Methods for Determining Soil Cation Exchange Capacity
Chapter 9
Recommended Methods for Determining
Soil Cation Exchange Capacity
Donald S. Ross and Quirine Ketterings
The cation exchange capacity (CEC) of a soil is a measure of the quantity of negatively charged sites on soil surfaces that can retain positively charged ions (cations) such as calcium
(Ca2+), magnesium (Mg2+), and potassium (K+), by electrostatic forces. Cations retained electrostatically are easily exchangeable with cations in the soil solution so a soil with a higher
CEC has a greater capacity to maintain adequate quantities of Ca2+, Mg2+ and K+ than a soil with a low CEC. A soil with a higher CEC may not necessarily be more fertile because a soil’s CEC can also be occupied by acid cations such as hydrogen (H+) and aluminum (Al3+). However, when combined with other measures of soil fertility, CEC is a good indicator of soil quality and productivity. Cation exchange sites are found primarily on clay minerals and organic matter (OM) surfaces. Soil OM will develop a greater CEC at near-neutral pH than under acidic conditions
(pH-dependent CEC). Thus, addition of an organic material will likely increase a soil's CEC over time. On the other hand, a soil’s CEC can decrease with time as well, through e.g. natural or fertilizer-induced acidification and/or OM decomposition.
Soil CEC is normally expressed in one of two numerically equivalent sets of units: meq/100 g (milliequivalents of charge per 100 g of dry soil) or cmolc/kg (centimoles of charge per kilogram of dry soil).
Because of the differing methods to estimate CEC, it is important to know the intended use of the data. For soil classification purposes, a soil’s CEC is often measured at a standard pH value. Examples are the ammonium acetate method of Schollenberger and Dreibelbis (1930) which is buffered at pH 7, and the barium chloride-triethanolamine method of Mehlich (1938) which is buffered at pH 8.2. Such CEC measures can result in values very different from the
CEC of
Recommended Methods for Determining
Soil Cation Exchange Capacity
Donald S. Ross and Quirine Ketterings
The cation exchange capacity (CEC) of a soil is a measure of the quantity of negatively charged sites on soil surfaces that can retain positively charged ions (cations) such as calcium
(Ca2+), magnesium (Mg2+), and potassium (K+), by electrostatic forces. Cations retained electrostatically are easily exchangeable with cations in the soil solution so a soil with a higher
CEC has a greater capacity to maintain adequate quantities of Ca2+, Mg2+ and K+ than a soil with a low CEC. A soil with a higher CEC may not necessarily be more fertile because a soil’s CEC can also be occupied by acid cations such as hydrogen (H+) and aluminum (Al3+). However, when combined with other measures of soil fertility, CEC is a good indicator of soil quality and productivity. Cation exchange sites are found primarily on clay minerals and organic matter (OM) surfaces. Soil OM will develop a greater CEC at near-neutral pH than under acidic conditions
(pH-dependent CEC). Thus, addition of an organic material will likely increase a soil's CEC over time. On the other hand, a soil’s CEC can decrease with time as well, through e.g. natural or fertilizer-induced acidification and/or OM decomposition.
Soil CEC is normally expressed in one of two numerically equivalent sets of units: meq/100 g (milliequivalents of charge per 100 g of dry soil) or cmolc/kg (centimoles of charge per kilogram of dry soil).
Because of the differing methods to estimate CEC, it is important to know the intended use of the data. For soil classification purposes, a soil’s CEC is often measured at a standard pH value. Examples are the ammonium acetate method of Schollenberger and Dreibelbis (1930) which is buffered at pH 7, and the barium chloride-triethanolamine method of Mehlich (1938) which is buffered at pH 8.2. Such CEC measures can result in values very different from the
CEC of