154 CHEMICAL FERTILIZERS 



into pyro-phosphoric acid; some will have gone as far as 

 meta-phosphoric acid. Unless such an overheated super- 

 phosphate so obtained is boiled at some period of its treat- 

 ment with acid during analysis in the laboratory, the analysis 

 will be misleading, since pyro- and meta-phosphates do not 

 give the same precipitates as ortho-phosphates. Meta- and 

 pyro-phosphates are not immediately absorbed by plants, 

 but they revert to ortho-phosphoric acid in the soil at a 

 sufficient rate to become fully utilized by crops. 



Retrogression. Superphosphate keeps very well from 

 one season to another, provided the phosphate from which 

 it was made does not contain more than 2 % of oxide of iron 

 and alumina. Retrogression is the result of many causes, 

 some chemical and some physical. The heat and pressure 

 engendered in the heap of superphosphate are important in 

 increasing the rate at which retrogression takes place. The 

 granules, of which superphosphate consists, may adhere on 

 contact, when pressure is applied, unless the granules have been 

 dusted as described on p. 150. As the depth of the layer, 

 in which superphosphate is stored, increases, the substance 

 becomes compressed by its own weight, so that finally the 

 particles may be compacted together. These changes are 

 more rapid in moist, hot superphosphate than in superphos- 

 phate which has been dried and cooled. 



The crystallization of calcium sulphate is by no means 

 complete when the den is emptied hastily, and it continues 

 in the heap. Mono-calcium phosphate also reacts with the 

 sulphate of iron and alumina in the heap, producing retro- 

 gression and bad physical conditions. Pressure in the heap 

 is therefore one of the evils to be avoided as far as possible. 

 It is consequently not desirable to pile the superphosphate 

 in large heaps until it has been made many hours. The 

 method of drying superphosphate by spreading in a thin 

 layer on a hot plate has the advantage that it entirely 

 prevents pressure taking place before the superphosphate 

 has reached a stage when it is less likely to revert. The 

 degree to which superphosphate may retrograde may be 

 discovered by artificial pressure. A pressure of about 4 



