43 



47. A series of analyses of the guanosupplied from the 

 Government stores in the years 1891 to 1894 gives per- 

 centages of ammonia varying from 7 to 19 per cent., the 

 average of fifteen samples being 13*53. Besides this 

 ammonia, guano contains phosphate of lime and potash to 

 an average amount of 16 per cent. It must be remem- 

 bered that this phosphate of lime is only very slowly 

 soluble, being in the tribasic condition. 



48. It is a pity that hitherto no mineral phosphates similar 

 to apatite have been found within the limits of the Colony. 

 There are two sources whence phosphates can be con- 

 veniently obtained. These are, bones in the form of bone- 

 meal or reduced to powder by fermentation, and a 

 certain by-product of the iron manufacture, known in 

 commerce as Thomas' slag. Although bones contain 

 precisely that substance which is required in our 

 soils more than any other, they are as a manure, ex- 

 tremely slow in their action, becoming available as 

 plant-food only after being mixed with the soil for a very 

 considerable time. Nor do all soils act on bones with equal 

 rapidity. In well aerated open soils with a good deal of 

 organic matter present, they decompose soonest ; in clay 

 soils they will lie for years without change. Everything 

 depends upon the degree of fineness to which the bones 

 are comminuted. Hence bone-flour acts most quickly, 

 bone-meal next, while the ordinary crushed bone, consisting 

 of large pieces, acts very slowly. This is because the 

 phosphoric acid in the bone has combined with the lime 

 with great force, forming what is known as insoluble or 

 tribasic phosphate. If it were not that the water which 

 percolates through the soil is impregnated with carbonic 

 acid, the bone-phosphate would remain unaltered indefinitely, 

 but the slight percentage of carbonic acid in the water 

 carries away one molecule of lime and leaves the phosphate 

 in a new and more soluble form, known as dibasic or soluble 

 phosphate. What is slowly done by the carbonic acid in 

 open soils, may be done rapidly by mixing the bones, 

 whether crushed or meal, with sulphuric acid. This 

 powerful substance, when added to the extent of about one 

 third of the weight of the bones, takes up the lime at once 

 ami leaves the valuable phosphate in a form in which it is 



