Soil Water
Soil Water Potential
The soil water content discussed above should not be confused with the soil water potential, which refers to the potential energy contained in the soil water available for plant uptake. The soil water potential (also called soil water tension) is the amount of work required to move (extract) water from the soil, and it is the driving force of water in soil. More simply, one can think of potential as the tendency of water to flow or move freely in the soil; the higher the water potential, the more freely it can move. The more freely water can move, the more available it is to plants. Water moves from locations of high potential to locations of lower potential, from wetter to drier soil. An important point about the soil-water potential concept is that what matters to a plant is not so much the amount of water in the soil as it is the energy, or potential, of that water.
Gravitational Potential
Gravitational potential is due to the force of gravity pulling on the water elevated above the water table. To achieve a lower energy state, water simply percolates through the soil to a lower elevation. Gravitational potential is usually a positive value.
Matric Potential
When a soil is unsaturated and contains no gravitational water, the matric potential is the energy required by plants to extract water from soil pores to overcome adhesive and cohesive forces. Soil matric potential represents the relative availability of the amount of water held in the soil profile for plant uptake/use. In more practical terms, soil matric potential is a direct indication of how much energy must be exerted (application of pressure) by plants to extract the water molecules from soil particles.
Osmotic Potential
The third force, osmotic potential, is most important in soils of high salt content. For soils high in salts, the osmotic potential of the soil solution will contribute to the total moisture stress along with the matric potential. The greater the concentration of salts, the more osmotic potential is lowered in effect reducing water uptake by roots, seeds, and microorganisms.
Water Uptake from Soils by Roots
Most of the water that enters the plant roots does not stay in the plant. Less than 1 percent of the water withdrawn by the plant actually is used in photosynthesis (assimilated by the plant). The rest of the water moves to the leaf surfaces, where it transpires (evaporates) to the atmosphere. The rate at which a plant takes up water is controlled by its physical characteristics, the atmosphere, and soil environment. As water moves from the soil into the roots, through the stem, into the leaves and through the leaf stomata to the air, it moves from a low water tension to a high-water tension (Figure 7.3).
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