Rate of Soil Drainage Following an Irrigation.: Ii. Effects on Determination of Rate of Consumptive Use

An investigation was conducted to determine the effects of soil drainage and other factors on consumptive use as measured by the soil sampling procedure. Four pairs of adjoining plots were established in four different fields in which the soil varied from coarse loamy sand to clay loam. One plot of each pair was kept in crop and the other was kept bare and covered. They were irrigated thoroughly, and their soil was sampled periodically for 20 days or longer to determine the soil moisture contents. Between samplings the bare plot was kept covered with a tarpaulin. The following results were obtained:1. No really satisfactory method was developed for separating consumptive use from drainage during the initial period of 2 or more days following an irrigation. During this initial period, consumptive use occurred largely at the expense of free water that would otherwise have drained away. This situation continued longer with a fine-textured soil than with a coarse-textured soil.2. By the procedure used in these tests, consumptive use was separated from drainage satisfactorily following the initial period after irrigation. After this initial period the rate of consumptive use decreased with time until the end of each test.3. The rate of drainage was less in the cropped plots than in the covered plots, but the rate of total moisture reduction was greater.4. When the total moisture lost from a bare covered plot was deducted from that lost from a cropped plot, the values thus obtained for consumptive use were much too low.5. When the total moisture lost from the cropped plots was used as a measure of consumptive use, the results were often much too high.It is concluded that no reliable method has yet been developed for routine determination of the rate of consumptive use by soil sampling.