114 CHEMISTRY OF FARM PRACTICE 



insoluble compounds in the soil, in this way depleting in 

 time the local supply. Land plaster acts similarly to 

 lime, in that it may furnish calcium, an essential plant 

 food element. It also assists in liberating nitrogen from 

 organic compounds and also frees insoluble potash and 

 phosphorus, but it does not correct acidity. 



84. Thomas Phosphate or Basic Slag. In the manu- 

 facture of steel, various processes are resorted to for the 

 purpose of removing the phosphorus from the pig-iron 

 from which the steel is manufactured. Essentially all 

 processes consist of lining the furnaces with dolomite, a 

 calcium magnesium limestone, before the pig-iron is put 

 in. The mass is subjected to a high heat, and the mag- 

 nesium limestone slags off the phosphorus as calcium phos- 

 phate, which rises to the surface. This slag is drawn off 

 from the converter, cooled, broken, finely ground and placed 

 on the market. It is stated that slag consists of trical- 

 cium phosphate and calcium silicate in proportions shown 

 by the formula (CaO)5P2OsSiO2. This material may con- 

 tain some free lime, and more lime may become soluble 

 by repeated washings. Thomas slag gives good results 

 on sour lands that contain organic matter, on lands rich 

 in humus, and on lands deficient in lime. It must not be 

 mixed with any material containing salts of ammonia 

 for example, sulphate of ammonia because the free lime 

 will liberate the ammonia as a gas, which will be lost. 



85. Bone. Among the sources of phosphorus, both 

 raw and steamed bone have held an important place. 

 Raw bone contains 3 to 4 per cent of nitrogen in the form 

 of organic matter, and from 20 to 25 per cent of phos- 

 phoric acid. About half of this phosphoric acid is avail- 

 able, more becoming available as the bone decays. The 

 composition of the bone varies with the age of the animal, 

 the bones of old animals usually containing more phos- 

 phorus and less nitrogen. 



Bones are steamed for the purpose of removing the 



