220 THE SYSTEM OF FABM-YAKD MANURING. 



constituents is retained, which is not returned to the 

 soil in the farm-yard manure. The loss of phosphoric 

 acid and potash which the soil sustains by the sale of 

 cattle and of animal products (wool, cheese, &c.), may 

 be estimated at one-tenth of the quantity of these min- 

 eral constituents contained in the potatoes, turnips, or 

 clover ; and even this estimate is, perhaps, 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 

 mineral 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-constitu- 

 ents 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 re- 

 main in their bodies, are very nearly identical, in 

 quantity and quality, with those of the cereals ; hence 

 the loss sustained by the land may be estimated as 

 equal to the corn-crops sold, plus the corn-constituents 

 which 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 surface 

 soil. 



If farm-yard manure contained only the constituents 

 of straw and potatoes, and nothing else, manuring a 



