THE PEINCIPLES OF MANUEING 195 



the least. The large proportion of silica in the former 

 is regarded as a proof that silicates are available to 

 these crops that are not available to the latter. Whether 

 this is the true and whole meaning of the fact or not, 

 it is a matter of common knowledge that grass and 

 grain crops which are only a kind of grass after all 

 can be grown on land that would not yield a crop of 

 roots at all without manure. 



Cereals and Grass. The graminaceous crops require 

 less manure than the others both because they require 

 smaller quantities of the fertilising ingredients, and be- 

 cause they are better able to ass'imilate them directly 

 from the soil. If, on a soil of average good quality, 

 the fertility is maintained at its normal level by the 

 restoration of the plant foods in the farmyard manure 

 at the end of each rotation, very good crops of grain 

 and grass can as a rule be raised without the use 

 of any special manure. The amount of produce can 

 however, generally be increased by the application of a 

 moderate top dressing of nitrogenous manure. Nitrate 

 of soda appears to be the most suitable form. The bene- 

 ficial effects of nitrogenous manures on these crops are 

 probably due to the fact that the most active growth 

 takes place in the early spring, i.e., just after the avail- 

 able compounds of nitrogen nitrates have been ex- 

 hausted by the winter drainage and before they have 

 been replaced Jby the process of nitrification. If the 

 fertility is not properly maintained by farmyard manure, 

 phosphatic as well as nitrogenous manures should be 

 applied to these crops. 



Root Crops. The root crops require very large quan- 

 tities of plant food and their assimilative capacity is so 

 inferior that, even on the naturally richest soil, full crops 

 cannot be obtained unless they are well manured. It 

 is partly for this reason that they are usually taken 



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