Chemical Properties of Soil
Cation Exchange Capacity
Cation exchange is the interchange between a cation in solution and another cation on the surface of any negatively charged material colloid. Cation exchange capacity (CEC) is a measure of the total negative charges within the soil that adsorb plant nutrient cations such as ammonium nitrogen (NH4⁺), calcium (Ca2⁺), magnesium (Mg2⁺), potassium (K⁺), iron (Fe2⁺), manganese (Mn2⁺), zinc (Zn2⁺), and copper (Cu2⁺). Other cations of importance are hydrogen (H⁺), aluminum (Al3⁺), and sodium (Na⁺). For example, each adsorbed K⁺ ion contributes one + charge, and each adsorbed Ca2⁺ contributes two + charges to the CEC.
Nature of Cation Exchange
In soil, the negative and positive surface charges on the colloids attract and hold a complex array of cations and anions. In temperate-region soils, cations are absorbed in much larger quantities than anions because these soils generally contain predominately 2:1-type silicate clays (e.g., smectite and vermiculite) on which negative charges predominate. In tropical regions adsorption of anions predominate because soils are more highly weathered, acid, and consist of 1:1 clays (e.g., kaolinite, halloysite) of iron and aluminum oxides.
Cation Exchange Capacities of Soils
Cation exchange capacity of a soil is the function of the amount and type of colloids (clay, organic matter) and soil pH. There is an increase in CEC as organic matter becomes more decomposed or more humified with clays have great variation in CEC. As a consequence, the CEC of soils is affected mainly by the amount and type of clay and the amount and degree of decomposition of the organic matter. In the A horizons of mineral soils, the organic matter and clay frequently make similar contributions to the CEC.
Soil Colloids Effect on pH CEC
Cation exchange capacity is related to several factors including the amount and type of clay and organic matter in soils. The CEC of organic matter is highly pH-dependent whereas most of 2:1 clays is permanent or non-pH dependent.
Buffering Capacity
A high CEC soil will also have a greater buffering capacity, increasing the soil’s ability to resist rapid and large changes in pH, which also protects nutrient availability and plant health in soil. Soils with high amounts of clay and/or organic matter will typically have higher CEC and buffering capacities than more silty or sandy soils. As CEC increases for a soil, it is able to retain more of these plant nutrients and reduces the potential for leaching.
Base Saturation
Soil fertility is influenced not only by CEC (how many cations it can store) but also by how much of the CEC is actually filled with plant nutrients. Exchange sites may be filled by members of two groups of cations. One group consists of H⁺ and Al3⁺ ions, which are not plant nutrients. Their primary contribution is to acidify the soil. The other cations are called exchangeable bases and include elements such as Ca2⁺, Mg2⁺, K⁺ ions, and Na⁺.
Managing Soil CEC
The higher the CEC the more clay or organic matter present in the soil. This usually means that high CEC (clay) soils have a greater water holding capacity than low CEC (sandy) soils. Low CEC soils are more likely to develop potassium and magnesium (and other cation) deficiencies, while high CEC soils are less susceptible to leaching losses of these cations. Cation exchange capacity influences fertilization practices. High-CEC soils have greater potential to hold cationic nutrients than low-CEC media. Smaller amounts of fertilizer, applied more often, are needed in low-CEC soils to prevent leaching losses, while larger amounts may be applied less frequently in high-CEC soils.
Click on the following topics for more information on chemical properties of soil.
