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Sustainable Agriculture: Programs

Soil Chemical Characteristics

We often tend to associate soil fertility with chemical properties of soils. Soil chemical properties are affected by physical properties (organic matter and texture for example) and by soil biology. Soils in Georgia are old, meaning they are highly weathered. Thousands of years of physical and chemical weathering have taken the original parent materials and formed secondary minerals and colloids. Colloids include clay minerals, iron oxides, and humus. This part of the soil is high in surface area and therefore is the most reactive part of the soil.

There are many different types of clay minerals, each with different properties and effects on soils. In Georgia, kaolinite is the most common clay mineral. Relatively speaking, kaolinite is low in surface area and charge. This means that the clay minerals in our soils have little ability to retain nutrients or a low cation exchange capacity.

Picture of a bucket and soil sampling deviceCation exchange capacity (CEC) is the ability of a soil to retain nutrients. Cations are positively charged ions such as calcium (Ca++), magnesium (Mg++), and potassium (K+). Because soils in Georgia are sandy or typically have clays with low CEC, management is important to maximize sustainability. Liming increases CEC by allowing ions such as calcium, potassium and others to bind to iron oxides. Soil organic matter (SOM) also has an important impact on increasing CEC and improving soil fertility. SOM is more chemically complex than iron oxides, and can hold several different chemical groups that have different optimal pH ranges. The take-home message for improving the fertility in soils and retaining nutrients is to increase the amount of SOM in your soil to provide more sites for nutrient retention and keep soil pH (range of 6.0-6.5 for most plants) to maintain a healthy amount of nutrients available to plants. You can determine the fertility of your soil by collecting soil samples and having them analyzed by a lab (More info ).