Soil Water Content

Soil water affects the moisture available to organisms as well as soil aeration status, the nature and amount of soluble materials, the osmotic pressure, and the pH of the soil solution. Water acts physically as an agent of transport by mass flow and as a medium through which reactants diffuse to and from sites of reaction. It acts chemically as a solvent, as a reactant in important chemical and biological reactions, and as a chemical buffer fixing the activity of water in soil solution at about 1. Of special significance in the soil system and to microbial cells in particular is the fact that water adsorbs strongly to itself and to surfaces of soil particles by hydrogen bonding and dipole interactions. The thin layer of adsorbed water remains unfrozen even at temperatures <0°C.

Soil water content can be measured on a mass or volume basis. Gravimetric soil water content is the mass of water in the soil, measured as the mass loss in a soil dried at 105°C (oven-dry weight) and is expressed per unit mass of oven-dry soil. Volumetric soil water content is the volume of water per unit volume of soil. Soil water is also described in terms of its potential free energy, based on the concept of matric, osmotic, and gravitational forces affecting water potential. Soil water potential is expressed in units of pascals (Pa) or, more commonly, kilopascals (kPa). Matric forces are attributed to the adhesive or adsorption forces of water attraction to surfaces of mineral and organic particles and to cohesive forces or attraction to itself. Since these forces reduce the free energy status of the water, matric forces reduce the water potential to values less than 0; that is, matric forces result in negative water potentials. Solutes dissolved in soil solution also contribute to a reduction in the free energy of water and give rise to an osmotic potential that too is negative. Combined, the matric and osmotic forces are responsible for the retention of water in soils. They act against gravitational forces tending to draw water downward and out of the soil. Gravitational forces are usually positive.

When the gravitational forces draining water downward are exactly counterbalanced by the matric and osmotic forces holding onto the water, the soil is said to be at field capacity or at its water holding capacity. This will occur after irrigation, after a heavy rainfall, or after spring thaw, which leave the soil saturated and having a soil water potential of 0 kPa. Gravitational forces begin immediately to drain away water in excess of that which can be retained by matric + osmotic forces, leaving the soil after 1-2 days at field capacity. By definition the field capacity for loam and clay loam soils is a soil water potential of -33 kPa, and for sandy soils -10 kPa.

Water retention or soil water content at a given soil water potential is a function of the size of pores present in the soil, or pore size distribution. Soils of different textures have very different water contents even though they have the same water potential (Table 2.2). An important property of water influencing its behavior in soil pores is surface tension. Because of the strong cohesive forces, water has a high surface tension. Based on matric forces and properties of surface tension, the maximum diameter of pores filled with water at a given soil water potential can be estimated using the Young-Laplace equation:

Maximum pore diameter retaining water (pm)

Those soil pores greater than about 10 pm diameter drain under the influence of gravitation forces, given that the soil water potential at field capacity is -33 kPa.

Soil water potential determines the energy that an organism must expend to obtain water from the soil solution. Generally aerobic microbial activity in soil is optimal at a soil water potential of about -50 kPa and decreases as the soil either becomes wetter and saturated, i.e., waterlogged, or dries (Fig. 2.9).

While plants are at their wilting point when the soil water potential reaches -1500 kPa, relative rates of soil microbial activity can still be quite high.

Water potential (kPa)

FIGURE 2.9 Generalized aerobic microbial activity response to soil water potential, assuming soil temperature is not limiting.

Water potential (kPa)

FIGURE 2.9 Generalized aerobic microbial activity response to soil water potential, assuming soil temperature is not limiting.

TABLE 2.5 Ability of Different Organisms to Tolerate Water Stress (from Paul and Clark, 1996)

kPa

Aw

Organism

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