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The objective of this work was to identify differences in soil water availability and plant available water, and morphological changes of sorghum and soybean crops submitted to irrigation and terminal soil water deficit in three soil texture classes. Two experiments were conducted in the experimental field of the Rural Engineering Department of the Federal University of Santa Maria-RS, during 1997/98 (sorghum) and 1998/99 (soybean) growing seasons. A set of drainage lisymeters was used. A rain shelter was used to avoid rainfall on the experiment site. A completely randomized statistical design with two replications was used. Treatments applied were two water management and three soil texture classes. Irrigations were applied when cumulative maximum crop evapotranspiration reached 25mm; the terminal soil water deficit for sorghum and soybean plants was imposed during V3 and V8 vegetative growth, respectively. The three soil texture classes used were clayey, silty clay loam and sandy loam. Upper limit and lower limit of plant availability water to plants were evaluated in the lysimeters. Field capacity and permanent wilting point were used to determine soil water availability, using the conventional methodology.

Results indicated that the soil water availability was 44, 66, and 72 mm for the clayey, silty clay loam and sandy loam soil classes, respectively. However, the plant available water to sorghum and soybean plants, average of two years, was 95, 154 and 124 mm. Values of plant available water were 109, 133 and 72% higher than soil water availability determined by conventional methodology. Reduction in leaf area index, plant height and number of nodes in sorghum and soybean plants submitted to terminal soil water deficit was more expressive in the clayey soil, compared with irrigated plants. These results clearly indicated that plant available water needs to be determined in the field, for each soil classes, soil management and crop species.