Physical and Chemical Properties of a Haploxeroll after Fifty Years of Residue Management

Abstract

Greater inference about the value of long‐term crop residue management requires that soil chemical properties be measured together with soil physical properties. After 54 yr of several different crop residue management treatments in a winter wheat (Triticum aestivum L.)‐fallow system in the Pacific Northwest, soil bulk density (ρ_b), saturated hydraulic conductivity (K_sat), soil water desorption curves (SWDC), soil organic carbon, and soil pH were all used to quantify the soil physical and chemical state. All wheat straw, chaff, and manure additions were incorporated by spring plowing in an alternating winter wheat‐fallow rotation. The residue treatments were: (i) straw, (ii) straw plus 22 Mg/ha strawy manure, (iii) straw burned in the fall, and (iv) straw plus 90 kg/ha of N as NH₄NO₃ broadcast before seeding. Straw return in treatments 1, 2, and 4 depended especially on the N added. Soil ρ_b at 0.02‐m increments was used to locate compacted layers. Respective tillage pan ρ_b for treatments 1, 2, 3, and 4 were 1.3, 1.2, 1.3, and 1.2 Mg/m³; respective K_sat, using a double tube method, were 0.34, 0.85, 0.29, 0.23 µm/s. Respective K_sat, above the tillage pan, using an air‐entry permeameter were 1.08, 2.24, 1.52, and 2.36 µm/s. Average ρ_b and K_sat distinguished between treatments with low vs. high return of residue. SWDCs in the plow layer and tillage pan reflected the degree of soil compaction within and among treatments. No treatment differences in K_sat or ρ_b were found below the tillage pan. The K_sat in the tillage pan, relative to overlying and underlying layers, indicated that the tillage pan governs water flow in this soil. Soil pH of the Ap horizon was 5.6, 6.1, 5.4, and 4.8 for treatments 1, 2, 3, and 4, respectively; respective percent organic carbon was 1.14, 1.53, 1.07, and 1.25. The K_sat in the tillage pan was maintained at a high level only where residue additions were increased and soil pH maintained above 5.6.