Plant exudates improve the mechanical conditions for root penetration through compacted soils

Plant exudates greatly affect the physical behaviour of soil, but measurements of the impact of exudates on compression characteristics are missing. Our aim is to provide these data and explore how plant exudates may enhance the restructuring of compacted soils following cycles of wetting and drying. Two soils were amended with Chia (Salvia hispanica) seed exudate at 5 concentrations, compacted in cores to 200 kPa stress (equivalent to tractor stress), equilibrated to -50 kPa matric potential, and then compacted to 600 kPa (equivalent to axial root stress) followed by 3 cycles of wetting and drying and recompression to 600 kPa at -50 kPa matric potential. Penetration resistance (PR), compression index (C-C) and pore characteristics were measured at various steps. PR decreased and C-C increased with increasing exudate concentration. At 600 kPa compression, 1.85 mg exudate g(-1) soil increased C-C from 0.37 to 0.43 for sandy loam soil and from 0.50 to 0.54 for clay loam soil. After 3 wetting-drying cycles the clay loam was more resillient than the sandy loam soil, with resilience increasing with greater exudate concentration. Root growth modelled on PR data suggested plant exudates significantly eased root elongation in soil. Plant exudates improve compression characteristics of soils, easing penetration and enhancing recovery of root induced soil compaction.