184 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



of compounds in the soil. Apatite, phosphate rock, ferric phos- 

 phate (precipitated), aluminium phosphate, vivianite, and triplite 

 are practically equally soluble. We also feel justified in saying 

 that acid phosphate would be completely dissolved. But no one 

 can yet claim that these materials possess the same value to plants. 

 Fifth-normal nitric acid may not distinguish between minerals 

 which have unequal values to plants. We have no solvent which 

 would dissolve phosphoric acid from the phosphates mentioned, in 

 the same proportions as would be taken from them by plants. 

 What we cannot do with known mineral phosphates of known 

 character outside of the soil, we could not expect to do with the 

 same phosphates after they are put into the soil, and with the un- 

 known phosphates already within the soil. 



Soils may, therefore, contain equal quantities of phosphoric 

 acid soluble in fifth-normal nitric acid, and yet give up unequal 

 quantities of phosphoric acid to plants on account of differences 

 in the phosphates present. This consideration must give rise to 

 caution in comparing the results of all kinds of soils with one 

 another. 



Only those soils should be compared which probably contain 

 the same kinds of phosphates. Soils widely dissimilar in origin 

 and character should not be compared, unless there is evidence 

 that they contain similar phosphates. 



Study of potash dissolved from minerals in a similar way shows 

 that very little is taken from the felspars, microcline and 

 orthoclase, less than ten per cent, from glauconite and biotite, and 

 from 16 to 60 per cent, from muscovite, nephelite, leucite, 

 phillipsite, and apophyllite. Potash absorbed by chabazite and 

 some other minerals is extracted to the extent of 70 per cent. 

 The dissolved potash thus represents a large portion of some 

 easily dissolved mineral, or a small portion of some difficultly 

 attacked mineral. 



Fixation of the Dissolved Phosphoric Acid and Potash. The soil 

 has the power to withdraw potash and phosphoric acid from 

 solution, both in water and in acids. The method of studying 

 this factor consists in extracting two portions of the soil of 



