The clay-humus complex or CEC (Cation Exchange Capacity, binding capacity of the clay-humus complex) is a measure of the soil’s ability to hold nutrients and water and deliver them during the season.
Clay (lutum) particles and humus particles have a negative charge, so they attract and adsorb positively charged nutrients, such as potassium (K), magnesium (Mg), calcium (Ca), ammonium (NH4) and sodium (Na) (the binding of nutrients to soil particles with a surface charge such as clay and organic matter). With adsorption, nutrients are better protected from leaching. The adsorbed nutrients form a reserve available to the crop. At a high CEC, the binding and subsequent delivery capacity is greater than at a low CEC.
Clay soils generally have a higher CEC because the clay particles have many binding sites. In sandy soils, the CEC is overwhelmingly formed by organic matter. Sand particles themselves are inert and bind virtually no nutrients. Clay soils therefore have a higher CEC than sandy soils. The charge and hence the binding capacity of lutum particles is pH-dependent: the binding capacity increases with higher pH. The lutum particles then line up more neatly, creating more space between them.
Image courtesy of BLGG, 2007
Rottink, A., A. Termorshuizen, A. Reijneveld, P. van Vliet, I. Ketelaar en M. Hermans (2007). De bodem doorgrond. BLGG Oosterbeek. ISBN 978-90-812265-1-6. 96 p.