286 Bacteria in Relation to Country Life 



the assumption that the excess of lime not only com- 

 bines with the organic acids generated in the humus, 

 but, also, with the carbon dioxid to form bicarbonate of 

 lime. These substances are, therefore, prevented from 

 exerting their solvent action on the insoluble bone 

 phosphate. On the other hand, fairly satisfactory re- 

 turns are secured from bone on sour soils. Its use on 

 the reclaimed moor (peat) soils of Europe has been found 

 to be profitable. Since lime is known to encourage 

 bacterial activities in the soil, it might seem that the 

 decreased availability of the phosphoric acid in bone, 

 consequent upon liming, is due to the more vigorous 

 growth of the bacteria. This seeming contradiction is, 

 however, explained by the foregoing remarks. With a 

 plentiful supply of humus there is but slight difficulty 

 to be apprehended from this source. 



The effect of using sulfuric acid. — The practice of 

 treating phosphate rock with sulfuric acid in the manu- 

 facture of superphosphate (acid phosphate or dissolved 

 rock) is justified by the greater availability of the phos- 

 phoric acid in the resulting product. The tricalcic phos- 

 phate of the crude rock is changed by the acid into the 

 water-soluble or into the reverted phosphate, both forms 

 available to the crops. Superphosphate is, therefore, 

 essentially a mixture of water-soluble, reverted and 

 insoluble lime-phosphate and gypsum. The soluble 

 phosphate is "fixed" in the soil, that is, it is changed 

 back into an insoluble form. 



It may seem rather wasteful to dissolve the crude 

 phosphate rock in order to have its phosphoric acid 

 return to the insoluble state in the soil. As a matter 



