process of growth. Such plants are the lupins and 

 vetches. Peas, clover, alfalfa and the native wild 

 lupins when grown as catch or cover crops and 

 ploughed under maintain the store of nitrogen in 

 soils. But, in this case, as with other organic sub- 

 stances, the two processes of decay and nitrification 

 are necessary before the nitrogen thus gathered be- 

 comes available. As nitrogen is the most expensive 

 of all fertilizing elements the importance and economy 

 of a green cover crop ploughed under is considerable. 



(See also "Humus.") 



SOURCES OF PHOSPHORIC ACID. 



Phosphoric acid, or phosphorous, in fertilizers, is 

 always found in combination with other elements. 

 Usually it is obtained from bone or phosphate rocks. 

 As rock it cannot become readily available without 

 treatment with sulphuric acid. As bone, unacidu- 

 lated, it must be very finely ground to be available, 

 and when thus ground is undoubtedly the best form 

 for citrus culture, as it is all equally available and its 

 ability to rot or ferment has not been destroyed by 

 the acid. 



ACIDULATED PHOSPHATES. 



These are made by treating bone or phosphate 

 rock with sulphuric acid. Their value may vary 

 according to the amount of acid used by the manu- 

 facturer. If 800 pounds of acid were used with 1200 



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