<P> Since regions of soil are usually not divided by a semipermeable membrane, the osmotic potential typically has a negligible influence on the mass movement of water in soils . On the other hand, osmotic potential has an extreme influence on the rate of water uptake by plants . If soils are high in soluble salts, the osmotic potential is likely to be lower in the soil solution than in the plant root cells . In such cases, the soil solution would severely restrict the rate of water uptake by plants . In salty soils, the osmotic potential of soil water may be so low that the cells in young seedlings start to collapse (plasmolyze). </P> <P> When water is in contact with solid particles (e.g., clay or sand particles within soil), adhesive intermolecular forces between the water and the solid can be large and important . The forces between the water molecules and the solid particles in combination with attraction among water molecules promote surface tension and the formation of menisci within the solid matrix . Force is then required to break these menisci . The magnitude of matrix potential depends on the distances between solid particles--the width of the menisci (also capillary action and differing Pa at ends of capillary)--and the chemical composition of the solid matrix (meniscus, macroscopic motion due to ionic attraction). </P> <P> In many cases, absolute value of matrix potential can be relatively large in comparison to the other components of water potential discussed above . Matrix potential markedly reduces the energy state of water near particle surfaces . Although water movement due to matrix potential may be slow, it is still extremely important in supplying water to plant roots and in engineering applications . The matrix potential is always negative because the water attracted by the soil matrix has an energy state lower than that of pure water . Matrix potential only occurs in unsaturated soil above the water table . If the matrix potential approaches a value of zero, nearly all soil pores are completely filled with water, i.e. fully saturated and at maximum retentive capacity . The matrix potential can vary considerably among soils . In the case that water drains into less - moist soil zones of similar porosity, the matrix potential is generally in the range of − 10 to − 30 kPa . </P> <P> At a potential of 0 kPa, soil is in a state of saturation . At saturation, all soil pores are filled with water, and water typically drains from large pores by gravity . At a potential of − 33 kPa, or − 1 / 3 bar, (− 10 kPa for sand), soil is at field capacity . Typically, at field capacity, air is in the macropores and water in micropores . Field capacity is viewed as the optimal condition for plant growth and microbial activity . At a potential of − 1500 kPa, soil is at its permanent wilting point, meaning that soil water is held by solid particles as a "water film" that is retained too tightly to be taken up by plants . </P>

What is the van hoff factor of pure water