In this article we will discuss about:- 1. Introduction to Cation Exchange 2. Cation Exchange Capacity (C.E.C.) of Soils 3. Factors Affecting Cation Exchange Capacity 4. Importance.
Introduction to Cation Exchange:
Cation exchange is a reversible process in which the cations are exchanged or interchanged between the solid and liquid phases or between the solid and solid phases when they come in close contact. Cation exchange is one of the most common and most important of soil reaction. Soil colloids are the seat of reactions.
In a near neutral soil, calcium remains adsorbed on the colloidal particles. A considerable amount of organic and mineral acids are formed due to decomposition of organic matter in the soil. The Hydrogen ions thus generated will tend to replace the exchangeable calcium of the colloidal complex. In colloid, hydrogen (H+) is adsorbed more strongly than the calcium (Ca++). The reaction may be shown simply as follows, where only one ion of the adsorbed calcium is represented as being displaced.
The reaction takes place fairly rapidly and the interchange of calcium and hydrogen is chemically equivalent. This phenomenon of the exchange of cations between soil and soil solution is known as cation exchange and the cations that take part in this reaction are called exchangeable cations.
Clay colloids are negatively charged and they attack positively charged ions—cations or anions which are also replaceable. Due to the presence of negative charges, cations are adsorbed on the surface of the clay micelle. Divalent cations are more strongly adsorbed than monovalent cations.
Cation exchange reactions are reversible. Hence, if some form of lime stone or other basic calcium compound is applied to an acid soil, the reverse of the replacement just given above occurs. The active calcium ions replace the hydrogen and other cations by mass action. As a result, the clay becomes higher in exchangeable calcium and lower in adsorbed hydrogen and aluminium.
When the fertilizer is applied in a soil, an exchange such as follows, may occur:
Cation Exchange Capacity (C.E.C.) of Soils:
C.E.C. is the milliequivalent of cations which can be adsorbed in the exchangeable phase per 100 gm of the soil. It is also termed as “cation adsorption capacity of soil.” The exchange of cation takes place between.
(i) Cation in soil solution and those one on the surface day crystals and humus.
(ii) Cations on the surface of either two clay crystals, two humus particles or a clay crystal and a humus particle.
(iii) Cation release by plant roots and those on the surface of clay crystals and humus.
If the clay colloid, saturated with calcium, is treated with Sodium chloride (NaCl), Calcium can be displaced. However, the resulting Calcium chloride (CaCI) is not removed from the medium, it would replace Sodium (Na) from the exchangeable complex until an equilibrium is obtained whereby the colloid would be charged with both Calcium (Ca++) and Sodium (Na+). If it is desired to replace all Calcium (Ca++) from exchange complex, the soil is leached with sodium chloride. Then the Calcium is constantly being removed and no chance to react with soil colloid to displace Sodium (Na++).
The reaction is as follows:
The potassium (K+) ions, for example, are more powerful replacer than sodium (Na+) ions and is readily adsorbed by the clay. Generally the divalent cations (i.e. Ca++, Mg++) are more effective than monovalent (i.e. Na+, K+, etc.) ones. Hydrogen is an exception which is tenaciously held by colloids and it is more powerful replacer. Thus the power of replacement is as follow –
Factors Affecting Cation Exchange Capacity of the Soil:
There are some factors that affect cation exchange capacity of the soil as follows:
(i) Soil pH – The cation exchange in most soils increases with soil pH. The C.E.C. of a soil is relatively lower than at high pH. In most cases, the C.E.C. is determined at a pH of 7.0
(ii) Nature of clay – The C.E.C. of soil depends on the nature of clay minerals. The C.E.C of soil varies with clay minerals is as follows (Table 4.2).
(iii) Soil texture – Generally the negatively charged clay micelle attacks positively charged cations and hold them. The C.E.C of soil increases when the percentage of clay is increased. The cation exchange capacity as influenced by soil texture is given below (Table 4.3).
(iv) Humus- C.E.C. of soil depends on the amount of humus present in the soil as the humic micelle is negatively charged and attacks cation. The C.E.C. of humus varies from 200-400 meq/100 gm.
Importance of Cation Exchange Capacity:
In the domain of agriculture, the importance of cation exchange phenomenon is next to photosynthesis.
The cation exchange phenomenon is involved in different activities in the soil as follows:
(i) Retention of Nutrients:
The nutrients cations after application in the soil in soluble form are retained by the soil colloid after adsorption by cation exchange process and thus the loss of nutrient by leaching is prevented. As for example, when Ammonium sulphate [(NH4)2SO4] is applied in the soil, the first effect is that Ammonium (NH4+) ion are adsorbed on soil colloid replacing other cation mainly Calcium. In this way, the nitrogen is retained in the soil for further utilization by plant. Otherwise nitrogen would have been lost from surface soil by leaching.
Similarly, the other nutrient cations like Calcium (Ca++), Magnesium (Mg++), Potassium (K+) etc. are also retained in the soil after their application through cation exchange process.
(ii) Absorption of Nutrients by Plants:
Soil water containing soluble salts and nutrients ions is known as Soil Solution. The nutrients that are in solution phase and exchangeable phase are readily available to plant. The nutrients, from both these two phases, are utilized by plant through the cation exchange process. The nutrient cation from soil solution is adsorbed by root hair by replacing the hydrogen ion originally present on the root surface through the cation exchange process. Once the nutrients are adsorbed on root surface, they are then taken within the plants.
The nutrient cations that are in exchangeable phase are utilized by the plant in two ways as follows:
(a) The nutrient cations are utilized by exchanging with other cation i.e. hydrogen. The nutrient cation are adsorbed on root surface of the plant by exchanging the hydrogen (H+) present on the root when they come into soil solution from exchangeable phase.
(b) The nutrient cations of the exchangeable phase may directly also be utilized by plant without bringing them into soil solution. When the root surface come in close contact with soil colloid, direct exchange may take place between the hydrogen (H+) on the root surface and nutrients cation on the colloidal surface and the nutrients are utilized by the plant after this exchange. This phenomenon is known as ‘Contact exchange theory’.
(iii) Reclamation of Acid and Alkali Soils:
The reclamation of acid and alkali soil is possible by application of suitable soil amendment due to the cation exchange phenomenon of the soil. In acid soil, calcium is applied in the form of lime for its reclamation. The Calcium (Ca++) cation of lime replaces the Hydrogen (H+) adsorbed on soil colloid and the soil colloids are saturated with Calcium (Ca++) increasing the pH of the soil.
On the other hand, the alkali soils, which contain high percentage of Sodium (Na+), is possible to reclaim by addition of Calcium (Ca++) in the form of Gypsum (CaSO4.2H2O). The Calcium (Ca++) replaces the Sodium (Na+) through the cation exchange process and the alkali soil is reclaimed in this way.