Soil Water Depletion and Root Distribution of Three Dryland Crops

Abstract

Characterization of plant uptake of soil water at different points in time and space are important in evaluating seasonal water use as well as rotational dryland cropping strategies. The objective of this study was to characterize root length density (RLD) and soil water depletion patterns of cowpea [Vigna unguiculata (L.) Walp.], grain sorghum [Sorghum bicolor (L.) Moench], and sunflower (Halianthus annuus (L.)] under no tillage (NT) and stubble mulch tillage (SMT) systems in a Torrertic Paleustoll. Root length density of crops was measured from scanned images of washed root samples obtained from soil cores extracted several times during each of two growing seasons. Soil water contents were measured with a neutron moisture meter to a depth of 2.3 m at weekly intervals throughout each growing season. The RLDs of sorghum and sunflower near the soil surface increased rapidly after planting but thereafter declined whereas subsoil RLD increased throughout the growing season. Residual water contents at harvest were 28 to 93 mm greater (P < 0.05) under cowpea as compared to sorghum. As compared to the other crops, most of the additional residual water under cowpea was confined to soil depths between 0.5 and 1.7 m. Soil depths of 1.0 to 1.8 m were the most important source of stored water for sorghum and sunflower towards the end of the season. Deeper rooting and greater soil water extraction below 1.2 m depth were observed for NT as compared to SMT (P < 0.05). Results suggest that a rotation of cowpea with sorghum or sunflower would permit the stratified use of soil water and that the storage and crop use of water deep in the profile would be optimized under NT.