Submitted to: Soil Science Society of America Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/4/1998
Publication Date: N/A
Citation: Interpretive Summary: Tillage systems and associated wheel traffic may produce different soil structures that may impede or accelerate water percolation and degrade or improve root environment. Studies under controlled conditions indicate that root environment is sensitive to available water, aeration, and mechanical resistance to root proliferation. Rarely do we know from field measurements swhich factor is limiting or controlling. A least limiting approach can provide both site specific and generic guidance in the field based upon a collective analysis of all the individual limitations. Tests of the least limiting water range on a poorly drained but tiled soil in southern Minnesota indicate the adverse soil environment produced by compaction and poor internal drainage in no tillage systems and in all tillage systems where there is wheel tracking without immediate tillage in the wheel tracks. The information is being used by research scientists, farmers, and extension workers to propose tillage and controlled wheel tracking systems that improve internal soil drainage and improve rooting environment in reduced/conservation tillage systems. Even though the traditional moldboard plowing had the best short-term potential for internal drainage and root environment, it produced negative long-term soil quality attributes in this soil related to infiltration, soil erosion, and soil organic carbon needed to reduce atmospheric CO2.
Technical Abstract: Root growth and function is a continuum response within some threshold soil physical environment, but root responses in the field are sufficiently complex to require a least limiting approach. Individual rooting limitat- ions of available water, soil aeration, and penetration resistance are linked to construct a least limiting water range (LLWR) as a function of bulk density. The LLWR concept (and associated measurements) was evaluated for two depths in a poorly-drained clay loam: 1) the 5 to 10-cm depth in non-tracked and tracked interrows of three long-term tillage treatments (chisel plow-CH, moldboard plow-MB, and no tillage-NT); and 2) a plow pan at 25 to 30-cm depth. Laboratory measurements of bulk density, soil shrinkage, saturated hydraulic conductivity, water retention characteristic (WRC), and penetration resistance characteristic (PRC) were all obtained from undisturbed soil cores (5 cm long by 5 cm dia.). Both the WRC and PRC included bulk density as an independent variable. Linearized fits of the WRC and PRC, with R**2 greater than 0.70, were sensitive to tracking and to CH vs MB tillage; compaction in the plow pan and tracking in the NT reduced by 75 percent the impact of bulk density on the PRC. Tracking reduced the LLWR especially in CH and NT treatments, but had a lesser effect in MB than CH tillage. Penetration resistance was more limiting than the wilting point in the NT treatment and the plow pan, but not in the other treatments. Aeration was more limiting than available water in the NT and plow pan compared to MB and CH. The LLWR portrayed a major potential for soil structure impact on physical control of rooting. The LLWR also explained a significant problem with conservation tillage that can be linked to shallow penetration of tillage tools in a fine-textured poorly drained soil.