Notes from ISA exam study session Soils function as chemically active systems largely because soil particles carry electrical charges. Clay minerals and organic matter possess a net negative charge, which allows them to attract and temporarily retain positively charged nutrient ions. This property underlies cation exchange capacity, the soil’s ability to hold and exchange nutrient cations such as calcium (Ca²⁺), magnesium (Mg²⁺), potassium (K⁺), ammonium (NH₄⁺), iron (Fe

Plant growth depends on mineral nutrients that occur in soil as dissolved ions or weakly bound to soil particles. While carbon is obtained from atmospheric carbon dioxide and hydrogen and oxygen from water, all remaining essential nutrients are absorbed through roots in specific chemical forms. Nitrogen is taken up primarily as nitrate (NO₃⁻), a highly mobile anion, and as ammonium (NH₄⁺), a cation that can be retained on soil exchange sites.