LEAST LIMITING WATER RANGE - TRAFFIC AND LONG-TERM TILLAGE INFLUENCESIN 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 lea st limiting approach. Rooting limitations of available water, soil aer ation, and penetration resistance are Linked into a least limiting wat er range (LLWR) as a function of bulk density (rho(b)). The LLWR conce pt and associated measurements were used to evaluate the impact of til lage 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 an d 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 conduc tivity, water retention characteristic (WRC), and penetration resistan ce 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; compacti on 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 treatm ent was <0.02 m(3) m(-3). Penetration resistance was more limiting tha n the wilting point in the NT treatment and the plow pan, but not in t reatments receiving annual tillage. Aeration was more Limiting than av ailable water in the NT and plow pan than in the MB and CH. The LLWR p ortrayed a major soil structural impact on physical control of rooting . Soil hydraulic properties associated with the LLWR explained a compa ction and drainage problem with conservation tillage that can be Linke d to shallow penetration of tillage tools in this poorly drained soil.