Soil Water Sensor-Based and Evapotranspiration-Based Irrigation Scheduling for Soybean Production on a Blackland Prairie Soil in Humid Climate.

In east-central Mississippi, annual rainfall was 1307 mm and reference evapotranspiration (ET_o) was 1210 mm for the 120-year period from 1894 to 2014. From May to October, when major crops are typically grown in this area, monthly rainfall ranged from 72 to 118 mm, and monthly ET_o from 94 to 146 mm which corresponds to a monthly rainfall deficit of 22 to 62 mm. Periods of drought during crop growth season support evidence that irrigation can stabilize economic return and increase crop yield by at least 20% in the region. The Blackland Prairie of northeast and east-central Mississippi accounts for 14% of Mississippi state crop lands. Across the state, soybean acreage was increased by 24% from 2008 to 2014, and irrigated acreage was increased by 20% from 2011 to 2012. Growers interested in irrigation systems are obviously eager to learn when and how much to water is needed by crops. Wide spreading irrigated land and improper irrigation scheduling due to lack of enough knowledge and right tool have resulted in serious issue of agricultural water availability. To determine better irrigation scheduling method for soils in the Blackland Prairie of humid southeastern USA, two commonly used irrigation scheduling methods, soil moisture via in-situ sensors (SM) and evapotranspiration (ET), were evaluated at Brooksville Experiment Stations of Mississippi State University. Three treatments with four replicates, SM, ET and rainfed (RF) were completely randomized to four blocks on a Brooksville silty clay soil in a three-acre, furrow-irrigated field. A group IV soybean cultivar, Asgrow 4632, was planted at 120,000 seeds per acre in a 1-m row spacing on May 19, 2015. Soil water content and potential in the layers of 0-15 cm, 15-30 cm, 30-45 cm and 45-60 cm were measured using TDR and Watermark sensors in each plot (6 rows × 111 m). Soil was sampled at these same layers and a pressure extracted was used to measure soil water content at field capacity (FC, 33 kPa) and wilting point (WP, 1500 kPa). Then Management Allowable Depletion (MAD) curve in soybean dynamic rooting zone was determined as 50% of the measured plant available water (difference between FC and WP water content). In the SM treatment, we compared daily soil water storage in rooting zone with the MAD value and once storage was equal or less than MAD, irrigation was applied to recharge soil water profile to its FC. The ET method compared soil water storage estimated through calculated daily ET and an initial soil water content and applied sufficient irrigation water to replenish the soil FC when the soil water  was equal or less than MAD. Rainfed treatment was not irrigated in the entire season. Water balance was estimated using a pen lysimeter (Soil Moisture, Corp) in each plot and microflume runoff collectors located at the tail end (north side) of the plots. The effective rainfall and canopy interception are estimated using a rain gage above the canopy and a complementary gage below the canopy of different plots. A weather station with an automatic evaporation pan was installed near the field to measure potential ET and evaporation. Crop growth stage, height and canopy cover, rooting depth, leaf area index, and dry biomass were determined weekly. Results comparing soybean ET, water use efficiency and yield among the three irrigation treatments should result in a better irrigation scheduling method for the relatively shallow and heavy clay soil in the Blackland Prairie humid region.