458 NATURE AND PROPERTIES OF SOILS 



Basic slag comes on the market as a heavy dark gray pow- 

 der, extremely alkaline to litmus, and contains from 14 to 

 20 per cent, of P2O5. The phosphorus of basic slag is almost 

 all soluble in citric acid and, therefore, is rated as available 

 phosphoric acid. It does not revert in the soil as does acid 

 phosphate, but is immediately attacked by carbon dioxide and 

 rendered rather quickly available. A possible reaction is as 

 below : 



(CaO)5.PA-Si02 + 8CO2 + 6H2O = CaH^CPOJs + 



4CaHo(C03)2 + Si02 



258. Relative availability of phosphate fertilizers. — 



Acid phosphate carries most of its phosphoric acid in a water- 

 soluble form and although the phosphates revert to the tri- 

 calcium form immediately when added to the soil, they are 

 rather readily available to plants. This is due to the charac- 

 ter of the freshly precipitated salt and the surface exposed 

 for solution activities. To insure a good distribution in the 

 soil of the phosphoric acid and a rapid influence on crops, acid 

 phosphate should be well mixed with the soil. 



Basic slag, since its phosphoric acid is largely citrate sol- 

 uble, is generally considered as next to acid phosphate in 

 availability. Steamed bone-meal usually gives better results 

 than raw rock phosphate and rates third, with rock phos- 

 phate fourth in availability. The degree of fineness makes a 

 great difference in the availability of the less soluble phos- 

 phate fertilizers, especially the ground bone and raw rock 

 phosphate. The latter material should be ground fine enough 

 to pass through a sieve having at least one hundred meshes to 

 the inch. 



259. Eaw rock phosphate versus acid phosphate. — Con- 

 siderable discussion as well as controversy has of late arisen 

 regarding the relative merits of acid phosphate and raw rock 

 phosphate not only when applied on the basis of equal amounts 

 of phosphoric acid but also when compared on the basis of 



