128 THE SOIL. 



as much (860 kilogrammes) of phosphoric acid as the 

 rye soil, or perhaps even more. 



Although these figures refer to an ideal soil of 

 strictly definite composition, yet the conclusion which 

 we draw is true for all classes of soil. 



It is an undoubted fact, that the ground must al- 

 ways, and under all circumstances, contain a larger 

 amount of nutritive substances than the crop grown on 

 it. Supposing the soil to contain, instead of the hun- 

 dred-fold, only the seventy or fifty -fold quantity of the 

 nutritive elements in the crop, we infer from the law 

 of the immobility of these elements, that, to double the 

 crop, we must add to the field seventy or fifty times 

 the quantity of mineral constituents contained in the 

 produce. In practice the case is different, for no actual 

 field, like our ideal one, contains phosphoric acid, pot- 

 ash, and silicic acid in exactly the relative proportions 

 in which they exist in the ash of rye or wheat. Most 

 fields which are suitable for cereals are fruitful also for 

 potatoes, clover, or turnips, which extract from the soil 

 much more potash than the cereals. 



Therefore to convert a rye soil containing more than 

 3900 kilogrammes of potash, per hectare (2-J- acres), into 

 a wheat soil, would not require an addition of 1300 

 kilogrammes of potash, but a proportionately less 

 amount would fully answer the purpose. 



"We shall hereafter discuss at greater length the re- 

 lations existing between the composition of a soil and 

 its fertility. The main conclusion, which the above fig- 

 ures are intended to illustrate, is the practical impossi- 

 bility of converting a rye soil into a wheat soil by supply- 

 ing the deficient ash constituents, or of making a wheat 

 field by the same means produce half as much again as 

 an average crop. Admitting this might be readily ac- 

 complished, .experimentally, on a small area, yet the 

 price of phosphoric acid, potash, or even of soluble silica, 

 and the impossibility of procuring them for a large num- 

 ber of fields, though in a given field only one of these 

 substances had to be increased in the proportion stated, 



