Least limiting water range: Traffic and long-term tillage influences in a Webster soil

Rooting is a continuous response to the soil physical environment, but root responses in the field are sufficiently complex to require a least limiting approach. Rooting limitations of available water, soil aeration, and penetration resistance are Linked into a least limiting water range (LLWR) as a function of bulk density (rho(b)). The LLWR concept and associated measurements were used to evaluate the impact of tillage and tracking on root and hydrologic environment at two depths in a poorly drained clay loam: (i) the 5- to 10-cm depth in nontracked and tracked interrows of three long-term tillage treatments (chisel plow, CH; moldboard plow, MB; and no tillage, NT); and (ii) a plow pan at 25- to 30-cm depth. Soil shrinkage, rho(b), saturated hydraulic conductivity, water retention characteristic (WRC), and penetration resistance characteristic (PRC) were all measured in undisturbed soil cores (5 cm long by 5 cm in diameter). Both the WRC and PRC included rho(b) as an independent variable. Linearized fits of the WRC and PRC, with R-2 > 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% the impact of rho(b) on the PRC. Tracking reduced the LLWR as much as 0.04 to 0.06 m(3) m(-3) in CH and NT treatments, but the reduction in the MB treatment was <0.02 m(3) m(-3). Penetration resistance was more limiting than the wilting point in the NT treatment and the plow pan, but not in treatments receiving annual tillage. Aeration was more Limiting than available water in the NT and plow pan than in the MB and CH. The LLWR portrayed a major soil structural impact on physical control of rooting. Soil hydraulic properties associated with the LLWR explained a compaction and drainage problem with conservation tillage that can be Linked to shallow penetration of tillage tools in this poorly drained soil.