Development of Irrigation Management Practices for Optimum Yield and Water Use Efficiency of Soybean in East Central Mississippi.

In the US second largest annual rainfall state of Mississippi, during soybean growing season May to September, the average monthly rainfall between 2002 to 2013 at Macon in northeast Mississippi ranges from 84 to 131 mm, while monthly mean reference evapotranspiration calculated by the FAO-56 Penman-Monteith worldwide accepted standard method ranges from as high as 145 to 193 mm. 35-98 mm average monthly water deficit was estimated between soybean water requirement and rainfall supply. Additionally, the difference between the lowest and the highest monthly rainfall averaged over the past 12 years in the same month ranged from 171 to 233 mm, uncertainty is fairly high. Therefore, supplemental irrigation is still required to increase and stabilize soybean productivity. Producers in east central Mississippi have steadily increased their utilization of irrigation in recent years and appear eager to learn irrigation management practices for ensuring that the water stored in surface ponds is applied at the proper time and rate. A project supported by Mississippi Soybean Promotion Board was begun in 2014 on a 17-acre, typical pivot-irrigated field located at Good Farm in Noxubee County, MS. A group IV cultivar, Asgrow 4632, was planted at 120,000 seeds per acre on May 8, 2014. The irrigated area, which contains Vaiden, Okolona, and Demopolis soil types (9.4, 5.8, and 2.3 acres, respectively; NRCS data) was divided into eight, equally-sized pies to accommodate three treatments in each soil type of (i) ‘Rainfed’, not irrigated (ii) ‘SM’, irrigation at 50% of plant available water in the root zone, and (iii) 'ET', irrigation at 80% of calculated daily crop evapotranspiration (ET) during the previous day, giving nine experimental plots. In each of the nine experimental plots (6 rows × 5 m) we have installed soil matric potential sensors (Watermark, Irrometer) used widely by local producers at 15, 30, 60 and 90 cm depths. To determine water balance, we installed a pen lysimeter (Soil Moisture, Corp) in each plot and microflume runoff collectors on representative slopes for each soil type. In Vaiden soil and SM treatment plot, 5TM soil moisture sensor coupled to a datalogger and three commercially available soil matric potential sensors (Stevens, Decagon and Campbell Scientific) were installed to compare sensor reliability in irrigation scheduling. Crop growth stage, height, canopy cover, rooting depth, dry biomass and tissue nutrient concentrations are determined periodically. Soils are sampled for pH and extractable nutrients at bed and furrow locations in each plot at 0-15, 15-30, 30-45, and 45-60 cm depths. Soil hydraulic conductivity, porosity, water at field capacity, and moisture retention curve are determined in undisturbed cores (1 and 6 cm thick) at 15, 30, and 60 cm depths in both the bed and furrow. Calculations in the scheduling tool based on soil moisture and weather data indicated the need for a single irrigation to SM and ET treatments of approximately 23 mm water, so this amount was applied during R5-6 stage on August 6, 2014. Results reported during the presentation will include soybean growth and development, grain yield, water use, and soil water balance under both irrigated and rainfed conditions, and research and grower assessments of the effectiveness of these irrigation scheduling methods.