EXHAUSTION OF A WHEAT SOIL. 175 



dii'ection of the vegetative force only through the soil, i.e. 

 by supplying his field with nutritive substances, in the right 

 proportions. For the production of the largest crop of 

 grain, the soil must contain a preponderating quantity of 

 the nutritive substances necessary for the formation of 

 seed. For leafy plants, turnips, and tuberous plants, the 

 proportion is reversed. 



It is therefore evident, that if on our field containing 

 25,000 kilogrammes of the ash-constituents of the wheat- 

 plant, we cultivate potatoes and clover, and take away 

 from the field the entire crop of tubers and clover, we 

 remove from the ground, in these two products, as much 

 phosphoric acid and three times as much potash as in 

 three wlieat crops. It is certain that the abstraction of 

 these important mineral constituents from the ground, by 

 the cultivation of another plant, must greatly affect the 

 fertihty of the soil for wheat ; the crops of wheat diminish 

 in amount and in number. 



But if, instead of this, we were to cultivate on our 

 field alternately, wheat one year, potatoes the next, 

 leaving the entire potato crop, tubers included, and the 

 wheat straw on the ground to be ploughed in, and if this 

 alternation of crops were continued for sixty years, the 

 crop of corn which the field was originally capable of 

 yielding woidd not in the slightest degree be altered or 

 increased. The field would gain nothing by the culti- 

 vation of potatoes ; and would lose nothing, because the 

 whole crop was left in the soil. Wlien by taking corn 

 crops from the field, the store of mineral constituents had 

 been reduced to- three-fourths of the original quantity, the 

 field would cease to fui'nish remunerative crops, supposing 

 that three-fourths of an average harvest leave no margni 

 of profit for tlie farmer. The same results would f()llo^v, 

 if instead of potatoes we interpose clover, and constantly 



