The clay fraction in soil is made up of the smallest soil particles (<2um). The smaller the size of a particle, the greater the relative surface area for adsorption. Clays – and there are many types of clay with different properties – have a large external surface area and some also have a large internal surface area between layers. Due to this, clay particles can hold nutrients such as magnesium, potassium and calcium well and provide a basis for a fertile soil.
Clay (%) is the percentage of clay that is present in the soil. The presence of clay influences the soil texture. A higher percentage of clay increases the water-holding capacity and creates smaller pores which are beneficial for certain micro-organisms like bacteria and protozoa.
The clay / SOC ratio is an indicator of the structural condition of the soil. Generally, the soil structure quality increases with a decreasing clay / SOC ratio. A clay / SOC ratio of 8:1 is an average for a very good structure quality, whereas a clay / SOC ratio of 13:1 or higher is less favourable (Johannes et al., 2017) .