The distribution and abundance of wheat roots in a dense, structured subsoil – implications for water uptake

We analysed the abundance, spatial distribution and soil contact of wheat roots in dense, structured subsoil to determine whether incomplete extraction of subsoil water was due to root system limitations. Intact soil cores were collected to 1.6 m below wheat crops at maturity on a red Kandosol in southern Australia. Wheat roots, remnant roots, soil pores and root–soil contact were quantified at fresh breaks in the soil cores. In surface soil layers (<0.6 m) 30–40% of roots were clumped within pores and cracks in the soil, increasing to 85–100% in the subsoil (>0.6 m), where 44% of roots were in pores with at least three other roots. Most pores contained no roots, with occupancy declining from 20% in surface layers to 5% in subsoil. Wheat roots clumped into pores contacted the surrounding soil via numerous root hairs, whereas roots in cracks were appressed to the soil surface and had very few root hairs. Calculations assuming good root–soil contact indicated that root density was sufficient to extract available subsoil water, suggesting that uptake is constrained at the root–soil interface. To increase extraction of subsoil water, genetic targets could include increasing root–soil contact with denser root hairs, and increasing root proliferation to utilize existing soil pores.