180 THE STUDY OF PLANT COMMUNITIES * Chapter VII 



affect plant distribution and growth of wild species. An important 

 part of the mineral nutrition of native and cultivated vegetation is 

 derived from the exchangeable bases or cations adsorbed on the 

 surfaces of the soil colloids. When these vary considerably in 

 amount or kind, there may be marked differences in the type of 

 vegetation or at least in rate of growth. For example, it has been 

 shown that, in soils derived from hydrothermally altered rocks in 

 the Great Basin, sagebrush and its associated species fail to grow 

 because of the very low percentage of exchangeable bases as com- 

 pared with the normal brown soils of the sagebrush zone. 22 



The colloidal portion of the soil is composed primarily of alum- 

 ino-silicates. These colloidal particles are almost always negatively 

 charged, and upon their surfaces are adsorbed great numbers of 

 cations. These cations are principally H + , Ca ++ , Mg ++ , K + , and Na + , 

 named in the decreasing order of tenacity with which the cations 

 are held. The hydrogen ion is held more tightly than calcium and 

 replaces calcium more readily than calcium will replace hydrogen. 

 This same relationship holds between calcium and magnesium, and 

 so on down the series. The displaced cation usually enters the soil 

 solution. This phenomenon, in which one cation may replace an- 

 other on the colloidal particle, is called base exchange. 



Plants are almost entirely dependent on this process of base ex- 

 change for their supply of calcium, magnesium, and potassium. Of 

 the anions, only POi is held to any extent by colloidal adsorp- 

 tion, the other anions, such as NO.r, being readily soluble in the 

 soil solution and therefore, readily leached. One source of the H 

 ions that can displace the bases and make them available is the car- 

 bonic acid formed when carbon dioxide from root respiration is 

 released into the soil solution. This was shown experimentally for 

 the calcium ion. 132 Another common source is the organic acids 

 derived from humus. 



Soils differ widely in their ability to supply cations because of 

 the effects of climate, parent material, and vegetation. The maxi- 

 mum amount of exchangeable cations a soil can hold is called 

 the base exchange capacity of the soil. Obviously, a soil high in 

 colloids will have a high capacity as compared with one low in 

 colloids, as, for example, a sand. Even the kind of clay may make 

 a great difference in the base exchange capacity of a soil. For ex- 



