I go SYSTEMS OF PERMANENT AGRICULTURE 



phosphate, acid dicalcium phosphate, or the neutral tricalcium 

 phosphate, respectively, carbonic acid being liberated, which 

 promptly decomposes into water and the gas, carbon dioxid. 



Tetracalcium phosphate is thought by some to be the compound 

 in which phosphorus exists in basic slag phosphate, being essen- 

 tially tricalcium phosphate loosely united with the CaO group 

 (see under basic slag phosphate). 



In the manufacture of commercial acid phosphate, the phosphorus 

 material most commonly used in mixed commercial fertilizers, one 

 ton of ground raw rock phosphate is treated with about one ton of 

 sulfuric acid, and the resulting material consists chiefly of mono- 

 calcium phosphate and calcium sulfate (land-plaster), together 

 with all of the impurities contained in the original materials, and 

 this mixture is the ordinary acid-phosphate fertilizer: 



Ca 3 (PO 4 ) 2 + 2 H 2 SO 4 = CaH 4 (PO 4 ) 2 + 2 CaSO 4 . 



This equation shows only the general reaction between the 

 chemical compounds, tricalcium phosphate and sulfuric acid, but 

 impurities are always present, and both the impurities and the 

 calcium sulfate are included in acid phosphate, in which the phos- 

 phorus is held chiefly in the water-soluble compound, monocalcium 

 phosphate. The reaction may be expressed in two equations, the 

 two molecules of sulfuric acid being added separately, thus showing 

 dicalcium phosphate as an intermediate product. Small amounts 

 of both dicalcium phosphate and tricalcium phosphate usually 

 remain in acid phosphate, and a considerable part of the sulfuric 

 acid used reacts with impurities which consist chiefly of silicates 

 of the abundant metals, aluminum, iron, calcium, magnesium, 

 potassium, and sodium. Sometimes calcium carbonate is among 

 the impurities. As a rule, about one fourth of acid phosphate 

 consists of phosphates (chiefly monocalcium phosphate), while 

 three fourths consist of calcium sulfate and impurities. 



The readily available phosphorus in acid phosphate has a market 

 value of about 12 cents a pound. This includes the phosphorus 

 soluble in water and also that dissolved by ammonium citrate 

 solution, which is sometimes called the " citrate-soluble " or the 

 " reverted." The term reverted is properly applied to dicalcium 



