ABSORPTION OF MINERAL ELEMENTS BY PLANTS 253 



pounds of different elements, which are continually reacting with 

 each other. Only a small part of the mineral salts found in the 

 soil are present in water-soluble form and directly available to 

 the plant. The greater part are either adsorbed by the colloidal 

 particle or else remain in a solid state as water-insoluble minerals 

 or organic substances. 



It has been known for some time that the water solution 

 obtained from even the most fertile soils contains such minute 

 quantities of nutrient salts that it cannot produce a normal 

 development in plants. This can be easily proved by growing 

 plants in a solution leached from a layer of soil. From this 

 it is clear that plants must be able to feed upon adsorbed and 

 insoluble mineral substances. 



The property of the soil to adsorb and to retain firmly dis- 

 solved substances has been termed its ''adsorbing capacity," 

 and the colloidal part of the soil determining this capacity is 

 called the '' soil-adsorbing complex." The phenomenon of the 

 absorption of different substances by the soil, especially of 

 cations, has been studied in detail by Gedroitz, whose classical 

 work threw more fight on this question, which was previously in 

 a very confused state. Gedroitz distinguishes five various 

 absorbing capacities: (1) mechanical, (2) physical, (3) physico- 

 chemical, (4) chemical, and (5) biological. The most important 

 is the physicochemical adsorbing capacity, otherwise termed by 

 Gedroitz the ''exchanging capacity." The phenomena of base 

 exchange in soils have been much clarified by recent American 

 investigations. 



The mechanical absorbing capacity is shown by soils just 

 as by any porous body. This is the simple filtering out of fine 

 particles held suspended in the water when it is being percolated 

 through the soil. Physical adsorption is the result of the action 

 of the force of surface tension at the interface between the solid 

 phase and the soil solution. This, if positive, causes an increase 

 of the concentration of the dissolved substances near the surface 

 of the solid soil particles. This phenomenon is termed "adsorp- 

 tion" and is T\'idely used in chemical technology, for instance, in 

 purifjdng sugar solutions by means of animal charcoal. The 

 magnitude of physical adsorption is dependent upon the size 

 of soil particles. The finer the particles, the larger the total 

 surface and the greater the adsorption. 



