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Conservation Tillage Plus Subsoiling Equals Enhanced Soybean Production on Medium- to Coarse-Textured Soils in the Midsouth

Conservation tillage is defined as a tillage system that consists of “limited mechanical operations with implements that result in the soil surface being covered with >30% plant residue (older standard) or a STIR of ≤80, and does not use a moldboard plow. Conservation tillage practices generally include mulch-till, strip-till, and no-till.”

conventional tillage system is defined as a tillage system that includes a “combination of mechanical operations with implements that result in a seedbed that is essentially free of weeds and plant residue cover. This is the antithesis of conservation tillage. Tillage management practices in this system result in a STIR of >80.”

stale seedbed is “a seedbed that has received no seedbed preparation tillage just prior to planting. It may or may not have been tilled since harvest of the preceding crop. Any tillage conducted in the fall, winter, or early spring will have occurred sufficiently ahead of planting time to allow the seedbed to settle or become “stale”. A crop is planted in this unprepared seedbed, and weeds present before or at planting are killed with herbicides. This system does not preclude tillage because it is a minimum or reduced tillage concept rather than a no-till concept.”

Subsoiling refers to primary tillage operations that affect soil deeper than 6 in. These operations are used to fracture or loosen deep soil barriers, improve rainfall infiltration, and mix residue and nutrients deep into the profile. Deep tillage can be part of a conservation tillage system if it minimally disturbs the soil surface and leaves more than 30% of the soil covered by plant residue. The fuel requirement for deep tillage is high (essentially the same as for moldboard plowing), so it should be strategically incorporated into a conservation tillage system only to remedy known subsurface soil problems.

Results from a study conducted for 4 years (2015-2018) on a Dubbs silt loam soil at Stoneville, Miss. are reported in an article titled “Conservation Soybean Production Systems in the Midsouthern USA: I. Transitioning from Conventional to Conservation Tillage” (CFTM– https://doi.org/10.1002/cft2.20055). Study objectives were to determine if the inclusion of subsoiling in a conservation tillage system with soybeans will affect seed yield, net returns, and water use efficiency (WUE) relative to that of conventional tillage. Details of the conduct of and results from this study follow.

•   Treatments were: 1) Conventional tillage/winter fallow (CT/WF)–fall disking (2 times), field cultivator 1 month prior to planting, spring bed creation, bed conditioner for seedbed preparation (served as control treatment); 2) Reduced tillage/winter fallow (RT/WF)–fall disking (1 time) and bed formation, natural winter vegetation chemically desiccated prior to planting, stale seedbed planting; and 3) Reduced tillage/subsoiling (RT/SS)–in-row fall subsoiling 22 in. deep–remaining tillage operations were as described for RT/WF.

•   Soybean planting each year occurred during May 9-14, and all plots were irrigated each year.

•   Average ground cover following desiccation of vegetation prior to planting in the spring was ≥42% in Treatments 2 (RT/WF) and 3 (RT/SS), and <3% in Treatment 1 (CT/WF).

•   Soybean seed yields from RT/SS were equal to or greater than those from CT/WF in 3 of 4 years of the study. RT/SS seed yields were equal to or greater than yields from RT/WF in all years of the study. RT/WF yields were less than those from CT/WF in 2 of 4 years of the study.

•   These results indicate that adoption of conservation tillage on a medium-textured soil such as that in this study will have no negative effect on soybean yield if subsoiling is included in the system.

•   Averaged across years, total specified costs for RT/SS ($307.18) were essentially equal to those for CT/WF ($307.98), and were only slightly higher than those for RT/WF ($292.33).

•   In the first year of the study, net returns from CT/WF were equal to those from both RT/WF and RT/SS; returns to RT/WF were greater than those from RT/SS. In year 2, returns to CT/WF were the greatest. In year 3, returns to RT/SS were similar to those from CT/WF and greater than those from RT/WF. In year 4, returns to RT/SS were the greatest.

•   WUE differences among treatments in the study followed essentially the same pattern as seed yield differences among treatments.

•   Based on the results from this study, integrating subsoiling into a conservation production system with soybean that is irrigated is recommended on medium- and coarse-textured soils in the Midsouthern U.S.

These results support the long-held practice of deep-tilling these soils for crop production, and specifically indicate that the inclusion of subsoiling will enhance production of soybeans that are grown in a conservation tillage system on non-clayey Delta soils.

Composed by Larry G. Heatherly, Aug. 2020, larryheatherly@bellsouth.net