Cation Exchange Capacity
The adsorption of a cation by a colloid nucleus or micelle and the accompanying release of one or more ions held by the micelle is termed as 'cation exchange'. The major cations are Ca2+Mg2+,K +,Na+, and NH4+. Trace amounts of other cations such as Cu2+M, n2+a nd Zn2+ are also present in the soil. Let us understand this with the help of an example. Assume that the clay micelle has one-half of its capacity satisfied with calcium ions, one-quarter with potassium ions, and one-quarter with hydrogen ions.
Suppose that the soil is treated with a strong solution of potassium chloride. After some time, the potassium ions froin potassium chloride will replace virtually all other cations and the micelle will get saturated with potassium. The calcium and hydrogen adsorbed on the micelle will be released and will enter the soil solution as chlorides. Since these cations (i.e., H+ and Ca2+) are rather easily displaced, they are termed as exchangeable ions. The efficiency with which ions will replace each other is determined by (a) relative concentration or number of the ions, (b) the number of charges on the ions and (c) the speed of movement or activity of the different ions.
The capacity of soil colloids to adsorb cations can be determined easily. The commonly used method involve replacement of adsorbed cations by saturating the exchangeable sites with barium or ammonium ions, and then the amount of adsorbed barium or ammonium is
determined. The cation exchange capacity is determined in terms of milliequivalent (m.e.) per 100 grams of soil. The total cation exchange capacity of the soil represents the total number of exchange sites both on humus and clay particles. If a clay has a cation exchange capacity of 1 milliequivalent (1 m.e./lOOg), it is capable of adsorbing and holding a milligram of hydrogen or its equivalent for every l00g of dry clay. You should note the term 'equivalent'. It indicates that other ions also may be expressed in terms of milliequivalents. For example, let us consider calcium. This elements has an atomic weight of 40 compared to 1 for hydrogen. Each calcium ion has two charges and thus is equivalent to 2H+. Therefore, the amount of calcium required to displace 1 milligram of hydrogen is 40/2 or 20 milligrams. This, then is the weight of 1 milliequivalent of calcium. If 100 grams of certain clay is capable of adsorbing a total of 250 milligrams of calcium, its cation exchange capacity is 250120 or 12.5 m.e./100 g. The cation exchange capacity ranges from less than 5 for soils containing very little clay or organic matter to about 200 for soils richer in organic matter.
The composition of these cations exert a powerful influence upon both the chemical and physical attributes of a soil. In temperate and cold climates with moderate or high rainfall, H+ ions are formed in abundance in the soil. They are of no direct use to the plants, and when they dominate the colloidal complex there is nothing to prevent the loss by leaching of important basic ions especially Ca, Mg ,and K and fertility is consequently low. This condition may arise as a result of the difference in the abilities of ions to replace each other, the order being H, Ca, Mg, K, NH, and Na. The degree of saturation with basic ions can always be reduced by cation exchange wherever there are enough H ions to bring about this displacement. Once displaced, the bases are carried away in drainage waters. Colloids are said to be unsaturated, when the adsorbed bases have thus been reduced to a very low level.
Cation Exchange Capacity = Exchangeable H + Exchangeable bases.