50 THE CHEMISTRY OF THE FARM. 



iodder and root crops is thus especially liable to varia- 

 tion. This subject is discussed on page 92, 



Cereal Crops. These contain much less nitrogen than 

 either leguminous or root crops ; about three-quarters 

 of the nitrogen is in the corn, and only one quarter in 

 the straw. The amount of phosphoric acid is not very 

 different from that found in other crops ; this ingredient 

 is, in fact, the most constant in quantity of all the con- 

 stituents of crops. The phosphoric acid is chiefly con- 

 centrated in the corn. Potash and lime are present in 

 much smaller quantity than in other crops; they are 

 chiefly found in the straw. 



The presence of a large amount of silica is character- 

 istic of the cereal crops; they possess apparently a 

 capacity for feeding on silicates not enjoyed by other 

 crops. The base of the silicate is made use of by the 

 plant, while the silica itself is excreted upon the surface 

 of the leaves and straw. It has been shown that silica 

 is by no means essential for the growth of cereals : they 

 take it up freely, but can also do without it. 



Owing to their small demands upon the soil, and 

 possibly also to their capacity for assimilating silicates, 

 cereal crops will for a long time continue to yield a 

 moderate produce upon exhausted unmanured land ; a 

 fact of great importance to the human race. 



The autumn-sown cereals (wheat and rye) have both 

 deeper roots and a longer period of growth than the 

 spring-sown cereals (barley and oats), and are conse- 

 quently better able than the latter to supply themselves 

 with the necessary ash constituents from the soil. Barley 

 possesses a considerable development of root near the 



