130 THE SOIL. 



If potash or lime alone is wanted to restore the right 

 proportion among the nutritive substances in the soil, a 

 supply of ash or lime will increase the produce of all the 

 crops — the additional supply of hme effecting, in this 

 case, an increase in the amount of phosphoric acid and 

 potash in the augmented produce. 



If we find that a soil will not bear a remunerative, 

 crop of cereal plants, though it remains fruitful for other 

 plants, such as potatoes, clover, or turnips, which require 

 just as much phosphoric acid, potash, and lime, as the 

 cereals, we may assume that the soil had the latter sub- 

 stances in excess, but was deficient in sihcic acid. 

 And if, in the course of two or three years, during 

 which other produce is cultivated on it, the land recovers 

 its fertility for cereals, this must be because it contained, 

 though unequally divided and distributed, an excess of 

 silicic acid also, which, during the fallow season, mi- 

 grated from the places where it was in excess to those 

 where it was deficient ; so that when the subsequent 

 period of cultivation began, there was in all these places 

 the right proportion of all the nutritive substances needed 

 by cereal plants. 



For similar reasons, if peas or beans can be cultivated 

 on a given field only at certain intervals, and experience 

 shows that skilful, industrious tillage is usually more 

 effective than manure in shortemng these intervals, we 

 may infer that m such cases the nutritive substances were 

 not deficient in total quantity in the whole field, but in 

 proper proportion in all parts of the field. 



