HOW TO MAINTAIN FERTILITY 



cured in several different forms, such as raw, 

 boiled or steamed, and fine bone. Upon its con- 

 dition when applied to the land will depend the 

 rapidity with which it will become available to the 

 plants. All the phosphoric acid in raw, broken 

 bone will not become available and used up by 

 the plants in less than four years, because the fat 

 retards decay. For this reason the bone is steamed 

 and ground, and in this condition all the phos- 

 phoric acid will have become available in about 

 two years. In steamed bone there is less nitrogen, 

 however, because some of the organic material con- 

 taining it is removed by the process. 



Other forms of bone which are sometimes used 

 as fertilisers are bone-black and bone-ash. They are 

 much less valuable, because in their preparation by 

 burning all the organic matter is driven off, there- 

 fore they contain no nitrogen, and it lessens the 

 amount of phosphoric acid somewhat. 



Tankage is the only other material containing 

 phosphoric acid which is sufficiently common in 

 the trade to warrant consideration. The fertilising 

 content of tankage varies according to what it 

 is made from; the higher its percentage of phos- 

 phoric acid the less nitrogen it contains and vice 



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