Ripping yarns: 25 years of variable responses to ripping clay soils in south-eastern Australia

Soil compaction has re-emerged as a potential limitation to further yield improvements on clay loam soils in southern NSW but the few published studies are restricted to sodic clay soils. A survey of 39 paddocks across the region in 2004/5 revealed that few had sub-surface soil strengths <2.0 MPa, a commonly accepted threshold at which root growth is restricted, while around 40% had levels >3.0 MPa. A review of crop responses to deep ripping to 40 cm in the area from 1980 to 2005 showed yield responses in only 5 of the 15 crops measured (from 11 to 48% increase), despite consistent reductions in soil strength to <1.0 MPa, and 20 to 30% increases in early vegetative biomass at some sites. Experiments conducted at 9 sites across the region in 2007, a drought year, showed that ripping consistently reduced sub-surface soil strength from 2.5 - 4.0 MPa to 0.5 - 1.3 MPa, but early vegetative biomass increased at only 2 of the sites. Ripping had no effect on yield at 5 sites and significantly reduced crop yield/biomass at the remaining 4 sites apparently due to water loss associated with soil disturbance. In the face of these variable responses, we discuss the role of structural cracks and biopores which enable roots to penetrate high-strength soil and the prospects of identifying paddocks where long-term economic yield responses to deep ripping can be achieved.