PLANT NUTRITION 267 



of cyanamide as a fertilizer, for the compound as such is toxic to 

 plants. Plant toxicity is avoided if cyanamide is uniformly distributed 

 and well mixed with the upper layer of soil and is applied at rates of 

 200 lb. per acre or less at least 10 days before planting. 



Urea and calcium cyanamide are called "non-proteid organic fertiliz- 

 ers" by fertilizer manufacturers to distinguish these products from 

 fertilizers of animal and vegetable origin. 



Phosphate Fertilizers 



The importance of phosjDhates is indicated by the fact, stated in a 

 previous chapter, that low crop production is due more often to a lack 

 of phosphorus than to a lack of any other element. All phosphatic 

 fertilizers contain phosphorus as salts of o-phosphoric acid. 



Rock phosphate. Rock phosphate, consisting largely of Cap, (P04)2, 

 occurs in large deposits throughout the world. In the United States 

 deposits of commercial importance occur in Florida, Tennessee, Idaho, 

 Montana, Utah, and Wyoming. A limited amount of finely ground 

 rock phosphate is used for direct application as a fertilizer. However, 

 it is not a readily available source of phosphorus because the solubility 

 of tricalcium phosphate is very low. Rock phosphate is not recom- 

 mended when monocalcium phosphate, ammonium phosphate, or 

 other more soluble forms can be obtained. The chief use of rock 

 phosphate is in the preparation of other forms of phosphatic fertilizers. 



Monocalcium phosphate. Monocalcium phosphate is one of the 

 products formed when rock phosphate is treated with sulfuric acid. 



1. Ca3(P04)o + 3H0SO4 + 6H2O -^ 2H3PO4 + 3CaS04 • 2H.2O 



tricalcium phosphoric calcium sulfate 



phosphate acid (gypsum) 



2. Ca3(P04)2 + 4H3PO4 -^ 3CaH4(P04)2 



tricalcium monocalcium 



phosphate phosphate 



A small amount of dicalcium phosphate, Ca2Ho(P04)2, is also 

 formed during the reaction. The mixture of products formed from 

 the above reactions contains about 50 per cent gypsum and 26 per cent 

 monocalcium phosphate. This mixture is sold as the fertilizer called 

 superphosphate or acid phosphate and contains 16 to 20 per cent avail- 

 able P^O-,. An application of monocalcium phosphate may result in 

 a temporary increase of soil acidity. 



Concentrated superphosphate, called double or treble superphos- 

 phate, containing from 40 to 48 per cent available P0O5, is also manu- 

 factured. This product results when the calcium sulfate is removed in 



