115 



" If potash or lime alone is wanted to restore the 

 right proportion among the nutritive substances in the 

 soil, a supply of ashes or lime will increase the produce of 

 all the crops the additional supply of lime 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 

 substances in excess, but was deficient in silicic 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, migrated 

 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 shortening these intervals 

 we may infer that in 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." 



It is the minimum quantity of what inay be a 

 single constituent which regulates the outturn of a 

 field and determines its fertility ; and it is by supply- 

 ing this minimum constituent it may be potash or 



