THE MANURING OF THE CANE 95 



Gypsum. This material is sulphate of lime, and, in a sense, can not be 

 regarded as a manure ; it acts indirectly as a source of potash, which it 

 sets free in soils ; it is also used as a corrective of soil alkalinity. 



Bone manures contain from 4 per cent, to 6 per cent, of nitrogen, and from 

 40 per cent, to 50 per cent, of phosphate of lime ; this form of manure is 

 sold as half-inch, quarter-inch, or as bone meal or dust, and is frequently 

 steamed to remove the fats. The nitrogen is of little availability, and the 

 phosphates, unless the bones are finely ground, are but slowly assimilated. 



Mineral phosphates contain from 25 per cent, to 35 per cent, of phosphoric 

 acid, and are occasionally used without previous treatment intended to 

 render the phosphoric acid soluble. 



Superphosphates usually contain about 20 per cent, soluble phosphoric 

 acid, and in the form known to the trade as " double superphosphate " 

 up to 40 to 50 per cent. They are prepared from mineral phosphates by 

 the action of sulphuric acid. 



Basic slag is the material obtained as a waste product in the " basic " 

 process of steel manufacture ; it usually contains from 15 to 20 per cent, 

 phosphoric acid, and from 40 to 50 per cent, of lime, a portion of which 

 exists as free lime. 



Reverted phosphate is the name given to a form of phosphate insoluble in 

 water but soluble in ammonium citrate solution, and which is valued at the 

 same figure as water-soluble phosphate. Superphosphates have a tendency 

 on storage to pass into reverted phosphate, and this form is also manufactured 

 and sold as precipitated phosphate, containing from 35 to. 40 per cent, 

 phosphoric acid soluble in ammonium citrate. 



Potash. Potash is applied in cane-growing countries as pure sulphate 

 containing about 48 per cent, potash. The chloride is occasionally used, 

 and kainit and other crude salts appear occasionally in mixed manures. 



Green Manuring. Green soiling or green manuring is a practice which- 

 has been carried on for generations past. In Europe the method employed 

 is to sow a catch crop of some quickly growing plant between the harvest 

 of the one and^ the seed time of the succeeding crop ; the catch crop is 

 ploughed into the soil and acts as a green manure to the following crop. 

 The principles of this practice are as follows. It had been known for a large 

 number of years that leguminous crops (beans, peas, clover, etc.), although 

 they contained large amounts of nitrogen, did not respond to nitrogenous 

 manurings, and even frequently gave a smaller crop when manured with 

 nitrogen than when unmanured. It was eventually established by Hellriegel 

 and Wilfarth in Germany, about 1886, that leguminous plants are able to 

 absorb nitrogen from the air. The absorption is not made directly by the* 

 plant, but by the agency of bacteria. If the roots of a leguminous plant be 

 examined, there will be found attached to its rootlets a number of wart-like 

 excrescences the size of a pin's head and upwards. These bodies, which are 

 termed nodules, on being crushed and examined under the microscope, 

 are found to consist of countless numbers of bacteria ; these bacteria, 

 living in symbiosis or commensalism with the host plant, supply it with, 

 at any rate, a part of its nitrogen. 



If then leguminous plants be sown and allowed to reach maturity, and 



