86 Elementary Principles of Agriculture 



while only one-fourth remains in the straw. Phos- 

 phorus compounds are widely distributed, though, 

 usually, in insoluble compounds. Phosphorus is found 

 in the soils combined with lime, magnesia, iron and 

 alumina. For fertilizing purposes phosphates are ob- 

 tained from bones, and rocks formed by the deposit of 

 similar remains. In bones it exists as the insoluble 

 lime phosphate. To overcome this, the rock or bone 

 phosphates are treated with sulphuric acid which con- 

 verts the insoluble into soluble compounds. When ap- 

 plied to the soil it soon returns to the insoluble salt, 

 dicalcium phosphate. This latter is soluble in the pres- 

 ence of carbonic acid formed by the roots and decaying 

 humus, and is hence readily available. (See U 76.) 

 Phosphorus fertilizers do not give beneficial results when 

 applied to soils containing an excess of lime, like most 

 of the "black waxy" soils. 



Bone-black, formed by heating raw bones in the 

 presence of air, is used in large quantities by sugar 

 refineries. When it has served its purpose, it becomes a 

 waste product and is sold for fertilizing. It has little 

 value until treated with sulphuric acid. Bone-meal is 

 the fresh bone ground and steamed and contains some 

 nitrogenous matters in addition to the phosphorus. 



The commercial supplies of phosphates are bones 

 and phosphate rocks. The latter are mined in large 

 quantities in South Carolina, Florida, Tennessee, Vir- 

 ginia and Pennsylvania. 



121. Nitrogenous Fertilizers. Nitrogen is absorbed 

 by plants as nitrates. The most readily available form 

 is the "Chili saltpeter," found in large quantities in 

 rainless regions on the western coast of South America. 

 As it occurs naturally in the "saltpeter beds" it contains 



