Ion exchange in soil

This will help you in understanding the concept of ion exchanging in the soil through surface physical adsorption.

1. 1 Ion-exchange in soil Mr . Muhammad Ullah Afridi

2. Soil Soil is the thin layer of organic and inorganic materials that covers the Earth's rocky surface. Soil is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life Basic soil properties can be divided into three categories. 1.physical properties(Texture,structure,bulk density etc) 2.chemical properties(salinity, ph,organic matter, mineral content etc) 3.biological properties (biomass,biodiversity etc) 3

3. Ion exchange Ion exchange is a cyclic process. That means the exchange of ions on soil surface with ions in soil solution. Ion exchange involves cations and anions that are adsorbed from solution onto negatively and postively charged surfaces, respectively . Such ions are readily replaced or ex-changed by other ions in the soil solution of similar charge, and thus, are described by the term, ion exchange. (Foth, 1990) There are two types of ions , they are cations and anions. Cation exchange is greather abundance in soil than anion e xchange . 6

4. Cation Exchange Capacity: Cation exchange capacity is the value of a soil to indicate how much cation nutriens have the soil. Cation exchange is the interchange between a cation in solution and another cation on the sur- face of any negatively charged material, such as clay colloid or organic colloid. Assume for purposes of illustration that X - represents a negatively charged ex-changer that has adsorbed a sodium ion (Na), producing NaX. When placed in a solution containing KC l , the following cation exchange reac-tion occurs: 7

5. In the reaction, K + in solution replaced or ex changed for adsorbed (exchangeable) Na', re-sulting in putting the adsorbed Na' in solution and leaving K + adsorbed as KX. Cations are ad-sorbed and exchanged on a chemically equiva-lent basis, that is, one K+ replaces one Na', and two K + are required to replace or exchange for one Ca ++ . There are many cation in soil but all are not absorbed in same amount . There is an sequence That is. :Ca > Mg > K > Na. Percentage is : * CEC is high in clay soil and low in sandy soil because in clay soil organic matter is high. 8 Ca(80%) > Mg(15%) > K(1.8%) > Na(1%)

6. Anion Ecxhange Capacity: * Anion exchange sites arise from the protonation of hydroxyls on the edges of silicate clays and on the surfaces of oxidic clays. The anion exchange capacity is inversely related to soil pH and is, perhaps, of greatest importance in acid soils dom- inated by oxidic clays. Nitrate (N0 3 - ) is very weakly adsorbed in soils and, as a consequence, remains in the soil solution where it is very sus-ceptible to leaching and removal from soils. In general, plants absorb as many anions as cations. The availability to plants of the anions nitrate, phosphate, and sulfate is related to miner-alization from organic matter, as well as anion exchange. Ion exchange capacity VS soil pH: CEC and AEC both depend on soil pH. Because pH indicates the soil's character as the soil is acidic or basic. To make valid comparisons of CECand AEC between soils and various materials, it is neces-sary to make the determination of CEC at a common pH. The CEC is positively correlated with pH; therefore, acid soils have a CEC le than the maximum potential CEC. On the other hand AEC is negatively correlated with pH. The CEC at the soils' current pH is called the effective cation exchange capacity (ECEC). The relative proportion of acidic or basic ions on the exchange sites determines the soil's pH value. * Sandy soils with lower pH are more subject to leaching of nutrients. 9

7. Soil pH: The common range of soil pH is 4 to 10. The pH is a negative logarithm of Hydrogen Ion activity and Soil pH is a measure of soil’s alkalinity or acidity The pH of a soil is one of the most important properties involved in plant growth. Soil pH is usually measured in a standard suspension of 1:2.5(weight to volume) of soil in distilled water, or in dilute solution of calcium chloride(0.01M). Determination of Soil pH: Soil pH is commonly determined by: mixing one part of soil with two parts of distilled water or neutral salt solution, occasional mixing over a period of 30 minutes to allow soil and water. to approach an equilibrium condition, and, measuring the pH of the soil-water suspension using a pH meter. 11

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