178 FAEM-YAED MANUEE. 



from tlie subsoil, to whicli the roots of the cereals very 

 seldom penetrate. When the fields have a subsoil favour- 

 able 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 constituents had been drawn 

 by them from the siu-face. 



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 w^hole of 

 the turnip and lucerne crops were ploughed in during 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. 



As, however, turnips and lucerne require for their de- 

 velopement a very great quantity of mineral constituents, 

 the subsoil is so much the sooner exhausted, when it con- 

 tains fewer of such constituents. Now as it is not actually 

 severed from the arable surface, but hes imderneath, it 

 can scarcely regain any of all the constituents which it 

 has lost, because the surface soil intercepts and retains the 

 portion supphed. Only that part of the potash, ammonia, 

 phosphoric acid, and silicic acid, which is not taken up 

 and fixed by the surface soil, can reach the subsoil. 



It is therefore possible, by the cultivation of these deep- 

 rooting plants, to gain an abundant supply of nutritive 

 substances for all plants drawing their nutriment chiefly 

 from the arable soil ; but this supply is not lasting, 

 and in a comparatively short time many fields will cease 



