LOSSES DURING ROTATION. 73 



much greater importance is the supply of nitrogen ob- 

 tained by the cultivation of leguminous crops. Where 

 such crops can be successfully grown, and are consumed 

 upon the farm, there should be little fear of a deteriora- 

 tion in the nitrogenous contents of the soil under the con- 

 ditions of rotation we have supposed. 



In the four- course manured rotation upon the heavy 

 land at Rothamsted, the nitrogen annually removed in the 

 crops, on an average of forty years, -has exceeded by 

 about 32 Ibs. the quantity supplied in the manure. If 

 the crops on this experimental rotation should be perma- 

 nently maintained in quantity, of which at present we 

 cannot be certain, we must conclude that these 32 Ibs. 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 by the crops or soil. 



When it is desired to make the utmost use of the 

 natural sources of fertility, the land is allowed to remain 

 more than one year in grass seeds ; or one green crop is 

 followed by another, as trifolium incarnatum by turnips ; 

 or a perennial leguminous crop is grown for several 

 years. The losses by sale of corn are thus diminished, 

 and the land is kept for some time under conditions 

 favourable to an accumulation of nitrogen in the surface 

 soil. 



We have supposed that only corn and meat are sold 

 off the land during the rotation ; it will often be economi- 

 cal 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 



