Soluble Phosphates 287 



of fact, however, the soluble phosphate distributes 

 itself more or less thoroughly in the soil before it is 

 fixed. Even then it exists in so fine a state of division 

 that it may be rapidly rendered available by the chemical 

 and bacteriological changes in the soil. In so far as the 

 bacteria are concerned, the action of superphosphate is 

 somewhat different from that of floats or ground bone, 

 for the superphosphate adds to the soil more or less 

 gypsum, besides the mixture of phosphates, which is 

 sUghtly acid. A certain effect is thereby produced on 

 the soil bacteria. This not only affects the decomposition 

 of the humus and the subsequent availabihty of the 

 phosphoric acid of the superphosphates, but, also, that 

 of the soil phosphates proper. In soils in which large 

 quantities of superphosphates are applied at frequent 

 intervals, the resulting bacteriological changes must 

 be far-reaching, as future investigations will probably 

 demonstrate. 



Thomas slag. — In European agriculture, particularly 

 that of Germany, France and Belgium, Thomas slag 

 serves as an important source of phosphoric acid. Thomas 

 slag is a by-product in the manufacture of steel from 

 phosphatic iron ores, and contains, besides the phos- 

 phate of lime, considerable quantities of iron and lime 

 carbonate. The material, as placed on the market, 

 is merely the finely ground slag. The availability of 

 the phosphoric acid in the slag depends largely on its 

 fineness of division. The bacterial relations in this case 

 are influenced by the iron and the lime contained in 

 the slag, as well as by its phosphoric acid. The latter 

 exists in combination with lime in the so-called tetra- 



