ABSORPTION OF MINERAL ELEMENTS BY PLANTS 255 



degree of sulphates. This has a great significance for the 

 fertihzation of soils with these salts. 



Biological absorption is the result of the vital activity of micro- 

 organisms, bacteria, fungi, and others, which populate the soil. 

 In absorbing cations and anions necessary for their nutrition, 

 these organisms remove them from the sphere of soil reactions 

 and thus contribute to the fixation of various compounds, which 

 return into general rotation only after the death of the organisms 

 and the disintegration of their cells in the processes of decay. 

 As a matter of fact, the absorption of soil constituents by the 

 roots of higher plants themselves may be considered as biological 

 absorption. 



The adsorption capacities of soils, especially those of physico- 

 chemical and physical nature are of great importance for the 

 nutrition of plants with mineral elements. Owing to this 

 phenomenon, potassium and ammonium fertilizers introduced 

 into the soil are retained by it and their leaching from the soil 

 is prevented. At the same time, they remain available to the 

 plant, for they may be easily displaced from the adsorbing com- 

 plex by means of base-exchange reactions. Physicochemical 

 together with physical adsorption prevents an excessive increase 

 in the concentration of the soil solution, for instance, during 

 drying of the soil or after the introduction of fertilizers, for it 

 transfers a part of the cations into an adsorbed state. It regu- 

 lates the chemical composition of the soil solution, providing 

 it with bivalent ions that balance the action of the univalent 

 Na+ and K+ ions. As a matter of fact, the whole process of the 

 absorption of salts from the soil by plant roots to a considerable 

 degree represents exchange reactions between the root cells and 

 the soil-adsorbing complex through the medium of the soil 

 solution. 



Since close contact between the absorbing and the adsorbing 

 surfaces contributes greatly to facilitate the exchange reactions, 

 root hairs usually adhere very closely to soil particles (Fig. 74). 

 That is why when young rootlets densely covered with root 

 hairs hold a coating of soil particles, it is almost impossible to 

 wash these away completely without injuring the tissues (Fig. 75). 



Such a close contact of the root hairs with soil particles has 

 still another significance. Not only can the roots utilize solutes 

 and adsorbed substances, but they also are able to dissolve 



