Water & SoilWater & SoilAssessment StatementsAssessment Statements

E.12.2E.12.2State what is meant by the termState what is meant by the termcation-exchange capacity (CEC) andcation-exchange capacity (CEC) andoutline its importanceoutline its importance

E.12.3E.12.3Discuss the effects of soil pH onDiscuss the effects of soil pH oncation-exchange capacity andcation-exchange capacity andavailability of nutrientsavailability of nutrients

E.12.2E.12.2 Cation-exchange capacity (CEC) is the capacity Cation-exchange capacity (CEC) is the capacity

of a soil for ion exchange of cations between the of a soil for ion exchange of cations between the soil and the soil solution. soil and the soil solution. Cation is a positively charged ion Cation is a positively charged ion

CEC is used as a measure of fertility, nutrient CEC is used as a measure of fertility, nutrient retention capacity, and the capacity to protect retention capacity, and the capacity to protect groundwater from cation contamination groundwater from cation contamination

The two main particles in the soil are clay and The two main particles in the soil are clay and humushumus

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The CEC of the soil is determined by The CEC of the soil is determined by the amount of clay and/or humus that the amount of clay and/or humus that is present. is present. Cation ‘warehouse’Cation ‘warehouse’ Nutrient and water holding capacityNutrient and water holding capacity

Sandy soils with very little organic Sandy soils with very little organic matter have a low CECmatter have a low CEC

Heavy clay soils with high levels of Heavy clay soils with high levels of organic matter have a much greater organic matter have a much greater capacity to hold cations. capacity to hold cations.

E.12.3E.12.3 Rise in pH = rise in CEC Rise in pH = rise in CEC  Acidic soil will have high concentrations Acidic soil will have high concentrations

of H+ and Al3+of H+ and Al3+ In neutral to moderately alkaline soils, In neutral to moderately alkaline soils,

Ca2+ and Mg2+ will be prevalent Ca2+ and Mg2+ will be prevalent Exchangeable cations are available to Exchangeable cations are available to

plantsplants pH can have the following effects:pH can have the following effects:

K uptake by plants is limited by high levels of K uptake by plants is limited by high levels of Ca in some soils.  Ca in some soils. 

High levels of K can in turn, limit Mg uptake High levels of K can in turn, limit Mg uptake even if Mg levels in soil are high. even if Mg levels in soil are high.