Soil water potential

Water-retention curve Graph showing soil-water content as a function of increasingly negative soil water potential. 

Fig. 1. Rates of CO2 assimilation, A (/miol s ) leaf conductance, g (mol m s ) intercellular partial pressure of CO2, Pi (Pa) soil water potential and leaf water potential, xp (MPa) during gas-exchange measurements of a 30-day-old cotton plant, plotted against day after watering was withheld. Measurements were made with 2 mmol m sec" photon flux density, 30 °C leaf temperature, and 2.0 kPa vapour pressure difference between leaf and air (S.C. Wong, unpublished data).

Jones, H.G. (1983). Estimation of an effective soil water potential at the root surface of transpiring plants. Plant, Cell and Environment, 6, 671-4. 

Finally, probably the most useful way to define aridity so that it includes also a reference to the capability of the land to sustain plant growth, is by using soil water potential. 

The osmotic effect due to salt in soil-water environments is related to water availability. Water availability is determined by the soil-water potential, w, which is composed of the osmotic potential, 0, the matrix potential, m, and the gravitational potential,... 

Wells, J. M., Thomas, J. Boddy, L. (2001). Soil water potential shifts developmental responses and dependence on phosphorus translocation by the saprotrophic, cord-forming basidiomycete Phanerochaete velutina. Mycological Research,... 

As the amount of soil water decreases, the air-water surface retreats into the crevice between the particles, the radius of curvature decreases, and the pressure accordingly becomes more negative. Because xPs°l1 is equal to pso 1 nSDl1 + pwgh (Eq. 2.13a), the soil water potential also becomes more negative as water is lost from such crevices but is retained in finer ones. 

First, we will consider the soil water potential, vFso 1. As indicated earlier in this chapter (Section 9.3A), vFs°l1 is usually dominated by PS01, which is negative because of surface tension effects at the numerous ail-liquid interfaces in the soil (Fig. 9-9). The magnitude of the water potential varies with environmental conditions and with the soil type. After a rainfall or in freshly irrigated soil, the soil water potential may be about -0.01 MPa (i.e., field... 

In field studies, soil moisture can affect decomposition dynamics in a number of ways. In systems where soil moisture is nonlimiting, researchers find little, if any, correlation between soil respiration and soil water potential (Davidson et al., 1998). However in the same study, Davidson et al. (1998) found that including matrix potential with soil temperature could significantly improve soil flux predictions at sites that are affected by seasonally low (Figure 36). Xu and Qi (2001) found a similar seasonal pattern in a ponderosa pine plantation in the Mediterranean climate of northern California—soil CO2 efflux increases sharply with increasing spring temperatures until the soil dries out a few months later and flux rates drop sharply. Bryant et al. (1998)... 

Figure 36 Seasonal variation in mean soil temperature and soil water content for well-drained soils at the Harvard Forest, MA. Also shown is the measured flux (error bars = 1 standard error) and predicted flux based on temperature alone (solid line) and based on temperature and soil water potential (dashed line) (after Davidson et aL, 1998).

It is also a measure of the moisture stress in plants or soil, measured in megapascals. A more negative value indicates greater moisture stress. Soils with no moisture stress have a water potential of 0 to -1 mPa. Two methods of measuring soil water potential are the heat-dissipation method or the electrical-resistance method. 

The disjoining pressure is a sum of several components (just as with soil water potential). The major components of the n(A)-isotherm in porous media are molecular, nm(h) electrostatic, ne(A) structural, ITS(A) and adsorptive IIa(A) ... 

Figure 8.9 Effect of soil water potential on morphology of Strvpharia caerulea extending from 4cm beech wood resources, across nonsterile soil compacted in 13 cm diameter trays at (a) -0.02 MPa (b) -0.06 MPa and (c) -0.006 MPa.

Plants were grown in small plots (1.5 x 4 m), under irrigated (soil water potential > -2.5 cbar) and non irrigated conditions. Treatments were set up in triplicate and measurements were carried out in January, on healthy, well developed plants. Twelve plants from each treatment were harvested and fresh and dry weight (after 24 h at 70 C) of roots, shoots and leaves were recorded. 

Cotton (Gossypium hirsutum L.) var. Stoneville 825 plants were grown outdoors, in 7 liter pots filled with sand, arranged in a field configuration (10 plants m ). The plants were drip-fertigated with nutrient solution 12 times per day. Soil water potential in the pots was maintained in the range of -5 to -40 IcPa. 

Soil water content and soil water potential were measured with a neutron probe and tensiometers. Plant water potential was measured... 

The Barrel Cactus, Ferocactus acanthodes, in natural stands in the Colorado Desert, is similar to Agave americana and Opuntia basilaris (Nobel, 1977 b). Water storage in its stem permitted to keep open stomata and CO2 uptake at night for about 172 months after the soil water potential had become lower than that of the plant, i.e., the plant had become unable to take up water from the soil. After 7 months of drought the stomata remained closed throughout the day/night cycle. 

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