SUBSOIL AMELIORATION BY PLANT-ROOTS - THE PROCESS AND THE EVIDENCE

Actively growing plant root systems have the potential to ameliorate s ubsoil in poor physical condition (biological drilling). Studies in wh ich improved crop growth has been attributed to biological drilling by previous crops are reviewed. Whilst we might expect that plants are a ble to modify subsoil pore size distribution and that subsequent crops will benefit from the improved structure, this has yet to be demonstr ated. 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 ef fects of biological drilling must be isolated from other confounding i nfluences such as disease reduction and improvements in plant nutritio n that might occur from crop rotation. The expected benefits from biol ogical drilling might not eventuate where roots are unable to function efficiently in large pores or are unable to exit from them into the s oil matrix. Model approaches can extend site and season specific obser vations 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 followin g wheat crops. Two seasons of canola did not create any measurable cha nges to soil structure at the top of the B horizon of a red brown eart h (Natric Palexeralf) at Temora, N.S.W., even though grain yield and w ater extraction were greater for wheat following canola compared with wheat following wheat (probably due to reduced incidence of root disea se). The canola appeared unable to create new pores due to the high st rength 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 [Medi cago sativa]) might be more effective at biological drilling because o f the longer time and wider range of water content conditions in which to establish a deep root system. It remains to clearly demonstrate bi ological drilling as an effective process for ameliorating these dense subsoils.