oa Agrochemophysica - A comparison of two methods for determining plant available water in a soil profile

Volume 12, Issue 3
  • ISSN : 0302-7112



Estimates were made of the amount of water readily available to mature maize on Jozini series soil at Fort Hare Farm using the field method recently proposed by Hensley & De Jager (1978) and the widely accepted method based on matric suction data (termed the laboratory method here). Values of 143 mm and 66 mm were obtained for the two methods respectively. The main reasons for the large discrepancy were found to be: (a) the assumption that all the readily available water had been extracted when the soil water content was midway between 0V at �10 kPa and 0V at �1500 kPa (i.e. at 50% depletion of � total available moisture�) was very unreliable, especially for the surface soil (0-600 mm) which was found in the field to dry out to considerably below the �1500 kPa water content before irrigation was considered necessary to obtain optimum yield; (b) as an estimate of field capacity the �10 kPa water content determined in the laboratory was not reliable enough ; (c) the assumption of an �effective� rooting depth of 1 000 mm for maize, within which water extraction is equally distributed with depth, is unsatisfactory for this soil. Determination of PAWC using the field method is recommended since it is reliable and easy to use. Guidance from experienced agronomists is essential for defining the lower limit of PAWC.

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