GRADUAL EXHAUSTION OF A WHEAT SOIL. 175 



This wheat field will now be able to produce three 

 full corn crops in three successive years, because the 

 conditions for the formation of straw have remained 

 unaltered, while those for the production of grain have 

 been increased three-fold. If the farmer by* this method 

 raises as much corn in three years as he could obtain 

 from the same fields in five years without the addition 

 and cooperation of the constituents contained in the clo- 

 ver and the potatoes, it is clear that his profit has been 

 greater, since with three seed-corns he has obtained as 

 good a harvest as in the other case with five. But 

 what the wheat field has gained in fertility, the other 

 two fields have lost ; and the final result is, that at less 

 cost of cultivation, and with more profit than before, 

 his three fields are brought to the period of exhaustion 

 which inevitably results from the continued removal of 

 the mineral constituents in the crops of corn. 



The last case which we have to consider is when 

 the farmer, instead of growing potatoes and clover, cul- 

 tivates turnips and lucerne, which by their long pene- 

 trating roots extract a great quantity of mineral con- 

 stituents from the subsoil, to which the roots of the 

 cereals very seldom penetrate. "When the fields have 

 a subsoil favourable to the growth of these plants, it is 

 as though the arable surface soil were doubled. If the 

 roots of these plants receive the half of their mineral 

 nutriment from the subsoil, and the other half from the 

 arable surface soil, the latter will lose by these crops 

 only half as much as they would, if all the mineral con- 

 stituents had been drawn by them from the surface. 



Thus the subsoil, considered as a field apart from 

 the arable soil, gives to turnips and lucerne a certain 

 quantity of mineral constituents. Now, if the whole 

 of the turnip and lucerne crops were ploughed in dur- 

 ing the autumn in a wheat field which had yielded an 

 average crop of wheat, so that the field should receive 

 back more than it had lost in the corn, it is clear that 

 this field might be maintained in an equable state of 

 fertility, at the expense of the subsoil, just so long as 

 the latter remained productive for turnips and lucerne. 



