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Cation Exchange Capacity and Base Saturation

How a little-known soil property affects plant productivity.

Did you know that the chemical form of a nutrient alters how that nutrient reacts with other nutrients in the soil? Knowing not only what types of nutrients your soil needs, but how they react together, is necessary to make your soil the best environment for your crops.

Cation basics

Interpreting cation exchange capacity on your soil test results is a key step to maximizing yield.

A cation is another term for a positively charged atom or molecule. Calcium, magnesium, sodium, potassium and ammonium are all cations found in soil.

The cation exchange capacity, or CEC, is the amount of positively charged cations that a given weight of soil can hold. Cations are held by negative charges in clay and organic matter. The type of clay, amount of organic matter and soil pH all determine the soil's CEC.

The CEC determines how the soil should be managed. A soil with low CEC (less than 5 meq/100g as shown on a soil test) typically has a low organic matter and clay content, holds less water, needs more lime and fertilizer, and is subject to leaching. Generally, soils with low CEC have lower yields, but intensive management can still produce a successful harvest. On a positive note, soils with a low CEC are easy to cultivate and make it easy for plants to quickly take up nutrients.

Consequently, soils with a high CEC (more than 20 meq/100g) have a high clay and organic matter content, can hold more water, need less lime and fertilizer, and are less prone to leaching. However, they might be more difficult to cultivate, irrigate or maintain good aeration, and these soils are more prone to a potassium fixation.

Base saturation

The base saturation rate is also important to know when understanding your soil. The cation base saturation rate is the proportion of the CEC occupied by potassium, calcium and magnesium. These elements are also known as base cations. The base saturation determines how available bases are for plant uptake and influences the soil pH. A high base saturation of CEC (70 percent to 80 percent) should be maintained. Lower base saturation levels will produce very acidic soils.

Your soil's CEC and base saturation rate can be determined from a soil test. For more assistance with interpreting soil tests, please consult your Southern States Agronomy Professional or local Extension office.

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