82 EFFECTS OF CROPPLNG, 



The v,i :ne thing would result from exclusive cul- 

 tivation of any of the other grains. Some soils bear 

 this severe treatment longer than others, hut there are 

 very few that would not eventually become exhausted. 

 If turnips or potatoes alone were grown, the loss would 

 be of another description, but equally injurious. In 

 this case, instead of phosphoric acid, it is potash and 

 soda that are exhausted, and no amount of phosphoric 

 acid would make good the deficiency. In the case of 

 trees, the demand would more probably be for lime. 



The general rule may from all of these facts be 

 considered as established, that cropping tends directly 

 to impoverish the soil. We see by Table I. that silica, 

 alumina, iron, and organic matter, in the soils there 

 given, amount to at least 90 lbs. of every 100. In 

 many soils they come up to at least 95 lbs. There is 

 no fear, then, of exhausting the silica; alumina, as has 

 been said, does not enter into the composition of plants, 

 and iron is not usually a prominent constituent. The 

 leading parts of the ash from the grain, the roots, and 

 all of those portions of plants most valuable for food, 

 are found not in the 90 to 95 lbs. made up by these 

 abundant substances, but in the 5 or 10 lbs. necessary 

 to make out the hundred. The quantities of these 

 important substances contained in most soils are there- 

 fore small; and hence as they are the very ones most 

 largely carried away, some one of them is usually first 

 exhausted, according to the class of crops that have 

 been chiefly cultivated, as indicated in the preceding 

 chapter. 



When one is gone, or reduced to a very small quan- 

 tity, the crops which particularly require that substance 

 will refuse to grow luxuriantly and to yield well : 

 suppose it to be wheat, and the wanting substance 

 phosphoric acid; there may be the greatest abundance 

 of every other necessary constituent, and yet all of 

 their good effects are more than neutralized by this 



