Abstract/Summary

The impact of soil compaction on soybean root growth and grain yield can be alleviated by the presence of biopores and root channels in the soil profile. We hypothesize that cover crops (ruzigrass and oats) are better than grain crops (wheat and maize) to reduce the soil physical limitation to soybean root growth. We aimed to identify which precrops have higher potential to reduce the mechanical and water stresses resulting from soil compaction and soil desegregation, and to enhance soybean (Glycine max L) root growth and yield in an Oxisol (Rhodic Eutrudox), with clayey soil texture. Soybean was grown after four precrops (ruzigrass, oats, wheat, or maize), under four soil compaction levels [soil chiselling (MTC), no-tillage (NT), NT additionally compacted with four passes of a tractor (NTCT), and NT additionally compacted with eight passes of a grain harvester (NTCH)]. Soil physical attributes (bulk density, macroporosity, water-filled pore space and soil penentration resistance) in the soil profile (0−50 cm) and soybean components (grain yield, cumulative root length density and root dry mass) were investigated. Soil physical attributes were improved over time due to the combined effects of natural wetting-drying cycles and biopores created by the roots of precrops. Ruzigrass increased soybean root biomass and length density, mainly at deeper soil layers of compacted treatments (NTCT and NTCH). The rate of increase of soybean root length density in the soil profile was higher after ruzigrass cultivation in comparison with maize and oats. Soil compaction effects on grain yield were partially (NTCH) or totally (NTCT) alleviated after two years and ruzigrass intensified the mitigation process. Ruzigrass also resulted in higher soybean yields in comparison with maize, whereas NTCH and MTC reduced yields in approximately 400 kg ha−1 (∼13 %) compared to NT and NTCT. Soil strengthening was more evident after ruzigrass and oats cultivation than maize or wheat cropping. Soil compaction in clayey Oxisols can be alleviated over time as a result of root channels provided by precrops combined with natural wetting-drying cycles. Among the evaluated precrops, ruzigrass is of particular interest, as it provides the most suitable soil physical environment for soybean root growth and grain yield. In contrast, chiselling was demonstrated to be a non-viable strategy to mitigate soil physical constraints for root growth and grain yields.