EXHAUSTION OF A WHEAT SOIL. 173 



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 contain- 

 ing 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 clo- 

 ver, we remove from the ground, in these two products, 

 as much phosphoric acid and three times as much pot- 

 ash as in three wheat crops. ' It is certain that the ab- 

 straction of these important mineral constituents from 

 the ground, by the cultivation of another plant, must 

 greatly affect the fertility 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 would not in the slightest degree be altered 

 or increased. The field would gain nothing by the 

 cultivation of potatoes ; and would lose nothing, be- 

 cause the whole crop was left in the soil. When 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 furnish re- 

 munerative crops, supposing that three-fourths of an 

 average har7est leave no margin of profit for the far- 

 mer. The same results would follow, if instead of po- 

 tatoes we interpose clover, and constantly ploughed it 

 in. We have assumed the field to be in the best phys- 

 ical condition, which therefore could not be improved 

 by the incorporation of the organic substances of the 

 clover and the potatoes. Even if we were to take the 

 potatoes from the field, to mow down and dry the clo- 

 ver, giving both to cattle in the farm-yard or making 

 any other use of them, and then to bring all back to 

 the field and plough them in, so as to restore to the 



