THE ABSORPTION OF MINERAL ELEMENTS 77 



and liquid phases. A state of equilibrium between the two sub- 

 stances is thus at once established. At low concentrations of the 

 solution, together with a large development of the adsorbing sur- 

 face, the substance to be adsorbed is almost completely extracted 

 from the solution. 



In soils containing organic and inorganic colloids in abundance, 

 the phenomenon of adsorption has an important function. It 

 permits the explanation of many of the well-known properties of 

 the soil, termed in general its "adsorbing capacity." This charac- 

 teristic property of the soil causes the retention of dissolved sub- 

 stances from a solution leached through the soil, allowing only the 

 solvent to pass. The adsorbing capacity of a soil may be very 

 clearly demonstrated by leaching through it dyes or other organic 

 substances. The cations and anions of various salts are rapidly 

 adsorbed by it; as, for instance, ammonia, phosphoric acid, potas- 

 sium, and magnesium. Calcium and sodium are adsorbed less; 

 and the anions of nitric and sulphuric acid, not at all. 



The adsorbed substances are very firmly retained by the soil. 

 They cannot be leached out completely by water, but they may be 

 displaced by other substances, especially those possessing adsorp- 

 tion capacity. Potassium, for instance, displaces calcium from its 

 adsorption compounds. Thus when soil is watered by a solution 

 of potassium sulphate, the solution leached out will contain cal- 

 cium sulphate. Similarly, sodium will replace potassium, this 

 being the reason why sometimes the addition of sodium chloride 

 can partly substitute potassium salts in fertilizers. Substances 

 that are easily adsorbed by soil remain largely in a bound state. 

 Compounds of ammonium and of phosphoric acid, for example, do 

 not go readily into solution from soil. 



Experiments with plants grown on soils rich in adsorbed sub- 

 stances, as, for instance, those fertilized with phosphate salts 

 and subsequently leached, have shown that plants are capable of 

 feeding on adsorbed substances. That plants can utilize sub- 

 stances adsorbed on soil colloids is due to the nature of the structure 

 of the cell walls of root hairs. They too are in a colloidal state and 

 therefore have a considerable adsorbing capacity. As adsorption 

 may take place only when the reacting substances are in close con- 

 tact, root hairs are usually closely appressed to the soil particles 

 (Fig. 30) . Consequently, young roots possessing such hairs, when 

 pulled out of the soil, are covered with a complete coating of soil 



