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Cation exchange capacity (CEC) and total exchangeable cations (TEC) are two significant concepts in soil fertility.  Cations refer to the positively charged nutrients in the soil, e.g. Ca2+ and K+. They are important as they give you an idea of how many cations a soil can potentially hold and how many cations are currently being held. Understanding exactly how these soil properties influence soil fertility and applying soil management systems that enhance these properties can assist in improving pasture quality and yield.

CEC is defined as the degree to which a soil can adsorb (hold/capture) and exchange cations with the soil solution1. This term is often confused with a soils TEC which refers to the number of basic cations that are held on the soil exchange sites (CEC sites) in comparison to the total sites and is usually reported in cmol(+)/kg soil. The ability of the soil to hold nutrients is greatly influenced by the soils organic matter (OM) content, which is mostly made up of carbon, as well as the clay content of the soil. The reason for the interaction between these two soil parameters is because cations are positively charged, and in order to be held in the soil, they require a negatively charged surface for them to be adsorbed. Clay and OM have negatively charged surfaces and therefore have the ability to hold cations. The more cation exchange capacity a soil has, the more likely the soil will have a higher fertility level.

The soils exchange sites are refilled from the soils reserves. The reserves for each specific nutrient can be determined by measuring the parts per million (ppm) concentration of the nutrient in the soil. The cations that are often accounted for when determining a soils TEC are Calcium (Ca2+), Potassium (K+), Magnesium (Mg2+), Sodium (Na+), Hydrogen (H+) and Aluminium (Al3+). This is because these cations usually take up 80 to 90% of the soils CEC.

These soil properties (CEC, TEC, OM and clay) were measured on three different farms in the Tsitsikamma region, Eastern Cape, South Africa. The amount of clay and OM is tabulated below for each farm.

Table 1: Clay and OM percent for farm A, farm B and farm C

  • Farms and Depths
  • Farm A (0-60 cm)
  • 0-15 cm
    15-30 cm
    30-45 cm
    45-60 cm
  • Farm B (0-60 cm)
  • 0-15 cm
    15-30 cm
    30-45 cm
    45-60 cm
  • Farm C (0-60 cm)
  • 0-15 cm
    15-30 cm
    30-45 cm
    45-60 cm
  • Clay %
  • 5
  • 2
    9
    6
    3
  • 6
  • 2
    8
    8
    5
  • 3
  • 2
    2
    3
    6
  • Organic Matter %
  • 3.7
  • 7.1
    4.5
    1.9
    1.3
  • 1
  • 2.3
    0.9
    0.5
    0.3
  • 1.3
  • 2.9
    1.6
    0.3
    0.5

Figure 1: Average CEC, TEC and OM throughout depth on Farm A

The soils on Farm A are predominantly sandy (an average of 90% sand has been measured with the remaining 10% being attributed to clay and silt).  However, because of the accumulation of OM, the soils have a reasonable nutrient holding capacity. The latter is confirmed by the availability of cation exchange sites in the soil.  It is important to note that unlike clayey soils which are naturally negatively charged and therefore have the ability to hold nutrients without carbon additions, sandy soils do not have that ability, unless carbon is built in the soil. The cation exchange sites have been observed to decrease as you move down the soil profile. This is because on the farm, OM has also been observed to decrease moving down the profile (Table 1).  This soil has a potential to hold more cations in the 0-15, 15-30 and 30-45cm depths. In the graph above it is shown that the exchange sites are not all completely filled, except in the 45-60cm depth, therefore this soil has the potential to hold more nutrients. This is clear evidence that nutrients are moving through the soil profile to deeper soil layers, where they may be lost if not fetched by deep rooted pasture crops.

Figure 2: Average CEC, TEC and OM throughout depth on Farm B

A similar trend to that of farm A in terms of OM is observed on farm B as well. Even though the soils are very low in OM, they have a relatively high CEC especially on the 15-30, 30-45, and 45-60 cm increments. This can be attributed to the high clay content measured on the lower profiles of farm B. The danger of having too much clay and too little OM is in that the soils will be susceptible to compaction. The result of that will be pruning of the grass roots growing due to the difficulty to explore the lower profiles because of too much clay that forms a root growth barrier. Also one other significant impact of high clay content would be that of poor water percolation, infiltration and aeration which is essential not only to the plant but also the organisms inhabiting the soil.

Figure 3: Average CEC, TEC and OM throughout depth on Farm C

Farm C closely resembles an average between farm A and B. It has a poor CEC because it has an average CEC of 3.6 cmol (+)/kg soil compared to the 5.5 and 6.6 cmol (+)/kg soil average of farms A and B. The farm has an average OM content of 1.3%, which is higher than the average of 1% on farm B, however, farm B has a higher average clay content than farm C and therefore having a slightly higher CEC than farm C.

All three farms show that the soils have cation exchange sites throughout the 60cm depth measured. This is a good indicator considering the sandy nature of the soils. There is still a need for ample additions of carbon in these soils in order to build up even greater nutrient accumulation and uptake by plants.

The data from this case study highlights the importance of building Carbon in soils, especially ones with low clay content, in order to ensure that the soil has the ability to hold larger quantities of nutrients, which can later be used by plants for growth.

References

1. Rengasamy P, Churchman GJ, (1999), Cation Exchange Capacity, Exchangeable Cations and Sodicity. In. Soil Analysis an Interpretation Manual. (Eds KI Peverill, LA Sparrow and DJ Reuter). CSIRO: Melbourne.

Portia Phohlo
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