All soils contain calcium ions (Ca₂+) and magnesium (Mg₂+) cations (positively charged ions) attracted to the negative exchange sites on clays and organic matter (cation exchange complex of the soil). The amount and relative proportion usually reflect the soil's parent materials. Calcium (Ca) and Mg are plant-essential nutrients, and the ionic form of each held on the soil exchange sites is the form taken up by plants. The usual approach for determining whether the soil supply is sufficient to meet crop needs is to extract soil with 1 molar (M) ammonium acetate (the same procedure used to determine soil test potassium) and evaluate the amount measured against critical levels.