of importance absorbed by the 

 soil is phosphoric acid, which combines with lime, iron, 

 and alumina, forming phosphates, — compounds of great 

 importance in plant nutrition, which are not removed 

 from the soil except through the growth of plants. 



This absorbing property of soils may be nicely illus- 

 trated by filling a cylinder of suitable length with a good 

 soil, and pouring upon it a dilute solution containing 

 one or more bases, including potash and lime, and both 

 nitric and phosphoric acid. An examination of the solu- 

 tion which passes through will show the presence of nitric 

 acid, and an absence, at least in any amount, of the potash 

 and phosphoric acid. Good soils fix all of the essential 

 constituents, except nitrogen when it is in the form of a 

 nitrate. 



The farmer can reduce the losses due to drainage by 

 careful management. The drainage waters contain least 

 nitrates when crops are growing and well cultivated. 

 This carrying away of plant-food constituents by the rain 

 into the drains may be regarded, therefore, as a natural 



