94 FIRST PRINCIPLES OF AGRICULTURE. 



is its first advantage, is chiefly valuable because of its ease 

 of self -distribution. When applied to the soil, it is taken 

 up by the water and more generally distributed than is 

 possible by any mechanical means, however fine the sub- 

 stance may be ground. The roots of plants come in con- 

 stant contact with it wherever they go. 



The dicalcic or reverted phosphoric acid is believed to 

 be quite as available to plants, but it remains exactly 

 where it is placed ; if the roots are there they can make 

 quite as ready use of it as the soluble. The main differ- 

 ence between the soluble and reverted is that in the former 

 case the phosphoric acid goes to the roots, while in the 

 latter case the roots must go to the phosphoric acid. The 

 same is true of the insoluble, though in a different degree. 

 Here the roots must not only go to the phosphoric acid, but 

 the amount that can be used is measured by the activity 

 of the roots in aiding its solution. 



A superphosphate, therefore, is valuable in proportion to 

 the amount of soluble phosphoric acid contained in it ; the 

 greater the amount of soluble, and the less the amount of 

 reverted and insoluble, the more valuable. If insoluble or 

 reverted forms are desired — and they are often quite as 

 useful as the soluble — they may be procured from un- 

 treated products. 



Fixation of Phosphates. — Phosphoric acid, though 

 soluble in water, is not washed from the soil ; it is fixed 

 there by combining with the lime and other minerals pres- 

 ent. It is believed to assume first, by the appropriation 

 of lime, the dicalcic form, though it is not positively cer- 

 tain that the insoluble tricalcic phosphate is not sometimes 

 formed. It may also combine with iron and alumina and 



