228 THE SYSTEM OF FARM-YARD MANURING. 



sustains by the sale of cattle and of animal products 

 (wool, cheese, &c.), may be estimated at one-tenth of the 

 quantity of these mineral constituents contained in the 

 potatoes, turnips, or clover ; and even this estimate is, per- 

 haps, too high. At all events, it is risking no great error 

 to assume that nine-tenths of all the constituents of the 

 clover, potatoes, or turnips, are returned to the field in 

 the farm-yard manure ; whence the arable surface soil, 

 after manuring, is richer for the new rotation in the mi- 

 neral constituents of potatoes, clover, and turnips, than it 

 was before, as the constituents of the two latter plants 

 have been brought up from the deeper layers of the 

 ground. 



The far greater portion of the active dung-constituents 

 is retained by the upper layers of the soil, the deeper 

 layers getting back very little of what has been taken 

 from them ; the power of the latter, therefore, to produce 

 as large crops of clover or turnips as before is not 

 restored. 



The soil constituents which the animals have derived 

 from the turnips, clover, potatoes, &c., and which remahi 

 in their bodies, are very nearly identical, in quantity and 

 quality, with those of the cereals ; hence the loss sus- 

 tained by the land may be estimated as equal to the corn- 

 crops sold, plus the corn-constituents wdiich the fodder- 

 plants have given up to the animals on the farm. 



The restoration of the power of a field to produce a 

 crop of corn as large as the last naturally presupposes 

 that the conditions required for the production of the new 

 crop should remain the same in the very layer of the soil 

 which supplied the preceding crop ; in other words, the 

 substances nutritive to corn which were taken away must 

 be fully returned to the arable smface soil. 



If form-yard manure contained only the constituents 



