Water Content

Cells need to be in water in order to function. Water is a liquid medium for cells to remain hydrated, and for nutrients to dissolve in. The biological activity of soil depends on aeration as well as adequate water in pore spaces. The source of water is from snow melt, rain and dew, which depend on temperature fluctuations and air humidity. In some locations, subterranean lakes or streams also contribute significantly to soil moisture. Most soils are hygroscopic and will absorb water vapour from the air.

Water is held on to organic matter, cell membranes and minerals by hygroscopic forces. These consist of hydrogen bonds, van der Waals forces, ion hydration spheres, molecular dipole forces and osmotic potential. Part of the water is bound chemically to soil particles. It is referred to as adsorbed water, having very strong negative potential, i.e. it is not naturally lost from soil and is held strongly. It is captured from air space humidity or retained by soil organic matter (SOM) and soil mineral matter. It provides a thin film 0.2 ^m thick which covers all particle surfaces. Film water is not available to roots and cells because it is too strongly held, but it guarantees a wet and humid layer for living cells in the soil. It is retained even in most arid soils. Capillary water is drawn into pore spaces by hygroscopic forces, but is held less strongly. It is retained even in dry conditions, especially in smaller pore spaces <10 ^m diameter. It is the main source of humidity in air spaces, so that pore space humidity is rarely below 98%. Gravitational water is free to drain through soil under the influence of gravitational pull. Vertical and lateral displacement of gravitational water occurs between 0.01 and -0.03 MPa soil water tension, and contributes to replenishing underground and above-ground water reserves. It is an important factor in loss of nutrients to leaching and erosion of the soil profile. Water pressure differences exist due to relative changes in elevation and atmospheric air pressure. These are exerted on the gravitational water. Total gravitational and capillary water content (%W) is measured as the percentage of weight loss due to oven drying to constant weight at 105°C. The percentage water content is obtained from the weight loss. It is expressed as (Sw - Sd)/Sd X 100, where Sw is the weight of the soil sample before drying; and Sd is the weight of oven-dried soil. Soil water potential is the sum of gravitational, matric (capillary and adsorbed water) and osmotic potentials. The force required to remove water from soil can be used to estimate how much water is available to living organisms. The soil water potential is affected by SOM content, but also soil texture and structure. The amount of water removed over a range of pressure differentials, by creating suction, is used to measure how strongly water is held by the soil. Alternatively, faster analysis is carried out with a dewpoint potentiometer (Decagon, Pullman, Washington, USA). Graphs of soil water content against the soil matric potential (i.e. the force applied in N/m2) are called 'soil water characteristic curves'. The shape and position of curves vary with soil texture and provide a measure of how difficult it is for the remaining water to be removed by living organisms (Fig. 2.6).

The osmotic potential of soil water indicates how easily dissolved nutrients can be removed by cells. It is the amount of pressure that needs to be applied to a solution of known molarity to raise the partial

Soil Water Release Curve

Water content (%)

Fig. 2.6. Soil water release curves. Matric potential curves for three arbitrary types of soil. As the suction increases (the matric potential in kPa), the amount of water remaining in the soil decreases. At low suction, silty and sandy soils retain less water than clay soils. Clay soils in general require more suction (higher matric potential of clay soil) to remove the soil water.

Water content (%)

Fig. 2.6. Soil water release curves. Matric potential curves for three arbitrary types of soil. As the suction increases (the matric potential in kPa), the amount of water remaining in the soil decreases. At low suction, silty and sandy soils retain less water than clay soils. Clay soils in general require more suction (higher matric potential of clay soil) to remove the soil water.

pressure of water vapour above the solution to that of pure water. The matric potential can be measured experimentally from water pressure changes between a soil solution and pure water. The less water, the more tightly the remaining water is held by charged ions and organic matter. In very dry soils, the remaining water is no longer available to cells, including plant root cells, once the water tension is too great. For dilute solutions, the osmotic pressure can be calculated as = RTC, where is the osmotic pressure in kilopascals (kPa), R is the gas constant (8.31 X 10-3 kPa m3/mol K), T is the absolute temperature in Kelvins (K) and C is the molarity in mol/m3.

One can calculate threshold osmotic potentials for each species. It is the potential above which that species (be it plant root, hyphae, protozoa or bacteria) is no longer able to acquire water from its environment. This value varies between species, and generalization are not possible (Harris, 1981). Certain species are better adapted for moist conditions, while others are only active in low soil moisture conditions. A consequence of changes in soil water content is to affect the rate of diffusion of solutes and gases. Diffusion is the movement of solutes (or particles in general) along a concentration gradient, from high to low. When there is sufficient capillary and gravimetric water, solutes can diffuse along concentration gradients. Diffusion of dissolved organic matter through the soil solution and its flow with gravitational water distributes nutrients through the pedosphere.

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