Subsoil amelioration by plant-roots - the process and the evidence

Actively growing plant root systems have the potential to ameliorate subsoil in poor physical condition (biological drilling). Studies in which improved crop growth has been attributed to biological drilling by previous crops are reviewed. Whilst we might expect that plants are able to modify subsoil pore size distribution and that subsequent crops will benefit from the improved structure, this has yet to be demonstrated. Improvements in root growth, water extraction and grain yield do not, on their own, definitively establish the occurrence or benefits of biological drilling. Firstly, specific measurements of soil pores, their size, number and continuity are required to establish that soil structural change occurs through biological drilling. Secondly, the effects of biological drilling must be isolated from other confounding influences such as disease reduction and improvements in plant nutrition that might occur from crop rotation. The expected benefits from biological drilling might not eventuate where roots are unable to function efficiently in large pores or are unable to exit from them into the soil matrix. Model approaches can extend site and season specific observations and link soil structural changes to soil-plant-water processes, thus improving assessment of the consequences of biological drilling. Results are presented from an investigation into biological drilling by canola (Brassica napus L.) and the subsequent benefits to following wheat crops. Two seasons of canola did not create any measurable changes to soil structure at the top of the B horizon of a red brown earth (Natric Palexeralf) at Temora, N.S.W., even though grain yield and water extraction were greater for wheat following canola compared with wheat following wheat (probably due to reduced incidence of root disease). The canola appeared unable to create new pores due to the high strength of the soil matrix, and thus relied on the pre-existing pores. This, and other studies, tend to indicate that tap rooted annual crops such as lupins or canola are unlikely to be able to improve B-horizon porosity in dense, duplex soil. Perennial species (e.g. lucerne [Medicago sativa]) might be more effective at biological drilling because of the longer time and wider range of water content conditions in which to establish a deep root system. It remains to clearly demonstrate biological drilling as an effective process for ameliorating these dense subsoils.