Basic Soil-Plant Relationships Chapter Basic Soil-Plant Relationships

Basic Soil-Plant Relationships

Absorption Plants absorb essential elements, usually in ionic form, from the soil solution. As ions are depleted in the soil solution, chemical and biological reactions occur that replenish the ions that were absorbed. Immobile soil nutrient ions are replenished by ion exchange reactions, soil particle surface desorption, dissolution of precipitated forms, and mineralization of organically bound forms of the nutrients. Mobile soil nutrients are replenished as soil water continues to move toward the root surface, carrying soluble nutrients from distant soil. Depending on the nutrient, these processes have varying degrees of importance.

CEC Most soils have a net negative charge because of the negative charges on layer silicates and organic matter

CEC and AEC

1:1 clays Ion exchange in soils Cation exchange 1:1 layered alumino silicates (clays), Kaolinite pH dependent charge

2:1 clays Idealized end members, in the spectrum of possible isomorphic substitutions, are clays without isomorphic substitution pyrophyllite, a dioctahedral mineral (all cations in the octahedral layer are Al 3+ , which occupy 2/3 of the “spaces” in the layer talc, a trioctahedral mineral (all cations in the octahedral layer are Mg 2+ , which occupy 3/3 of the “spaces” in the layer) illite: derived from the dioctahedral mineral muscovite where the octahedral layer is occupied by Al 3+ , and the trioctahedral mineral biotite, where Mg 2+ and Fe 2+ occupy the octahedral positions instead of Al 3+ . Negative charge (CEC) arises from substitution of Al 3+ for Si 4+ in the silicon tetrahedral layer. K + is similar in size to the “holes” in the base of the tetrahedral layer and becomes “fixed” between crystal lattices in the mineral. These minerals are a rich source of K. montmorillonite: Like pyrophyllite except that Mg 2+ has substituted for some of the Al 3+ in octahedral layer and accounts for high CEC.

Soil organic matter CEC arises as a result of H+ being dissociated from carboxylic, phynolic, and other hydroxyl groups. pH dependent charge related to CEC from organic matter and broken edges of clays.

Anion exchange Some highly weathered soils dominated by allophane and hydrous oxides may have a net positive charge at low pH (Anion Exchange Capacity) Strength of anion adsorption is: HPO42- > SO42- > NO3- = Cl- Important for phosphates and sulfates. Chlorides and nitrates are too weakly adsorbed to significantly compete with phosphates and sulfates. Most important in highly weathered, acid soils (tropics).

Movement of ions from soil to roots 1. Contact exchange or root interception roots have CEC which interacts with soil CEC Overlapping oscillation volumes of ions allows for ions to exchange places on sites of adsorption

Ion Movement 2. Diffusion 3. Mass flow Accounts for the majority of P and K movement from soil solution to root surface Concentration gradient is driving force Effective in ion movement over short distances (e.g. P 0.02 cm; K 0.2 cm) 3. Mass flow water movement in response to transpiration, evaporation, and rainfall (irrigation) are driving forces. Most important for ions in relative abundance in the soil solution (Ca, Mg, and NO3-N

Plant uptake of nutrients Passive uptake Nutrients moved from high concentration in the soil solution to lower concentration in the apparent free space (between cell in the cortex or outer layer of cells) by diffusion and ion exchange (cells have an internal negative charge). Indiscriminate & does not require expenditure of plant energy. The process occurs outside the Casparian strip and plasmalemma that are barriers to diffusion and ion exchange. Active uptake Nutrients are moved against a concentration gradient across the plasmalemma by a carrier mechanism. The process requires expenditure of plant energy.