The plant roots absorb mineral from the soil solution. In the soil, the minerals are present in two forms: bound and solution form. Those minerals existing as positively charged ions are adsorbed on clay particles because clay particles are negatively charged. Ions are absorbed from the soil by both passive and active transport. Minerals are present in the soil as charged particles [nitrate (NO3⁻), phosphate (HPO4⁻) and potassium ions (K⁺) that cannot move across the cell membrane, and the concentration of minerals in the soil is usually lower than the concentration of minerals in the root.