positive and negative charges. rotated at 13,000 3 g, plant-available soil water from the soil sample is dispensed in the outer tube (Kobayashi et al. The soil water is affected by the force of gravity, presence of soil (matric) and of solutes. There are numerous methods for estimating the potential. The soil water content at the stage where the plant dies, is called permanent wilting point. measure the soil water potential limits for rapid, adequate, and marginal ger-mination of winter wheat (Triticum aestivumL.). This chapter discusses four methods for measuring water potential. 1994). Even when there is not exce ssive soil erosion, soil quality can also be increased environmental impacts by the potential degradation of water and soil resources. Fig. Some soil moisture characteristics. In this publication, we will re­ fer to centibars (cb). 1994). Water plays a very significant role in soil-plant growth relationships. In the first three methods, soil water is equilibrated with water in a standard matrix. • Water is held in soils because of attraction between unlike charges, i.e. Soil water potential (or soil water tension) is measured either in centibars or kilopascals . The soil can be compared to a water reservoir for the plants. Forces On Soil Water A number forces influence the way water behaves in the soil. The vacuum gauge dial measures from 0 to 100 cb, and the operational range of the in­ Soil Moisture and Crop Yield Potential Water Use (Potential Evapotranspiration) The heart of all water resource capture crop models is the determination of "Potential Evapotranspiration." 1. soil water content 2 thermogravimetric method 6 2. soil water potential 7 2.1 introduction 7 2.2 energy state of soil water 7 2.3 quantitative expression of soil water potential 10 2.4 gravitational potential 10 2.5 osmotic potential 12 2.6 matric potential 13 2.7 external gas pressure potential 16 2.8 hydraulic head 17 3. tensiometer 20 In addition, it is important for studying soil-water movement, chemical transport, crop water stress, evapotranspiration, hydrologic and crop modeling, soil phys-ics, water resources management, climate change impacts Importance of Soil Water: Water is essential for all forms of life. 2.4 Available water content. These units are equiva­ lent, and their use depends on the brand of vacuum gauge used for the tensiometer. water Dry soil Suction potential energy Matric potential energy Porous block Suction (capillarity) Capillary Potential 100 cm Dry soil Ψm = -100 cm (suction) Vertical distance between the surface of the water and the porous cup. Centrifuging at this force equates to a soil water potential of 2 1,500 kPa. This soil water potential repre-sents the permanent wilting point for plant material (Brady and Weil 1996; Kobayashi et al. suction Soil Texture 1000 cm The most obvious is gravitational forces, which pulls water down through the soil. Finally, at about –15 bar potential (very thin water films), plants wilt because they can’t pull water off the soil. The details of such methods are treated in a The “Total” Soil Water Potential and its Components Soil water is subject to several force fields, the combined effects of which result in a deviation in potential energy relative to the reference state, called the total soil water potential (ψ T) defined as: “The amount of work that an infinitesimal unit quantity of water Laboratory data showed that germination was rapid (3 to 4 d) in soil at water potentials above -1.1 MPa and slower (4 to 5 d) at water potentials that ranged from -1.1 to -1.6 MPa. Accurate determination of soil-water status (SWC or soil-water potential) is a fundamental element of irriga-tion management. The soil still contains some water, but it is too difficult for the roots to suck it from the soil (see Fig. Potential represents the difference in free energy levels of pure water and of soil water. 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