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 (ρb). 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, ρb, 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 ρb as an independent variable. Linearized fits of the WRC and PRC, with R2 > 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 ρb on the PRC. Tracking reduced the LLWR as much as 0.04 to 0.06 m3 m‐3 in CH and NT treatments, but the reduction in the MB treatment was <0.02 m3 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.

Spatial characterization of crop residues and agrichemicals incorporated by tillage is needed to interpret biomass‐related soil processes. This study presents such a characterization. We first described oat residue (Avena sativa L.) and green‐sphere distributions produced by chisel plow (CP) and moldboard plow (MP) tillage and then described red‐sphere distributions when incorporated during secondary tillage. Finally the co‐location of incorporated oat residue and colored spheres was characterized. Green and red ceramic spheres were surrogates for shoot residue and incorporated agrichemical, respectively. Characterizations were derived from weights of soil and residue and counts of spheres in soil cores (1.84 cm diam. by 30 cm long, sectioned into 15 sublengths each 2 cm long). Penetration with MP was 25 cm, CP was 15 cm, and cultivator was 17 cm. The MP incorporated 67% of residue in the 10‐ to 20‐cm depth, while more that 90% of the incorporated residue in the CP was between 1 and 11 cm deep. Secondary tillage had negligible influence on these depth distributions. Green‐sphere distributions were related more closely to tool penetration than those of oat residue because root and shoot tissue were not distinguished. Fractional porosity of the 5.32‐cm3 volumes, adjusted to account for oat residue and sphere volumes, was increased ≈12% due to these materials. Many 5.32‐cm3 volumes contained no oat residue, whereas only a few volumes had concentrations ≥4 times the mean residue concentration. Although oat residue was incorporated with primary tillage, residue and herbicide surrogates were co‐located in 35% of the volumes, many of which contained large concentrations of residue. Exclusive use of either primary tillage with the same secondary tillage produces an environment for characteristically different bioactivity in the upper 10 cm.

and in the tillage pan was maintained at a high level only when crop residue returns increased and soil pH Long-term experiments are ideal for evaluating the influence of was maintained above 5.6 (Pikul and Allmaras, 1986). agricultural practices on soil organic carbon (SOC) accretion. Little is known about the influence of tillage and N fertilization on SOC Long-term use of N fertilizer also reduced soil pH (Rasdistribution and silica (Si) movement in a soil. This study: (i) deter-mussen and Rohde, 1989), as a linear function of N mined the effect of tillage and N fertilizer on SOC accretion in a Walla applied, in both the MP and SW systems. Acidifying Walla silt loam (coarse-silty, mixed, superactive, mesic Typic Haploxeffects were concentrated in the upper 7 and 22 cm of eroll), and (ii) examined the subsequent influence of fine organic the SW and MP system, respectively. matter (FOM) on Si movement. A long-term wheat (Triticum aesti-The Walla Walla silt loam is one of many Mollisols vum L.)-fallow experiment was established in 1940, in a randomized in the Pacific Northwest that contains a high concentrablock with split-plot design. Soil cores (2-cm increments) from two tion of potentially mobile Si, ranging from 40 to 100 mg tillages (moldboard plow, MP; and sweep, SW) and two N rates (45 and SiO 2 kg Ϫ1 soil in the form of H 4 SiO 4 (Douglas et al., 180 kg N ha Ϫ1) were used to measure coarse organic matter (COM), 1984). Douglas et al. (1984) showed that the concentra-FOM, pH, bulk density (b), water-soluble C (C ws), and water-soluble Si (Si ws). The FOM fraction (6.6 kg C m Ϫ2) in SW was 14% higher tion of H 4 SiO 4 in these soils increased as pH decreased, (5.8 kg C m Ϫ2) than in MP for the 180 kg N ha Ϫ1 rate. After 44 yr when base cations were leached out of the soil profile. of N additions, the SOC storage (6.2 kg C m Ϫ2) for the 180 kg N ha Ϫ1 Measurements of Si ws , soil pH, amorphous Si, and total rate increased 3% above that for the 45 kg N ha Ϫ1 (6.0 kg C m Ϫ2). acidity in the same Walla Walla silt loam (Baham and Total Si ws in the B horizon were 34 and 39% greater than in the Ap Al-Ismaily, 1996) indicated that Si had leached from horizon for the MP and SW systems, respectively. Interaction of tillage the upper Ap layer, illuviated into the lower Ap, and and N with Si ws suggests that SOC provides a mechanism to suppress deposited in the boundary between the Ap and B hori-Si solubility, which impacts siliceous pan formation, reduces soil mezons. Higher mechanical (penetrometer) resistance, eschanical resistance, and enhances drainage and plant growth.

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