176 FARM- YARD MANURE. 



As, however, turnips and lucerne require for their 

 dev elopement a very great quantity of mineral constitu- 

 ents, the subsoil is so much the sooner exhausted, when 

 it contains fewer of such constituents. Now as it is 

 not actually severed from the arable surface, but lies 

 underneath, it can scarcely regain any of all the con- 

 stituents which it has lost, because the surface soil in- 

 tercepts and retains the portion supplied. Only that 

 part of the potash, ammonia, phosphoric acid, and 

 silicic acid, which is not taken up and fixed by the sur- 

 face soil, can reach the subsoil. 



It is therefore possible, by the cultivation of these 

 deep-rooting plants, to gain an abundant supply of nu- 

 tritive substances for all plants drawing their nutriment 

 chiefly from the arable soil ; but this supply is not last- 

 ing, and in a comparatively short time many fields will 

 cease to bear crops, because the subsoil is exhausted, 

 and its fertility is not easily restored. 



If a farmer grows upon three fields, potatoes, corn, 

 and vetches or clover, alternately, or if he cultivates 

 one field with potatoes, corn, and vetches successively, 

 selling the crops, and going on in the same way for 

 many years, without manuring, any one can foresee 

 the end of such husbandry, because such a system can- 

 not possibly last. No matter what plants may be 

 selected, what variety of cereals, tuberous or other 

 plants, or in what rotation, the field will at length be 

 reduced to such a state that the cereals will yield no 

 more than the seed sown, the potatoes will give no 

 tubers, and the vetches or clover will die away after 

 barely appearing above ground. 



From these facts it follows indisputably, that there 

 is no plant which spares the ground, and none which 

 enriches it. The practical farmer is taught by innu- 

 merable instances that the success of a second crop de- 

 pends upon the previous one, and that it is by no 

 means a matter of indifference, in what order he culti- 

 vates his plants ; by previously cultivating some plant 

 with extensive ramification of roots, the soil is made 

 fitter for the growth of a succeeding cereal, which will 



