ACTIVE; PLANT FOOD, ETC. 183 



it is quite effectually protected from any solvent. If it is con- 

 tained within zeolites, it may be affected by some solvents and not 

 by others. If it is contained in carbonate of lime, the latter will 

 be dissolved by any acid solvents, with consequent exposure of the 

 included phosphate to the action of the solvent. 



The quantity of phosphoric acid or potash contained in the soil 

 extract thus depends upon three factors : 



(1) The quantity of phosphate or potash mineral exposed to 

 the solvent, and its solubility under the conditions of the extrac- 

 tion. 



(2) The solubility of the soil materials which protect or 

 enclose phosphates or potash compounds. 



(3) The power of the soil to fix phosphoric acid or potash 

 under the conditions of the extraction. 



The strength of the solvent, its nature, the period of digestion, 

 the temperature, and the proportion of soil to solvent, all affect 

 the quantity of phosphoric acid and potash contained in the soil 

 extract, but they have their effect through action on the three 

 factors mentioned above. 



Solubility of the Soil Minerals. This subject 1 is studied by 

 bringing phosphate or potash minerals in contact with N/5 nitric 

 acid, in the proportions in which these minerals may occur in the 

 soil, and under the conditions of the soil extraction. 



Phosphoric Acid. The phosphates of lime are completely 

 soluble, the precipitated phosphates of iron and aluminium are 

 completely soluble, and vivianite and triplite are nearly so, in N/5 

 nitric acid. The aluminium phosphates (variscite and wavellite) 

 and the basic ferric phosphates are comparatively slightly dis- 

 solved. 



It is hardly probable that ferrous phosphate (vivianite) is of 

 common occurrence in ordinary cultivated soils, though it may 

 exist in some soils which are not well aerated. Fifth-normal 

 nitric acid dissolves calcium phosphates completely, but dissolves 

 mineral aluminium phosphates or basic ferric phosphates only to 

 a slight extent. It thus distinguishes between these two classes 



1 Texas Station Bulletins 126-1.15. 



