OBEBAL CEOPS ?5 



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 constituents of crops. The phosphoric acid 

 is chiefly concentrated 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 charac- 

 teristic of gramineous 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 

 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 consequently better able than the latter to supply 

 themselves with the necessary ash constituents from 

 the soil. 



The spring tillage for barley and oats aids nitrifica- 

 tion in the soil, and consequently less nitrogenous 

 manure is required for these crops than for wheat. 

 Maize is not only spring-sown, but has also a much 



