EFFECT OF INCREASING ONE MINERAL CONSTITUENT. 133 



the other mineral constituents. The additional supply 

 of phosphoric acid serves in this case to increase the 

 amount of potash, lime, and silicic acid in the produce ; 

 but if this additional supply exceeds one-tenth of the 

 phosphoric acid present in the soil, the quantity in ex- 

 cess remains ineffective. Up to this limit, every pound 

 nay, every ounce of phosphoric acid supplied has, 

 in this case, a fully determinate action. 



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 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, dur- 

 ing 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 sub- 

 sequent 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 culti- 

 vated on a given field only at certain intervals, and ex- 

 perience shows that skilful, industrious tillage is usually 

 more effective than manure in shortening these inter- 

 vals, we may infer that in such cases the nutritive sub- 

 stances were not deficient in total quantity in the whole 

 field, but in proper proportion in all parts of the field. 



