516 SOILS: PROPERTIES AND MANAGEMENT 



the phosphoric acid becomes less easily soluble, and to 

 that extent the value of the fertilizer is decreased. This 

 change, known as reversion, is much more likely to occur 

 in superphosphates made from rock than in those derived 

 from bone. It will also vary in different samples, a well- 

 made article usually undergoing little change even after 

 long standing. It is supposed to be caused by the presence 

 of undecomposed tricalcium phosphate and of iron and 

 aluminium sulfates. 



434. Relative availability of phosphate fertilizers. — 

 Superphosphates and double superphosphates contain 

 their phosphorus in a form in which it can be taken up 

 by the plant at once. They are therefore best applied 

 at the time when the crop is planted, or shortly before, 

 or they may be applied when the crop is growing. Crude 

 phosphates, on the other hand, become available only 

 through the natural processes in the soil. They should 

 be applied in quantity sufficient to meet the needs of the 

 crops for a number of years. 



Reverted phosphorus, although not soluble in water, 

 is readily soluble in dilute acids. It is now generally 

 believed that in this form an available supply of phos- 

 phorus is furnished to the plant. In a statement of fer- 

 tilizer analyses reverted phosphorus is termed citrate- 

 soluble, and this and the water-soluble are termed available. 



The degree of fineness to which the material is ground 

 makes a great difference in the availability of the less 

 soluble phosphate fertilizers, especially in the ground-rock 

 phosphates and in ground bone. This material should be 

 ground fine enough to pass through a sieve having meshes 

 at least one-fiftieth of an inch in diameter. 



435. Changes that occur when superphosphate is added 

 to soils. — When incorporated with soils superphosphate 



