COMMERCIAL FERTILIZERS 517 



undergoes changes, the nature of which depends more or 

 less on the properties of the particular soil with which it 

 is mixed. No matter how readily soluble the phosphorus 

 may be in the fertilizer, it soon becomes insoluble in the 

 soil, only a fractional proportion of it being recoverable in 

 water extracts. Absorption by colloidal complexes is the 

 fate of a part of the phosphorus, in which condition it is 

 still available to plants, especially when the colloidal 

 matter becomes coagulated. The excess phosphorus en- 

 ters into combination with the calcium of the soil, form- 

 ing tricalcium phosphate and some dicalcium phosphate, 

 and with the iron or the aluminium, forming phosphates of 

 those metals. The latter compounds are less readily soluble 

 than the former, and probably do not serve as a direct 

 source of phosphorus for plants ; while tricalcium phosphate, 

 although acted upon by plant roots, is not so readily avail- 

 able as is the phosphorus held by the colloidal matter. 



It is desirable that there should be an abundant supply 

 of calcium in a soil to which a superphosphate is added, be- 

 cause the phosphorus not absorbed by the colloidal matter 

 of the soil will, under such circumstances, form more cal- 

 cium phosphate than if only a small supply of lime is pres- 

 ent, according to the law of mass action. The great loss 

 of availability through the conversion of phosphorus into 

 iron and aluminium phosphates may thus be mitigated. 



436. Other factors influencing the availability of tri- 

 calcium phosphate. — As this is the form in which phos- 

 phorus is probably most extensively held in the ordinary 

 soil, and as it is also a cheap form of phosphorus in manures, 

 it is a matter of some importance to know the most favor- 

 able conditions for its utilization by agricultural plants. 

 Experimentation by numerous investigators has estab- 

 lished at least four factors that influence the availability 



