114 Phenomenon of Absorption in its Relation to Soils 



physically combined in the soil, considering the latter only to be that 

 immediately available to plants. "The power of the soil to nourish 

 cultivated plants/' he states, "is therefore in exact proportion to the 

 quantity of nutritive substances which it contains in a state of physical 

 saturation. The quantity of other elements in a state of chemical 

 combination distributed through the ground is also highly important, 

 as serving to restore the state of saturation, when the nutritive sub- 

 stances in physical combination have been withdrawn from the soil 

 by a series of crops reaped from it^. " 



Knopfs compromise and analytical method. 



W. Knop^ in reviewing the subject attempted to combine Way's 

 chemical and Liebig's physical hypotheses. He explains the removal 

 of bases from solution by surface attraction and by combination with 

 silica or double aluminium silicates. The absorption of phosphates 

 he attributed to the precipitation of calcium phosphates in the first 

 instance and afterwards to the formation of iron and aluminium phos- 

 phates. Other acids were said to be held up also by iron and aluminium 

 hydroxides with the formation of basic salts. Knop recognised that 

 the chemical explanation was insufficient to account for the absorption 

 of the bases, because an equilibrium was invariably established between 

 the soil and the solution and the whole of the base could not be removed 

 from its solution no matter how dilute this might be. 



The chief facts concerning absorption by soils were accumulated by 

 about 1880. Since then very little has been added except on the 

 theoretical side. W. Henneberg and F. Stohmann^ treated soils with 

 different ammonium salts in varying amounts, and Peters* worked with 

 potassium salts. Both obtained the same results as Way : , as the 

 concentration of the salt was increased the amount of base taken up 

 by the soil increased also but was in lower proportion ; i.e. the relative 

 amount taken up decreased with the increase in concentration of the 

 solution. A vast amount of subsequent work has shown that this is 

 true of all absorption phenomena and the relationship has more recently 

 been expressed mathematically. Two methods of investigation have 

 been used: (1) the soil is brought into equilibrium with a definite 



^ Nati'ral Laws of Husbandry, 1863, pp. 67-69. 



2 Lchrbuch der AgriruHurchemie, Leipzig, 1868. 



3 Journ. f. Landw. 1859, 3. 25. 



« Landw. Versuchs-Stat. 1860, 2, 113. 



