HOME MANURE. 63 



soil is increased, for some of it is derived from the air. 

 If it is continually removed, however, the loss will ex- 

 ceed the natural increment ; and the soil will ultimately 

 fail, unless the substances removed are restored from 

 some other source in the form of manure. The poorer 

 the soil, the more complete must be the restoration of the 

 ingredients carried away in the crops, if fertility is to be 

 maintained or mcreased. But even the best soils are 

 made to yield larger crops with manure than they can do 

 without it. 



Practice and experiment in the growth of crops have 

 shown that nitrogen, phosphates, potash, and lime, in 

 assimilable form, are the substances which most strik- 

 ingly benefit land ; and chemical analysis has determined 

 in a measure the varying proportions in which different 

 crops draw upon these and other constituents of the soil. 



Acting on this knowledge, chemists have given speci- 

 fications for the preparation of manures for all the differ- 

 ent crops, these schemes being professedly based on the 

 composition of the crops themselves. But manuring on 

 this principle would often cost more than the consequent 

 increase of the crop would repay ; for it makes no allow- 

 ance for natural fertilitv, and it makes no distinction 

 between the composition of the crops grown and the 

 composition of the produce sold off the farm. AVe know 

 that soils are of very unequal fertility, that some have an 

 unlimited food-supply compared with others, and that it 

 is only the materials sold off the farm that the mainte- 

 nance of fertility requires to be restored. More than this, 

 crops differ greatly in their cai:»ability of self-su^^pl}-. 

 Take, as an example of the latter characteristic, the re- 

 lations of wheat and clover to nitrogen. Chemical an- 

 alysis shows that- clover contains more nitrogen than 

 wheat ; and yet the wheat finds its nitrogen with diffi- 

 culty, while the clover seems to have a power of self- 

 supply in this particular. Thus, in defiance of the 



