2G2 Uueeau of Faemees' Institutes. 



(b) Availability of Different Forms of Phosphoric Acid. 



The availability of phosphate compounds as plant food depends 

 upon several conditions, among which may he mentioned: 



(1) The form of compound; (2) the kind of material; (3) the 

 fineness of division; (4) the character soil, and (5) kind of crop. 



(1) The Availability of Phosphoric Acid Dependent on Form of 

 Compound. — The water-solubles form of phosphates is readily 

 distributed in the soil and is immediately absorbed and used by 

 roots as plant food. Therefore, water-soluble phosphates present 

 in dissolved rock, superphosphates, dissolved bone, etc., represent 

 the form most quickly available. 



The citrate-soluble or reverted form of phosphates does not dis- 

 solve easily in pure water, but does dissolve fairly well in soil 

 water and in the acid plant juices contained in and exuded by the 

 rootlets. The disadvantage in using reverted phosphate is, that 

 on account of its slow solubility it is not quickly, thoroughly and 

 uniformly distributed through the soil, and consequently does not 

 come so easily into contact with the roots of plants. 



The insoluble form of phosphates, like ground rock, is not easily 

 available under ordinarv conditions. 



(2) Availability of Phosphoric Acid Dependent on Kind of 

 Material Furnishing it. — Mineral phosphates untreated are 

 changed into soluble forms very slowly under usual conditions 

 and they become available as plant food much less easily and 

 quickly than do phosphates contained in animal or vegetable mat- 

 ter. In organic matter, as bone, for example, the phosphates are 

 associated intimately with material that decays easily, and, in the 

 process of such decay or rotting, the phosphate compounds are 

 more or less changed and made soluble. 



(3) Availability of Phosphate Compounds Affected by Fineness. 

 — It has been found in usual practice that the phosphate com- 

 pounds, especially of organic matter, are more quickly available 

 as they are more finely ground. Increased fineness increases the 

 rapidity with which the phosphate compounds can be acted upon 

 and rendered soluble by natural agencies. 



(.'/) Availability of Phosphate Compounds affected by Kind of 

 Soil. — Soils containing an abundance of decaying organic mat- 

 ter usually favor the more rapid change of insoluble to soluble 

 phosphates. On muck soils the application of undissolved rock 

 in the form of " floats " gives about as good results as the dis- 

 solved phosphate. The insoluble phosphates contained in the soil 



