62 THE CHEMISTRY OP THE FARM. 



100 200 Ibs., or even more, of nitrogen per acre. In 

 the case of permanent pasture the effect of long-continued 

 crop residues is strikingly manifest, the surface soil con- 

 taining twice as much nitrogen, and more than twice as 

 much carbon, as ordinary arable land. In the case of 

 pine forests, the accumulated residues of dead leaves, 

 <c., are also very large : chiefly by their means bare rock 

 is often transformed into fertile soil. It is obvious that 

 the more luxuriant a crop the greater will be the crop 

 residue : a series of good crops thus tends to enrich the 

 soil with nitrogenous humus, while under a series of bad 

 crops the soil will diminish in fertility. 



Weeds. The weeds of a farm form a natural crop ; 

 their influence is at times beneficial and at times injurious 

 to the farmer. That some vegetation should grow on the 

 land in the absence of a regular crop is most desirable. 

 The rapid growth of weeds after harvest will greatly 

 diminish the loss of nitrogen by drainage and be of use 

 in other ways as a green crop. When the weeds are 

 ploughed-in in the spring the valuable matter stored up 

 by them again becomes available as plant food. On the 

 other hand 3 it is obvious that a crop will have little chance 

 of obtaining plant food if it has to compete with growing 

 weeds which have obtained earlier possession of the soil. 

 The best plan is apparently to destroy weeds, and to ob- 

 tain the important benefits they yield by a judicious use 

 of green crops, sown so as to occupy the land after har- 

 vest. 



