Under no-tillage and stubble retention, soil water content and crop growth are poorly related to soil water repellency

Abstract

In southern and western Australia up to 10 million hectares of farmed land is water repellent or at risk of developing repellency. The majority of these soils are sandy. Their high susceptibility to erosion has led to the adoption of practices such as no-tillage and stubble retention. However, retention of stubbles can lead to increases in soil organic matter and consequently aggravate soil water repellency. In a 4-year study on sandy soils on the south coast of Western Australia, soil organic C (LECO), soil water repellency (measured by the Molarity of Ethanol Drop (MED) method), soil water contents (using a hand held time domain reflectometer (TDR)) and crop performance (emergence and grain yields) were monitored in four treatment combinations; no-tillage, stubble retained or burnt; cultivated, stubble retained or burnt. Over time, higher levels of soil organic C were measured under no-tillage than cultivation, and under stubble retention than stubble burning. Soil water repellency followed a similar pattern to soil organic C with the most severe repellency under no-tillage and stubble retention and least under stubble burning and cultivation (R2=0.67). However, soil water contents measured in the field contradicted the findings on water repellency and indicated that water infiltration was best under no-tillage and stubble retention and poorest under stubble burning and cultivation, and this impacted on crop performance. The results suggest that mechanisms other than just soil water repellency are involved in determining soil water content and crop performance. Visualisation of water infiltration using blue dye indicated that under no-tillage, old and current crop rows provide pathways for water movement in the soil, thereby by-passing the repellent surface layer. These findings challenge traditional thinking on soil water repellency and have implications for crop management.