92 Fertilizers 



scribed, and the free phosphoric acid combines with the 

 lime to form either a soluble or a reverted form. 



Phosphoric acid remains in the soil until taken out by plants. 



The phosphoric acid in superphosphates, though soluble 

 in water, is not readily washed from the soil. The real 

 object of making it soluble is to enable its better distribu- 

 tion. If it were possible to as cheaply prepare the dicalcic 

 or reverted form as the soluble, it would, perhaps, be quite 

 as useful from the standpoint of availability. After the 

 soluble is distributed in the soil, it is fixed there by com- 

 bining with the lime and other minerals present. It is 

 believed that it assumes, first, by the larger relative pro- 

 portion of lime usually present in soils, the dicalcic form, 

 though it is not positively certain that in the present of an 

 abundance of lime, or that in time, it may not assume the 

 insoluble tricalcic form. The soluble phosphoric acid may 

 also combine with the iron and alumina in the soil, and 

 form phosphates of these elements, though recent investi- 

 gations lead to the conclusion that these conditions are 

 much more rare than was at one time supposed. The 

 time required for the fixing of the phosphoric acid, as well 

 as the form it may eventually assume, depends chiefly 

 upon the character and composition of the soil. In those 

 rich in lime, the fixation is most rapid, though in no sense is 

 the fixation immediate, and in such soils the fixation is 

 probably largely completed in the course of a week. On 

 clay soils, containing a low percentage of lime, and in light 

 soils that contain little clay or organic matter, the fixation 

 is much slower, though even in these the chances are that 

 no serious loss of phosphoric acid occurs. Seldom do we 

 find more than traces of phosphoric acid in drainage waters, 

 even when heavy applications of soluble phosphoric acid 



