Clay particles and some organic soil constituents carry a negative charge that allows them to adsorb positively charged nutrients (cations) such as NH4+, K+, Ca2+, Mg^+, Fe3+, Mn2+, Cu2+, and Zn2+. Adsorbed cations can be exchanged, in chemically equivalent amounts, with those present in the soil solution. The adsorption of cations on soil particles minimizes their leaching losses to groundwater. The exchange replenishes the soil solution with nutrients depleted by root uptake. Soils have a low positive charge; hence, anions such as NO3-, BO3-, and Cl- are easily leached, but H2PO4-, MoO42-, and SO42-are specifically adsorbed or chemically react and precipitate and thus are effectively held by the soil.
Plant roots absorb nutrients primarily from the soil solution and secondarily from the soil exchange complex in contact with the root. Most of the nutrients reach a root's surface by mass flow or diffusion. The mass flow component is the product of the concentration of nutrients in the soil solution and the volume of plant water uptake. If a given nutrient is taken up faster than water, then that nutrient is gradually depleted in the immediate root vicinity, creating a diffusion gradient for its movement from bulk soil toward the root surface.
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