70 THE CHEMISTRY OF THE FARM 



When 14 tons of farmyard manure are annually applied 

 to wheat or barley at Kothamsted, about thirteen years 

 elapse before the maximum produce is reached, as the 

 residues up to this point are increasing in amount. 



The residues of phosphatic and potassic manures are 

 available for subsequent crops, but are distinctly less 

 effective than fresh applications of the same manures. 

 Dyer found that where 3^ cwts. of superphosphate 

 had been annually applied for forty to fifty years on the 

 heavy loam at Eothamsted, nearly the whole of the 

 unused phosphoric acid remained in the surface nine 

 inches, and that from one-third to one-half of this was 

 still soluble in weak citric acid. Where 200 lbs. of 

 sulphate of potassium had been annually applied for 

 forty to fifty years, only 40 — 65 per cent, of the unused 

 potash apparently remained in the surface nine inches ; 

 a considerable part of the remainder was found in the 

 subsoil, and a small part had escaped in the drainage 

 water. In twenty-seven inches of soil about one- 

 quarter of the unused potash was found soluble in 

 a 1 per cent, solution of citric acid. 



Where farmyard manure is continuously employed, 

 a somewhat larger proportion of the phosphoric acid 

 and potash which it contains passes into the subsoil, 

 than where superphosphate and potassium salts have 

 been applied. The distribution of the constituents in 

 question is apparently aided by the organic matter of 

 the manure. 



