Cation exchange capacity

The map provides information relating to the cation exchange capacity (CEC) of soils in the region described. The CEC of a soil is a measure of the cations that a soil can retain on its negatively charged surfaces, which are found on clay particles and on soil organic matter. As CEC in soils increase they can hold more nutrients such as K+, NH4+, Mg2+, Ca2+, Zn2+, particularly if pH is maintained around pH 7. Thus clay soils generally have higher CEC values due to their clay content and generally holding more organic matter, whereas sandy soils generally have lower CEC values, where greater leaching of nutrients is expected. Soils with high CEC values may also hold more water and will require more lime to correct to a given pH.

The CEC values were estimated from the organic matter determined as % LOI from the G-BASE survey and converted to % C using the recognised conversion factor of 0.58 and the mean % clay from classes of soil texture from the BGS Soil Parent Material Model. Estimates were calculated using the mean values for CEC produced from 3 different pedo-transfer functions used to predict CEC. The median values estimated as Cmolc kg-1 (Centimole of charge per Kg) from the G-BASE sample points associated with each soil parent material were calculated and assigned to one of three classes (low, medium, high).

For farmers, understanding soil CEC can lead to an improved understanding of the timing of fertiliser and lime applications, to improve efficiency.

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Soil with plant, high CEC content