Pressure plates are commonly used to measure the soil water retention curve. Low plate and soil conductance, lack of plate-soil contact, and soil dispersion, however, make this method often unreliable at low water potentials. We investigated how errors in the determination of the water retention curve affect the soil hydraulic properties and the computation of plant-available water. We first determined soil water retention data for a silt loam soil using pressure plates only and a combination of pressure plates and a dew point meter. The two sets of soil water retention data were then analyzed using different models for describing the hydraulic properties. The soil water retention curves determined with the two methods deviated at potentials less than about -20 m H sub(2)O, with the pressure plate apparatus data yielding larger water contents than the dew point meter at the same water potentials. These results indicate that soil water retention curves determined from pressure plates may be in error at potentials less than -20 m H sub(2)O, which may lead to substantial errors in water flow calculations. These errors can be eliminated by using a combination of pressure plates and dew point measurements to determine the water retention curve.
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机译：压力板通常用于测量土壤保水曲线。较低的土壤和土壤电导率，缺乏土壤与土壤的接触以及土壤的分散性使这种方法在低水位条件下常常不可靠。我们调查了保水曲线确定中的误差如何影响土壤的水力特性和植物可用水的计算。我们首先仅使用压力板以及压力板和露点仪的组合来确定粉质壤土的土壤保水数据。然后使用不同的模型分析了两组土壤保水数据，以描述水力特性。用两种方法测定的土壤保水曲线在小于约-20 m H sub（2）O的电势下发生偏离，在相同的水势下，压板设备数据产生的水含量比露点仪大。这些结果表明，由压板确定的土壤保水曲线在电势小于-20 m H sub（2）O时可能有误差，这可能导致水流量计算中的重大误差。通过组合使用压板和露点测量来确定保水曲线，可以消除这些误差。
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