Soil Cation Exchange Capacity (CEC) and Soil Sampling

Testing for nutrient status of soils that comprise sites that are to be cropped is one of the most-used and trusted processes conducted in support of crop production. The consensus is that should not change even in times of high prices for fertilizer nutrients that are needed to sustain optimum crop production. However, improvement of the soil sampling process should always be a priority since maximum agronomic and economic yield from a growing crop are needed to sustain optimum production and producer viability.

Two recent articles in FarmProgress titled “Data-driven decisions must start with good data” and “Group soils by CEC for testing” (both authored by Tom J. Bechman) present a valid point that should be considered when collecting samples for soil testing–i.e., in order for soil test results to be accurate, they must be based on sound agronomic sampling practices. This likely will cause some producers to implement management changes in their soil sampling process to ensure that a nutrient management plan for a given site/field is based on the use of quality data so that any recommended fertilizer additions are accurately applied to the different areas of a given field.

As the title of the second article indicates, soil samples should be collected and grouped by the CEC (cation exchange capacity) of soils in a production field. Some producers may be fortunate enough to have fields that have uniform soils so that the CECs of those soils have little variation. However, this is not likely the norm–i.e., most fields used for crop production in the Midsouth are populated with soils of different textures that will likely have considerable variation in their CECs.

Before going any further with this discussion about soil sampling and testing being based on CEC, information about soil CEC is presented.

•    CEC is a soil chemical property, and is the total capacity of a soil to hold or adsorb exchangeable cations such as K⁺, Ca²⁺, and Mg²⁺, which are the main or base cations associated with a soil’s CEC at or near neutral pH [7.0]. These basic cations are distinguished from the acid cations H⁺, Al³⁺, and Mn²⁺. Soils with a high percentage of the base cations (high base saturation) have a higher pH, and are generally more fertile because they have little or no acid cations. Percentage base saturation is usually calculated as %BS = [(Ca + Mg + K)/CEC] x 100.

•    CEC is a soil property that is related to that soil’s textural composition and organic matter (OM) content, and is not easily altered. Increasing a soil’s OM content will increase the CEC of that soil, but this is not easily accomplished in the short term.

•    CEC is a soil property that indicates that soil’s capacity to supply nutrients to the soil solution for plant uptake. Soil particles and OM tend to be negatively charged, and thus will attract positively charged molecules such as K, Ca, and Mg that will be available to replenish those that were in the soil solution but were taken up by plant roots.

•    CEC is the amount of exchangeable cations that a soil can absorb at a specific pH, expressed as centimoles of charge per kilogram [cmol(+)/kg] of soil or milliequivalents per 100 g of soil [meq/100 g soil].

•    CEC is directly related to soil composition. Soils that have a high percentage of sand particles have low CEC values, while soils with a high percentage of clay particles and organic matter have high CEC values.

•    The CEC of a soil indicates how well that soil holds on to amendments that are added to it. When applying nutrients to a low-CEC soil, it is best to apply smaller amounts at any one time. Otherwise, those not attached to soil particles will be leached through the soil into the ground water.

•    Soils with a low CEC are more likely to develop K, Ca, and Mg deficiencies because of the susceptibility of those cations to leaching in those soils.

•    Rates of numerous herbicides are CEC-dependent, which is why their label indicates they should be applied in an amount that is based on soil texture.

•    CEC of any soil can only be determined by a soil test, and is commonly measured at a pH of 7.0.

•    The CEC value of a soil obtained from a soil testing laboratory is usually calculated by adding together the concentrations of the base cations K, Ca, and Mg.

•    Soils with a large quantity of negative charges are more fertile because they retain more cations. Also, since soils with a higher CEC value generally have more clay and/or OM, they also have a greater water holding capacity than soils with lower CEC values.

•    As soils become more acidic (i.e., below about pH 5.4), some or all of the base cations are replaced by acid cations such as H⁺, Al³⁺, and Mn²⁺. High concentrations of Al at lower soil pH’s can be toxic to plants.

•    General CEC values at pH 7.0 of soils with different textures are: sand, 1-5; fine sandy loam, 5-10; loam, 5-15; clay loam, 15-30; and clay, >30.

Now back to the subject of the above two articles. Collecting soil samples for nutrient testing should be based on sound agronomic practices so that the resulting data from those samples will be of the highest quality. According to information in the above articles, the main driver for accurate soil test results is collecting samples for aggregation from field areas with similar CECs. Conversely, if samples from field areas with greatly different CECs are aggregated, then the results from subsequent soil tests aren’t going to be accurate for any portion of the sampled area. This will almost assuredly result in both over- and under-fertilization of areas within the sampled field. Thus, a first step in the soil sampling process is to identify sampling zones based on the CEC values of the soils in those zones.

Since CEC values are strongly related to soil texture, producers must be aware of whether or not a particular field has zones with divergent soil textures so that those zones can be sampled and aggregated separately from each other. This likely means that these zones within a field will need to be fertilized at different rates based on soil test results from samples taken from those zones.

Mapping a field according to CEC zones will only need to be done once since the soil texture of divergent zones in a particular field will not change over time. Once a field is mapped and categorized according to CEC zones, these zones can be sampled in a similar fashion from year to year, and changes in nutrient values can be compared across those years to determine trends in nutrient use and availability. Over the long term, this will result in accurate fertilization of a field regardless of crop, nutrients staying where they are applied, and the most economical use of applied fertilizers.

Composed by Larry G. Heatherly, June 2022, larryh91746@gmail.com