88 THE CHEMISTEY OF THE FARM 



continued crop residues is strikingly manifest, the 

 surface soil containing twice as much nitrogen, and 

 more than twice as much carbon, as ordinary arable 

 land. In the case of a pine forest, 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. At Kothamsted 

 the normal percentage of nitrogen and carbon is main- 

 tained in the soil where large crops of wheat ar^ 

 continuously grown with purely inorganic manures ; 

 but the soil has become greatly impoverished where 

 no manure, or where an insufficient manure, has been 

 applied (see p. 102). 



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 into the soil the valuable 

 matter stored up by them again becomes available as 

 plant food. On the other hand, it is obvious that a crop 

 will have little chance of obtaining plant food, or even 

 water, 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 obtain the 

 important benefits they yield by a judicious use of 

 green crops, sown so as to occupy the land after 

 harvest (see p. 101). 



