The soil’s ability to store certain essential elements for plant growth is measured by its TOTAL CATIONIC EXCHANGE CAPACITY. Elements that are positively charged when in their ionic form are called cations. Here are the cationic elements of interest for plant growth:
- Calcium (Ca++)
- Magnesium (Mg+)
- Potassium (K+)
- Sodium (Na+)
- Copper (Cu++)
- Iron (Fe+++)
- Manganese (Mn++)
- Zinc (Zn++)
- Ammonium (NH4+)
Clay particles and humus in the soil have negatively charged receptors on them that hold cations in the soil. The more clay and humus, the more capacity to hold cations = cationic exchange capacity. Clay is not quite as efficient as humus is, though. Humus can hold 2x to 4x more cations than can clay.
The technical definition of Total Cationic Exchange Capacity is: the sum of all the effective number of exchangeable cations per dry weight that a soil is capable of holding, at a given pH value, and available for exchange with the soil water solution.
If the TCEC is low, then the nutrients in the soil aren’t locked in place waiting to be used – instead, the nutrients are lost by leaching or other movement.
A good analogy is this: if the plants are feeding at a dining room table, the soil solution nutrients are like food on their plates; the TCEC is food in the pantry, ready to be put on the table when needed. The bigger the pantry, the longer dinner can go on.
TCEC is measured in milliequivalents (meq) per 100 g soil. For example, if the TCEC is 8, then it is capable of holding 8 meq of cations. To put this in terms for the garden, multiply the TCEC by the millequivalent weight for the elements of interest. For example, calcium is 400 lb/meq, so 8 meq is equal to 8 meq * 400 lb/meq = 3200 lb per acre. Keep in mind that it’s never just one element of interest, so then you would bring in the relative percentages desired for each element.
|Cationic Element||milliequivalent weight (meq)|
|Calcium (Ca++)||400 lb|
|Magnesium (Mg+)||240 lb|
|Potassium (K+)||780 lb|
|Sodium (Na+)||460 lb|
Alternatively, CEC can also be stated in cmolc/kg (centimoles of charge per kilogram of dry soil).
Estimating CEC from Soil Tests (CECsum)
The CEC can be estimated from a soil test’s measurements of Ca, K, and Mg, although it is not as accurate as direct laboratory test methods. This is especially true for calcareous soils and soils that have recently been amended with lime or fertilizer. If your soil pH is higher than 7.5, or has recently been limed or fertilized, do not use this method.
- Choose an equation based on the units your soil test results are in:
- ppm: CECsum (meq/100g soil) = (ppm Ca / 200) + (ppm Mg / 120) + (ppm K / 390)
- lb/acre: CECsum (meq/100g soil) = (ppm Ca / 200) + (ppm Mg / 120) + (ppm K / 390)
- If your soil has a pH of 6 or less, exchangeable acidity should also be included in the calculation: Most states estimate exchangeable acidity from a regression equation between 1 M KCl exchangeable acidity (Thomas, 1982) and the lime requirement buffer pH. This value is added to the CECsum from step 1. Contact the soil testing laboratory in your state for the appropriate equation to estimate exchangeable acidity from soil buffer pH.