LECT. iv THE FOOD OF CROPS 63 



is not so, but on the contrary proper applications 

 of those essentials feed the land. Crops exhaust the 

 soil deprive it of its minerals only when nitrate 

 of soda or sulphate of ammonia are applied alone and 

 continuously. 



Taking nitrate of soda as our nitrogenous manure, 

 we add to it muriate of potash, steamed bone flour 

 and mineral superphosphate, or Thomas's phosphate 

 powder, to form a satisfactory manure. The propor- 

 tion differs somewhat according to the crops, but for 

 an acre of land we may safely take 1 J cwt. nitrate 

 of soda, 1^ cwt. mineral superphosphate, or Thomas's 

 phosphate, J cwt. of muriate of potash, -^ cwt. 

 steamed bone flour. This mixture may be given at 

 the rate of 2 ozs. to the square yard, 4 Ibs. to a rod 

 of land, or 5 to 6 cwt. per acre, but half as much 

 more may be given to poor soil. 



It may be said that muriate of potash contains 

 41*9 per cent, of potash, and is therefore richer in that 

 element than is kainit, which varies from 9 to 14 per 

 cent, of potash. Yet kainit is cheaper, and has often 

 answered equally well. Potash is not washed out 

 of land by rain, the soil holding it most tenaci- 

 ously, but nitrate of soda is much quicker in action 

 and fleeting, therefore best applied to growing crops 

 in spring. It is of benefit to all, though least of all 

 to peas, beans, and other pod bearing plants. 



Steamed bone flour is obtained from bones which 

 have been subjected to steam, at high pressure, for the 

 extraction of glue or gelatine. The residue contains 

 from 56 to 65 per cent, phosphates, and from 1 to 2 

 per cent ammonia. It is white, friable, and can be 

 crushed with the hand. It is ground to a fine flour 

 and is the active form of bone manure. 



Mineral superphosphate is ground coprolite 

 treated with sulphuric acid. Coprolite is antediluvian 



