Variable responses of canola and cereals to deep ripping and liming of high-strength, acidic, subsurface soil layers

Context Acidification of hard subsurface layers has been observed in agricultural soils in southern Australia. This study tests hypotheses that subsurface acidity is widespread in the region, and that subsurface liming increases yields of wheat, canola and barley. It also investigates the interaction of roots and an acid subsurface. Methods In 19 field experiments where the subsurface soil pH was < 4.5, lime was injected into soil behind deep-ripping tines. Profiles of soil pH and strength were measured and the growth and yield of wheat, canola or barley determined. In a separate experiment subsurface soils collected from each field site were utilised for a pot study in which canola was grown in a pH 5.8 topsoil below which there were 0.10 m-thick subsurface layers varying in pH from 3.9 to 5.9, and subsoils all with pH 5.8. Results Deep ripping reduced soil strength and injected lime increased profile pH in the field experiments, but no consistent responses by vegetative growth or grain yield were observed. In the pot experiment, above-ground canola growth was neither affected by subsurface acidity measured as pH, nor as concentrations of Al or Mn; however, root growth was reduced by acidity. In both the field and pot experiments the roots of canola were observed growing through the acidic subsurface soil and proliferating below it. Conclusions Neither the field experiments nor pot study showed consistent effects on growth or yield of canola or cereals to subsurface acid layers where the soil above and below were not highly acidic. A possible explanation is that roots can penetrate a discrete acidic subsurface layer and gain access to nutrients and water from soil above and below the layer. Subsurface acidity, unless ameliorated, may worsen and deepen if unresolved.