PLANT FOOD AND MANURES 131 



These are sometimes finely ground and used as manure 

 without any further treatment, but, as certain changes 

 are necessary before this insoluble phosphate can be 

 used by plants, their action is slow. More frequently, 

 the phosphatic mineral, or the bones, is treated with 

 strong sulphuric acid, which renders the phosphate of 

 lime soluble. Thus prepared, the manure is known as 

 superphosphate. Superphosphate contains from 25 to 

 45 per cent, of phosphate of lime in a soluble condition. 

 Basic phosphate, Thomas' phosphate, or basic slag, 

 is a form of phosphate of lime obtained as a by-product 

 in the manufacture of steel. It is a heavy, brownish or 

 purplish-grey powder and should be as fine as flour. 

 Unlike superphosphate, which is acid, basic phosphate 

 is alkaline, hence, if mixed with sulphate of ammonia it 

 will liberate the ammonia. For this reason it must not, 

 when used as a manure, be put on with sulphate of 

 ammonia, but if these two substances are to be applied 

 to the same piece of ground the basic phosphate should 

 be put on first and worked into the soil, and, some days 

 later, the sulphate of ammonia should be added. 



Potassic Manures. 



Kainit. A mineral obtained from the Stassfurt 

 mines in Germany. It consists of sulphate of potash, 

 together with common salt and Epsom salts. The 

 actual amount of potash contained is usually about 

 12 per cent. 



Sulphate of potash. This is really a purer form of 

 kainit, containing about 50 per cent, of potash. 



