Conservation Tillage and Soil Properties

A tillage system is made up of operations that use a variety of implements to achieve a particular production environment and/or to remedy a defined problem that limits production potential. Tillage in soybean production systems is utilized to: 1) prepare a seedbed to ensure proper seed–soil contact; 2) remedy soil compaction, or disrupt soil layers that may restrict root development and/or water infiltration; 3) incorporate fertilizers and herbicides to facilitate their beneficial effects; 4) incorporate plant residues that may impede planting; 5) influence water movement both within and out of a production field; and 6) control problem weeds either before or after planting. Click here for a Tillage White Paper on this website.

Conventional tillage is generally synonymous with clean tillage. Operations involve primary tillage with implements that leave essentially no plant residue on the soil surface; thus, soil is exposed to maximum water runoff and erosion. Conversely, conservation tillage is a system where tillage is conducted with minimal soil disturbance–e.g. fertilizer application and planting are done in narrow strips, with minimal soil disturbance outside the application and planting zone. Weed control is accomplished exclusively with herbicides both before and after planting. More than 30% of the soil is covered by residue at any given time.

Conservation tillage systems are adopted to potentially improve soil health. The minimal tillage practices associated with these systems are utilized to enhance accepted soil health factors such as the cover provided by residue and an increase in soil carbon stocks and organic matter that will subsequently support an increase in soil microbial activity.

A little-researched facet of adopting conservation tillage to improve soil health factors such as those listed above is the time period needed for such improvement. In an article titled “Loamy sand soil approaches organic carbon saturation after 37 years of conservation tillage” by Novak et al. (Agronomy Journal 2020:112:3152-3162, https://doi.org/10.1002/agj2.20184), research results are presented that quantify this on a specific soil type in the southeastern U.S.

The results presented in this article are from measurements of soil organic carbon (SOC) and total nitrogen (TN) following 37 years (1978-2016) of crop production where either conventional (CNT) or conservation (CVT) tillage were used on a loamy sand soil. The study site was located near Florence, South Carolina [lat. 34°17'–same as Clarksdale and Verona , Miss., and similar to Stuttgart, Ark. (34°29' )]. Crops included corn, soybean, wheat, and cotton. For all crops, either CVT or CNT were used annually. In-row subsoiling was annually conducted in both tillage treatments. Surface tillage was never conducted in the CNT treatment; thus, crop residue was maintained on the soil surface.

•   The objectives of the study were to 1) quantify changes in SOC and TN content, distribution, and accumulation in the 0-6 in. soil layer, and 2) determine if organic carbon (OC) had reached its saturation level in this soil after using either CVT or CNT management for 37 years.

•   Within the 0-2 in. soil layer, SOC after both corn and a soybean/cotton rotation was 1.8 times greater for CNT than for CVT. Average SOC contents with CNT decreased in the 2-6 in. soil layer compared to SOC in this layer following CVT. Thus, the lack of mixing crop residues throughout the 0-6 in. soil zone with CNT resulted in this higher SOC in the shallow 0-2 in. soil layer.

•   Within the 0-2 in. soil layer, mean TN content was 1.6 times greater for CNT than for CVT. Despite this TN difference in this shallow soil layer, cumulative average values for the entire 0-6 in. soil layer were not different between CNT and CVT.

The authors make several significant statements based on their review of the literature. These statements follow.

•   “The OC concentration point is unique for every soil and varies as a function of texture, aggregation, and organic residue recalcitrance. Parent material, climate, and texture have major effects on SOC content and the ability of a specific soil to store OC.”

•   “Sandy soils generally have a low potential to store OC because they are warmer, better aerated, and have higher OC mineralization rates than finer-textured soils.”

•   “...the bonding of OC substances to minerals is also recognized as an important factor influencing a soil’s OC mineralization potential and thus the longevity of OC storage. Sand particles exhibit weak bonding affinities for OC substances because of the dominance of quartz particles. Weak binding between quartz particles and OC substances decreases the protection of those compounds from soil microbes.” Thus, when combined with the warm, humid climate of the Midsouth, “the OC substances can be rapidly oxidized to CO₂.”

•   “In comparison, OC will accumulate in fine-textured soils because OC substances can bind with silt and clay surfaces..., leading to the formation of clay-humic complexes.” This leads to a reduction in microbial mineralization and enhancement of OC accumulation.

•   “Coupled with a warm, humid climate, tillage plus low-residue crops such as cotton and soybean have resulted in low SOC and TN concentrations due to increased soil aeration and physical accessibility of plant residues to microbial decomposition.”

•   “ SOC accumulates rapidly during the first 5-10 years of conservation tillage, but after 15-20 years, the accumulation rate slows to where annual changes are minimal.”

The results from the above long-term study on a loamy sand soil with known management practices confirm that conservation tillage can significantly increase SOC and TN content in the shallow (0-2 in.) soil zone compared to conventional tillage. Specifically, the evaluation in this study showed that the sandy soil at the study site (using a wheat/corn/cereal rye cover crop system) required a longer period to reach OC saturation of a higher concentration when conservation tillage vs. conventional tillage was used, and that a conservation tillage system can bolster C sequestration and TN contents in this sandy soil.

These results support the following points.

•   Long-term use of conservation tillage can result in increased carbon sequestration and nitrogen content in soil.

•   The enhanced soil carbon sequestration that results from conservation tillage is a long-term process; i.e., the benefits from conservation tillage will not likely be realized with its short-term use.

•   The SOC enhancement that results from conservation tillage will reach a plateau, and the time to reach that plateau will vary depending on soil parent material and texture, and climate.

•   A conservation tillage system adopted over a long period vs. use of a conventional tillage system will result in greater sequestration of carbon in the soil.

•   Conservation tillage use in crop production systems must occur over an extended time for its perceived benefits to be realized.

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