MANURES. 329 



was shown that the amount of mineral plant food that is 

 actually present in the soil in an available form is extremely 

 limited. In a state of nature, our fields would produce only 

 such crops as could be fed by the small amount of this 

 plant food which is rendered available from year to year, 

 and there would be no diminution of production. On the 

 contrary, the decay of the crop of one year would probably 

 add to the supply available for the next year. The removal 

 of the crop lyy man, not the production of a crop which on 

 decay returns its elements to the soil, is what impoverishes 

 is what makes the use of manure vitally necessary on all but 

 virgin lands. 



The larger the crop provided it decays on the land the 

 more the fertility of the soil is increased. 



The larger the crop provided it is removed from the 

 land the more the fertility of the soil is diminished. 



If the crop is made larger by the use of manure, and is 

 removed from the land, the manure has caused a larger 

 amount of mineral plant food to be taken away. But if 

 the manure itself contains the full equivalent of what enters 

 into the crop, and so makes up for its drain upon the soil, 

 there will be no impoverishment. If, on the other hand, the 

 manure does not contain the equivalent of the ash-constitu- 

 ents of the crop, but has only stimulated it to take an extra 

 supply from the soil, the injury is obvious. 



In some cases, a soil that will produce 10 bushels of wheat 

 without manure will produce 25 bushels if dressed with 100 



