Irrigation and Tillage Effects on Soybean Yield in a Coastal Plain Soil 1

Tillage‐induced pans in many Atlantic Coastal Plain soils restrict soybean ( Glycine max L. Merr.) rooting. Consequently, one or more drought periods may reduce soybean yields. This study determined tillage and irrigation effects on soybean yields in one of these soils in a dry year. ‘Ransom’ soybeans were planted on 19 May 1977 on Wagram loamy sand (Arenic Paleudult) which has a traffic induced pan at 25 cm. One half of each plot was conventionally tilled (moldboard plus three diskings); the other half was subsoiled to 45 cm and bedded. The date at which irrigation began was considered the whole plot and was replicated four times in a randomized block design. Irrigations were scheduled using tensiometers whenever the soil moisture tension in the 15 to 30‐cm depth reached 0.8 bars. A gradient with respect to the amount of water applied was established across each plot; no irrigation water reached the far side of the plots. Dates at which irrigation began were 23 June (early), 29 July (after flowering began), and 1 September (after pod set started). Once irrigation was initiated, each treatment was irrigated as needed throughout the growing season. Yields of conventionally tilled soybeans were 1,728, 2,063, 2,124, and 1,686 kg/ha for no irrigation and for full irrigation initiated on 23 June, 29 July, and 29 September, respectively. No significant yield increase or decrease was observed for the irrigated, subsoiled plots, regardless of the date irrigation began. Average yield of the subsoiled treatment was 37% higber than the 1,802 kg/ha overall average yield for conventionally tilled soybeans. The fact that the irrigated, conventionally tilled treatments did not achieve yields equal to those of the nonirrigated, subsoiled treatment suggest that some other benefit(s) accrue from the subsoil and bedding operation in addition to increasing water availability.