Chemical properties

(1)CEC (Cation exchange capacity)

1.1 What is the CEC? 

                     CEC is the total capacity of a soil to hold exchangeable cations otherwise, CEC is capacity of the soil to hold on to positively charge cations.

                                   Cations are positively charged ions such as calcium (Ca2+), magnesium (Mg2+), and potassium (K+), sodium (Na+) hydrogen (H+), aluminum (Al3+), ironFe2+), manganese(Mn2+), zinc (Zn2+) and copper (Cu2+).These cations are held by negatively charged clay particles and organic matter.

Clay particles attract cations of Al, Si, and Fe to negatively charge sites by ionization of OH groups. The negative charge of the clay particles is balanced by the positive charge of the cations in the soil.

       Organic matter also attracts cations to their negatively charged groups by ionization of COOH, NH2 and OH- groups.

                 The cations on the CEC of the soil particles are easily exchangeable with other cations and as a result, they are plant available. Thus, the CEC of soil represents the total amount of exchangeable cations that the soil can adsorb. The cations used by plants in the largest amounts are calcium, magnesium, and potassium.

1.2CEC of different type of soils

Soil textural class                      CEC(cmolc/kg)

Sands(light color)                      2-4

Sands(dark color)                      8-16

Loams                                         10-15  

Silt loams                                   15-25

Clay and clay loams                  20-50

Organic soils                              50-100

Sand has no capacity to exchange cations because it has no electrical charge. This means sandy soils such as podzolic top soils have very low CEC, but this can be improved by adding organic matter.

                                      Minerals soil particles

Sand                                  Clay

 Si2O4                                               SiAlO4-

                             No charge.              Negatively charge.

             Does not retain Attracts     Attracts and retains 

             and retains cations.            Cations.

  Silica in clay soil is substituted by aluminum so clay soils get negatively charged sites therefore clay soils can hold positively charged cations at their negatively charged sites.

                    Organic matter has relatively larger CEC than clay soils because dissociation of organic acids, alkanes and alcohols got relatively more CEC sites than clay soils.        

                         1.3 Factors affecting CEC 

1) Amount of clay  

                                                         With the increasing of clay and

                                             organic matter, negative sites

                                                         per unit surface are is also high.

                                                                                               Therefore CEC is relatively higher.       
                                                                                     
2) Amount of organic matter

3) Type of clay- The chemical conditions that were present when the clay was crystallizing determine the amount of negative charge locked in crystals (called isomorphic substitution). This negative charge is a part of the clay and does not readily change.

4) PH- the CEC provided by organic matter is entirely determined by the soil pH. As the soil becomes more acidic, organic matter loses its ability to retain cations. When the pH of soil is netural, that soil has more CEC than pH of 5.5 of soils. The CEC of a soil is increased with the increasing of pH.Clay also has a pH-dependent charge. For some clay such as kaolinite, 50% or more the CEC can be pH dependent, while for other clays, such as montmorillonite or vermiculite, less than 10% of the CEC is affected by soil pH.

1.4Importance of CEC

1. Indicates the nutrient holding capacity of a soil. (With the increasing of CEC, fertility of soil is also increased therefore CEC can be used as an indicator to reorganize fertertile of soils.)

2. The amount and adding duration of lime is decided by using CEC.

 3. The CEC determines how to crop nutrients must be applied. Whether the material may be broadcast or placed in a band.