Cation and anion exchange capacity notes soil

Cation exchange capacity

1. Cation Exchange Capacity (CEC) Dr. Kiran Karthik Raj Assistant Professor COA, Vellayani

2. Definition 2 Cations are positively charged ions such as calcium (Ca 2+ ), magnesium (Mg 2+ ), and potassium (K + ), sodium (Na + ) hydrogen (H + ), aluminum (Al3+), iron (Fe 2+ ), manganese (Mn 2+ ), zinc (Zn 2+ ) and copper (Cu 2+ ). Cation exchange capacity (CEC) is the capacity of a soil to hold exchangeable cations ( hold positively charged ions) on its surface. Cations are held by the negatively charged clay and organic matter particles in the soil through electrostatic forces (negative soil particles attract the positive cations). Units CEC is conventionally expressed in meq /100 g which is numerically equal to centimoles of positive charge per kilogram of exchanger ( cmol (+)/kg ). The CEC of soils varies according the clay %, the type of clay, soil pH and amount of organic matter .

3. Why clay particles are negatively charged ? 3 pH Independent Charge / Isomorphous substitution: N et negative charge because of the substitution of silica (Si 4+ ) by aluminum (Al 3+) in the silicon tetrahedral sheet of mineral structure of the clay. The negative charges associated with isomorphous substitution are considered permanent, that is, the charges do not change with pH changes. the negative charge of the clay particles is balanced by the positive charge of the cations in the soil. pH Dependent Charge : Depends on the soil pH, de-protonation of surface hydroxyl groups. CEC associated with soil organic matter is called pH-dependent CEC . This means that the actual CEC of the soil will depend on the pH of the soil.

4. Cation exchange capacity (CEC) It influences the soil’s ability to hold onto essential nutrients and provides a buffer against soil acidification. Soils with a higher clay fraction tend to have a higher CEC. Organic matter has a very high CEC. Sandy soils rely heavily on the high CEC of organic matter for the retention of nutrients in the topsoil. 4 It influences the soil’s ability to hold onto essential nutrients and provides a buffer against soil acidification. Soils with a higher clay fraction tend to have a higher CEC. Organic matter has a very high CEC. Sandy soils rely heavily on the high CEC of organic matter for the retention of nutrients in the topsoil. It influences the soil’s ability to hold onto essential nutrients and provides a buffer against soil acidification. Soils with a higher clay fraction tend to have a higher CEC. Organic matter has a very high CEC. Sandy soils rely heavily on the high CEC of organic matter for the retention of nutrients in the topsoil.

5. Importance The higher the CEC the more clay or organic matter present in the soil. The clay mineral and organic matter components of soil have negatively charged sites on their surfaces which adsorb and hold positively charged ions (cations) by electrostatic force. This electrical charge is critical to the supply of nutrients to plants because many nutrients exist as cations (e.g. magnesium, potassium and calcium). In general terms, soils with large quantities of negative charge are more fertile because they retain more cations  A cid soil: Exchange acidity due to exchangeable H + , Al 3+  and Mn 2+ The lower the CEC of a soil, the faster the soil pH will decrease with time. Liming soils to higher than pH 6 will maintain exchangeable basic cations. 5

6. Importance Soils with a low CEC are more likely to develop deficiencies of calcium, magnesium and potassium. The higher the CEC, the larger the quantity of lime that must be added to increase the soil pH; sandy soils need less lime than clay soils to increase the pH to desired levels. 6

7. ANION Exchange Capacity (AEC)

8. Definition S um total of exchangeable anions that a soil can adsorb. In addition to predominantly negative charge sites which attract soil cations, all soil clays and organic matter simultaneously have a relatively small number of positive charge sites which retain anions in dynamic equilibrium with the soil solution. Unit: milli equivalents per 100 grams of soil ( meq /100 g soil ) or centimoles of negative charge per kilogram soil ( cmol (-)/kg ). Anion exchange capacity  (AEC) represents the positive charge available to attract  anions  in solution. In most soils CEC > AEC. 8

9. Reason Soils  that have an  anion exchange  capacity typically contain weathered kaolin minerals, iron and aluminum oxides, and amorphous materials.  Anion exchange  capacity is dependent upon the pH of the  soil  and increases as the pH of the  soil  decreases. Phosphorus, Sulphate and borate are held on anion exchange site. Govern fixation of phosphorus in soil. 9

10. pH Dependent Charge 10