138 MINERAL SALTS ABSORPTION IN PLANTS 



exchangeable, forms attached to soil particles. Since they are held 

 by positive charges originating from basic groups, e.g. in various 

 clays, the anion exchange capacity of soils increases with decreasing 

 pH (cf. Fig. 9, p. 38). It varies greatly for different anions; chloride 

 and nitrate are bound very slightly by most soils, except in extremely 

 acid conditions, whereas phosphate is adsorbed at high as well as 

 at low pH values. Sulphate is weakly adsorbed by most soils even at 

 low pH but appreciable adsorption occurs when the soil is rich in 

 hydrated oxides of iron or aluminium, because sulphate can 

 apparently displace hydroxyl ions from these substances. Phosphate 

 competes strongly with sulphate for these sites. 



2. Cation Exchange Capacity 



The "base exchange" or "cation exchange" capacity of a soil is 

 much greater than its anion exchange capacity and is of much greater 

 significance in plant nutrition. Cation exchange capacity (C.E.C.), 

 that is, the number of exchangeable cations in a given amount of soil, 

 ranges from 1-2 m eq./lOO g dry weight for sandy soils, to 60 m eq, 

 or more for clay soils, and for soils rich in organic matter (Table 14). 

 C.E.C. increases with increasing pH as the exchangeable ions are 

 held at negatively charged sites on amphoteric soil colloids. 



The bulk of the exchangeable cations in soils consists of Ca"*""*" 

 Mg,"*""*" K"*" and Na,"^ and of these, calcium ions usually predominate 

 in non-saline soils (Table 14). Hydrogen ions may comprise a 

 considerable proportion of the exchangeable cations in acid soils, 

 and this leads to infertility, since other essential cations are displaced 

 into the soil solution where they are subject to leaching. Of the major 

 exchangeable ions, calcium and magnesium are bound most firmly, 

 and sodium is most weakly adsorbed. The latter is thus particularly 

 Hable to be leached from soil and this is the reason for its considerable 

 accumulation in the sea. 



An equilibrium is maintained between exchangeable ions and the 

 soil solution, and if this is disturbed, for example, by the absorption 

 of salts from the soil solution by plants, desorption of exchangeable 

 ions occurs until an equilibrium is re-established. In this way, 

 exchangeable ions become available to plants. 



D. Non-exchangeable Ions 

 The greater part of the mineral salts occurring in soil is present 



