Monitoring Soybean Root Growth and Soil Water Depletion.

Predicting the soybean rooting depth is necessary to project the maximum depths of water depletion in the soil profile. This information will allow a better estimation of irrigation requirement when using a soil balance approach. The objectives of this study were: 1) to monitor soybean rooting parameters and soil water depletion at 15-cm depth increments to 1.2 m and 2) to investigate whether there is a predictable relationship between the above-ground and below-ground phenology.  Soil matric potential sensors were installed at 15-cm increments up to 1.2 meters in non-irrigated field plots located on the University of Nebraska-East Campus during two seasonal rainfall-differing years (2009-2010). Clear acrylic tubes were installed nearby at 30-degree angle in every 4^th row. Root imaging and scoring of phenology were carried out bi-weekly from emergence to seed-fill. Taproot depth into the soil increased linearly with time. The depth at which 5-mm long secondary roots emerged from the taproot was also linear, and also the depth when 2-mm long tertiary roots appeared, was also linear with time. The successive appearance of main stem nodes (Vn) was linear from the V1 stage to reproductive development R5 stage. Rooting depth at R3-stage, a critical R-stage for irrigation scheduling exceeded 100-cm in each year.