Cation exchange capacity is the ability of the medium to attract positive charged ions called cations and hold them. Cations are the chemical molecule of various minerals and other nutrients. The medium must be porous and have a negative chemical charge that attracts the positive charged cations. In simple terms it holds the nutrients like a sponge so that when it comes into contact with plant roots, the roots can absorb these nutrients.
"Organics" do not lower the pH. Organic acids do. Minerals, slag, gravels are not organic. Organics are vegetable, plant, and feces matter. Some organic matter such as peat and humus have CEC. Some media attracts negative ions which are called Anions. Some media retains small quantities of anions, negatively charged ions, in addition to cations. However, anion exchange capacities are usually negligible, allowing anions such as nitrate (NO3-), chloride (Cl-), sulphate (SO4-), and phosphate (H2PO4-) to leach from the media.
Important cations in the cation exchange complex in order of adsorption strength include calcium (Ca2+) > magnesium (Mg2+) > potassium (K+) > ammonium (NH4+), and sodium (Na+). Micronutrients which also are adsorbed to media particles include iron (Fe2+ and Fe3+), manganese (Mn2+), zinc (Zn2+), and copper (Cu2+).
Robert Paul Hudson
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