Effects of Bulk Density, Aggregate Size, and Soil Water Suction on Oxygen Diffusion, Redox Potentials, and Elongation of Corn Roots^†

Abstract

Corn seeds (Zea mays L. ‘WF9 × 38‐11’) were planted in plastic cylinders of soil at different bulk densities (0.93, 1.03, 1.13, and 1.23 g cm^‐3) or aggregates of different sizes (< 0.5, 0.5–1, 1–2, 2–3, and 3–6 mm). Soil water suction during germination was maintained at constant levels (0, 3, 18, 48, and 68 cm of water).

Germination began and root elongation rates sharply increased from zero to a maximum in each soil sample as soil water suction increased above the air‐entry value or bubbling pressure. Over these ranges of suction, diffusion of O₂ controlled rate of root elongation. Diffusion of O₂ in soil was determined primarily by air porosity. Bulk density and aggregate size per se had little effect on diffusion or concentration of O₂ at seed depth. Bulk density and aggregate size greatly influenced soil water desorption, the depth of soil to which O₂ could diffuse, and the thickness of water films at equal air porosities. Between approximately 0 and 20 cm of suction, thickness of water films limited root elongation. Measured O₂ concentrations in soil agreed with concentrations predicted by diffusion theory. Redox potentials were correlated with O₂ levels. Root elongation rates decreased in some samples at the highest suctions used, probably because of increased soil strength.