CONVERSION OF EYE INTO WHEAT SOIL. 



127 



much nutriment we must add to convert it into a per- 

 manently productive wheat soil, the answer would be 

 not hypothetical, but perfectly trustworthy and exact. 

 If 



Hence, to a rye soil of a given condition and productive 

 power, we should have to add, in some form or other, 

 one-half more phosphoric and silicic acid, and one-third 

 more potash, than it already contains, to make it capa- 

 ble of producing average crops of wheat grain and 

 straw. 



And to obtain permanently from a wheat soil a 

 crop half as large again as an average harvest, we 

 should add one-half more of nutritive substances than 

 it already contains. 



These speculations have no other object than to 

 show that a small difference in the absolute quantity of 

 a nutritive element, required by one kind of plant more 

 than by another, presupposes a great excess in the 

 amount of this constituent in the soil. A wheat crop 

 takes from the soil, per hectare (2-J acres), only 8*6 kil- 

 ogrammes (19 Ibs.) more phosphoric acid than a rye 

 crop ; but that the wheat-roots may appropriate these 

 8*6 kilogrammes, the soil must contain a hundred times 



