RATE OF SOIL DRAINAGE FOLLOWING AN IRRIGATION.: III. A NEW CONCEPT OF THE UPPER LIMIT OF AVAILABLE MOISTURE

Abstract

The upper limit of available moisture is defined as the highest moisture content that includes all moisture available for consumptive use but excludes all drainage below the root area. In studies of the upper limit, four pairs of bare and cropped plots in deep well-drained soils were irrigated, then the soil was sampled periodically to determine moisture contents. The soil texture varied from loamy sand to clay loam. The bare plots were kept covered between samplings. Moisture losses in the cropped plots were broken down into consumptive use and drainage. The upper limit was calculated by determining the moisture content when drainage had been reduced to 0.001 inch per day, and adding the consumptive use occurring up to that time. The upper limit values did not accord with field capacity as usually defined. No close relationship was found between the upper limits and breaking points, levelling-off points or other characteristics of either the moisture-time curves from the covered plots or those from the cropped plots. Sampling the soil at 1 to 4 days after an irrigation, depending on the soil texture, gave a close approximation to the upper limit.