108 CHEMISTRY OF FARM PRACTICE 



The inorganic phosphorus present in the soil is generally 

 in the form of the normal calcium phosphate combined 

 with fluorine and chlorine (CasCPO^CaFCl), although 

 some of it is found as the phosphates of iron or aluminium. 

 Aluminium phosphate is a normal constituent of rock phos 

 phate, but phosphates containing iron, even in very small 

 amounts, are rejected, as the iron has the power in the soil, 

 even after the treatment with sulphuric acid, to take away 

 phosphoric acid from acid phosphates and render them 

 insoluble. A small amount of the total phosphorus of the 

 soil is found combined in organic compounds and is liberated 

 by the decay of organic matter. 



Experimental results have indicated that 1 per cent 

 of the total phosphorus present in the soil is available 

 during the course of a year. Obviously this availability 

 will depend upon several factors, the form of combination 

 of the phosphorus, the amount of soil moisture, the content 

 of decaying organic matter, and the influence of added 

 fertilizers on the solubility of the material present. It 

 must be kept in mind that, in explanation of the low phos- 

 phorus content of normal soils, a large per cent of the 

 phosphorus used is stored in the seed of the plant, which 

 is generally the product sold off the farm. Assuming 

 T^O of one per cent of phosphorus and 2,000,000 pounds of 

 soil in the surface area per acre, a total of only 1000 pounds 

 of phosphorus will be present, of which perhaps 1 per 

 cent will become available, furnishing 10 pounds of phos- 

 phorus combined in soluble compounds. Ten pounds of 

 soluble phosphorus, providing that none leached out, would 

 furnish per acre phosphorus sufficient to make 43 bushels 

 of corn, or 62 bushels of oats, or 31 bushels of wheat, or 

 cotton sufficient to amount to 375 pounds of lint. As 

 a matter of fact, many soils do not contain as much as 

 Tihr of 1 per cent of phosphorus, and some of the available 

 phosphorus is lost through leaching. 



81. Commercial Sources. The farmer has at his dis- 



