124 THE SOIL. 



the wheat-roots may appropriate these 8-6 kilogrammes, 

 the soil must contain a hundred times as much (860 

 kilogrammes) of phosphoric acid as the rye soil, or per- 

 haps 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 always, 

 and under all circumstances, contain a larger amount of 

 nutritive substances than the crop grown on it. Sup- 

 posing the soil to contain, instead of the hundred-fold, 

 only the seventy or fifty-fold quantity of the nutritive 

 elements in the crop, we infer from the law of the immo- 

 bility of these elements, that, to double the crop, we 

 must add to the field seventy or fifty times the quantity 

 of mmeral constituents contained in the produce. In 

 practice the case is different, for no actual field, like our 

 ideal one, contains phosphoric acid, potash, 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 fi'om the soil much more potash 

 than the cereals. 



Therefore, to convert a rj^e soil containing more than 

 3900 kilogrammes of potash, per hectare (2^ 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 rela- 

 tions existing between the composition of a soil and its 

 fertility. The main conclusion, which the above figures 

 are intended to illustrate, is the practical impossibility of 

 converting a rye soil into a wheat soil by supplying the 

 deficient ash constituents, or of making a wheat field by 



