Soil Structure under No-Tillage Enhances Soybean Root Growth and Access to Subsoil Water

Soil structure can mitigate both mechanical impedance and water stress, thereby modulating root elongation and access to deep soil water. Although process-based root growth models represent soil–root interactions, they rarely account explicitly for structural conditions and their consequences for the combined effects of water and mechanical stresses on root growth. We quantify how soil structure under no-tillage influences soybean root elongation, effective rooting depth, and water-deficit mitigation, and we parameterize these effects in a biophysical root-growth model. A long-term field experiment established in 2016 compared three cropping systems preceding soybean (Glycine max): ruzigrass (Urochloa ruziziensis), maize (Zea mays), and fallow. Soybean root length density and soil physical attributes were measured in nine layers down to 210 cm. Effective rooting depth was defined as the depth containing 95% of total root length. Plant-available water was computed from soil water retention between −60 and −15,000 hPa, and readily available water was assumed as 50% of plant-available water within the rooted zone. Grain yield was determined at harvest. In addition, soybean root elongation rate was measured in the laboratory using core from field and repacked samples across gradients of degree of saturation and soil penetration resistance. The structural effect was incorporated as a parameter in a biophysical model that combines water and mechanical limitations to root elongation. Increasing soil penetration resistance from 1.0 to 3.5 MPa reduced relative root elongation by 46% in preserved structure, whereas reductions reached 76% in repacked soil. At 0.5 MPa and 60% degree of saturation, elongation in repacked soil was 29% higher than in preserved structure, but both structural conditions converged as soil penetration resistance increased to 1.0 MPa. Under 90% degree of saturation, elongation in preserved structure was nearly threefold that in repacked soil. In the field, effective soybean rooting depth (in a trench of 210 cm depth) differed among previous cropping systems, with ruzigrass promoting substantially deeper roots (154.7 cm at 95% cumulative distribution) compared with maize (127.9 cm) and fallow (121.0 cm). Root length density in the 0 to 10 cm layer was highest after ruzigrass (4.72 cm cm^-3), followed by maize (3.33 cm cm^-3) and fallow (2.48 cm cm^-3). Cumulative root length in the soil profile from 0 to 210 cm reached 202.2 cm cm^-2 after ruzigrass, compared with 128.4 cm cm^-2 after maize and 94.3 cm cm^-2 after fallow. Soybean yield was 2.9 (after ruzigrass), 2.6 (after maize), and 2.1 Mg ha^-1 (after fallow). Plant-available water in the soybean root zone was 175 mm after ruzigrass, compared with 145 mm after maize and 140 mm after fallow, indicating a 25% increase relative to fallow. Assuming evapotranspiration of 7 mm d^-1, this represents approximately 15 days of water supply after ruzigrass versus 12 days after fallow. Preserved soil structure improved soybean root performance under strong physical constraints and increased deep water access. Explicitly representing soil structural conditions in simulation models can improve predictions of rooting depth and drought mitigation under no-tillage.

Acknowledgements: AGRISUS Foundation [PA 3534/23], CNPq [409621/2023-4] and FAPESP [23/10427-3 and 23/11945-8].