268 PLANT BIOCHEMISTRY 



reaction 1 above and free phosphoric acid reacts with rock phosphate 

 as in reaction 2. The mixture contains very little gypsum but may 

 have a considerable quantity of free phosphoric acid. 



We have already mentioned the fact that monammonium phosphate 

 is produced when superphosphate is treated with anhydrous ammonia 

 or ammonia solutions. The mixture of products obtained by this 

 reaction is marketed as ammoniated superphosphate and contains from 

 16 to 18 per cent PoO-,. 



Basic slag, a by-product of the steel industry, is a relatively insoluble 

 form of phosphatic fertilizer. But its phosphorus is far more available 

 than that of rock phosphate. It contains from 8 to 25 per cent PoO.r;, as 

 much as 50 per cent CaO, and varying amounts of Fe^O;., SiO^, and 

 other compounds. In ternrs of its principal constituents its composi- 

 tion is sometimes given as P20--4CaO. 



In the soil, basic slag is made available by the following reaction: 



P2O5 • 4CaO + 6CO2 + 5H,0 -^ CaH4(P04).. + 3Ca(HC03)2 



basic monocalciuiu calcium 



slag phosphate bicarbonate 



Bones are used as the parent material for several different products 

 sold as phosphatic fertilizers. Raw bone meal, steamed bone meal, and 

 precipitated bone are such products. The form of phosphorus present 

 in bones is thought to be Ca3(P04)2. Dicalcium phosphate results 

 when precipitated bone is made as a by-product of glue manirfacture. 

 Both raw and steamed bone meal contain nitrogen as well as phos- 

 phorus, but the cost of these elements per unit is higher when pur- 

 chased in bone meal than when bought in other forms. 



The term available phosphorus, or available phosphoric acid, as 

 used in the fertilizer industry has a definite meaning. Available phos- 

 phorus is the sum of the water-soluble phosphorus and the ammonium 

 citrate-soluble phosphorus. The value of available PoO^ obtained in 

 this manner approximates the amount of phosphorus in a fertilizer 

 which will be usable by plants. 



Potash Fertilizers 



Deposits of potassium salts are found in various parts of the world, 

 but only those in France, Germany, and the United States are mined 

 extensively. Sources of potash in the United States include the natural 

 brines of inland lakes in Nebraska and California and deposits of 

 sylvite (KCl) and kainite (MgS04-KCl-.'}H.O) in New Mexico. 

 Greensand, found in the eastern states, particularly New Jersey, is 

 present in quantities sufficient to supply the potassium needs of the 

 United States for hundreds of years. Many of these deposits are near 



