ROTATION OF CROPS. 59 



maintained in quantity, of which at present we cannot be 

 certain, we must conclude that this 35 lb. of nitrogen, 

 together with the unknown additional quantity lost as 

 nitrate by drainage, have been annually derived from 

 the atmosphere — partly as rain, but mostly by direct 

 absorption b}^ soil or crop. It appears very probable 

 that on many soils the amount of nitrogen contributed 

 by the atmosphere in the course of a rotation is very 

 considerable. 



We have supposed that only corn and meat are sold off 

 the land during the rotation ; it will often be economical 

 to sell a larger part of the produce, and to purchase 

 manure in its place. The sale of straw will be attended 

 with little practical loss on heavy land ; but on light land 

 both the loss of potash, and the diminution in the bulk of 

 the manure will be more or less felt. The sale of hay or 

 roots is far more exhaustive, and except on the most 

 fertile soils, must demand a considerable purchase of 

 manure or cattle food to replenish the soil with plant 

 nourishment. 



