Measurement of the suction of soil water by Portland stone absorbers calibrated by a new method for determining vapour pressures near to saturation

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Abstract
1. A filter apparatus cannot be relied upon to give a correct reading of the suction of soil water when this exceeds about 400 cm. of water.2. Owing to the contribution of dissolved matter to the freezing-point depression the suction in a soil of normal low salt content can only be obtained from the freezing-point depression with reasonable accuracy when it exceeds about 4000 cm. of water (freezing-point depression greater than 0–3° C).3. Suctions in the range 400 to 4000 cm. have been measured with the aid of calibrated absorbers consisting of thin plates of Portland stone.4. The plates were calibrated up to 1000 cm. by the application of suction through a filter, and above 1000 cm. by measuring the vapour-pressure depression by a new technique.5. When applied direct to soil, the new vapour-pressure technique is more reliable than the freezing-point method, and can be applied to materials which do not exhibit a well-defined freezing-point.6. A suitable soil was washed free from salts and brought to 1000 cm. suction on a filter. Measurements of the vapour-pressure depression and the freezing-point depression checked well with the thermodynamic formulae.
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