What factors influence soil's cation exchange capacity?

Soil's cation exchange capacity is influenced by its clay content, organic matter content, and soil pH.

Cation exchange capacity (CEC) is a critical property of soil that determines its ability to hold and exchange cations (positively charged ions). The three main factors that influence this capacity are the soil's clay content, organic matter content, and pH level.

Firstly, the clay content of the soil plays a significant role in its CEC. Clay particles are negatively charged, which allows them to attract and hold onto positively charged cations. Different types of clay have different CEC values. For instance, smectite clay has a high CEC while kaolinite clay has a low CEC. Therefore, soils with a higher proportion of clay, particularly those types with a high CEC, will have a greater capacity to hold and exchange cations. For more detailed insights into how these components interact within different soil types, you can refer to Soil Composition Basics.

Secondly, the organic matter content of the soil also influences its CEC. Organic matter, like clay, is negatively charged and can attract and hold cations. The more organic matter a soil contains, the higher its CEC will be. Organic matter not only contributes to the CEC but also improves the soil structure, water holding capacity and provides nutrients to the plants. Understanding the role of Soil Organic Matter can provide further clarity on its impact.

Lastly, the pH level of the soil can affect its CEC. In general, soils with a higher pH (more alkaline) have a higher CEC. This is because alkaline conditions increase the negative charge on clay and organic matter, allowing them to hold more cations. Conversely, in acidic soils, the H+ ions compete with other cations for the exchange sites, reducing the CEC. The influence of soil structure and layering on these properties can be explored further in Soil Profiles and Horizons.

IB ESS Tutor Summary: Soil's ability to hold and exchange nutrients, known as cation exchange capacity (CEC), depends mainly on its clay and organic matter content, and pH level. More clay and organic matter increase CEC, as does a higher pH (more alkaline soil). Simply put, soil with more clay, organic matter, and a less acidic environment can better nourish plants by holding onto essential nutrients.