564 VARIOUS FERTILITY FACTORS 



The fixation of soluble phosphates involves a very different re- 

 action, which may be illustrated as follows: 



CaH 4 (PO 4 ) 2 + CaCOg = Ca 2 H 2 (PO 4 ) 2 + CO 2 + H 2 O. 

 + CaCO 8 = Ca 3 (PO 4 ) 2 + CO 2 + H 2 O. 



Compounds of iron or aluminum may take the place of the cal- 

 cium carbonate in the fixation of phosphates. 



Thus, we may apply to ordinary soil a solution containing soluble 

 salts of potassium, ammonium, or phosphorus, but the liquid which 

 passes through a soil stratum three inches or more in thickness will 

 be found to contain very little of the salts applied. The chief 

 value attached to soluble fertilizers is due to their thorough dis- 

 tribution in the soil before passing into insoluble forms. If every 

 soil grain touched by the soluble fertilizer becomes coated with the 

 insoluble product, it presents to the plant roots a very much 

 greater surface than if the fertilizer is applied in small solid par- 

 ticles, as in ground rock phosphate. 



