Faba beans with improved varieties and high standard agronomy not only are yielding high more consistently but also are commonly the best grain legume for building soil nitrogen.
But a key requirement for success from faba beans is effective nodulation and root growth not adversely affected by either surface or subsoil acidity.
A 2015 research study highlights the impact of acid soils on growth and yield of faba beans with clear links between successful nodulation and soil pH in commercial crops in southern NSW, VIC and SA. Dr Mark Norton NSW Department of Primary Industries leads the joint NSW DPI/Grains Research and Development Corporation research aimed at improving the performance of legumes in the southern region high rainfall zone. Helen Burns, NSW DPI development office is also vitally involved in the project.
Soil pH below 5.2 (Ca chloride method) is confirmed by the research as a critical factor. Faba bean and its specific Group F rhizobia are especially sensitive to pH below about 5.2. Also especially critical is that applied lime without careful incorporation through the top 0 -10 cm layer can result in an elevated pH in the top 5.0 cm layer with a band of low pH in the 5.0 – 10 cm layer badly impacting on nodulation, root growth and faba bean performance. Also critical is soil pH below the 10cm layer. Faba beans are unlikely to nodulate satisfactorily in the subsoil if pH is below 5.0 with root growth less able to use subsoil moisture and nutrients or build soil nitrogen.
Helen Burns points out that pH from a bulked 0-10 cm soil sample may be misleading as unincorporated lime moves slowly into the subsurface layers. Lime unincorporated increases pH of the soil surface but has limited effect on subsurface pH in the short to medium term.
Faba bean crops with poor nodulation and vigour had a soil pH below 5.0. A sowing depth of about 4.0-5.0 cm meant rhizobia inoculated seed were placed in an acid soil layer (either recently applied lime not well incorporated or not limed at all). This is likely to have affected rhizobia survival, root growth and nodulation. Group F rhizobia are sensitive to pH below 5.0 with the optimal pH being above 6.0. Most growers have a minimum tillage or zero tillage farming program and rarely incorporate lime. If incorporation is not an option she stresses it is essential lime be applied before sowing acid sensitive species such as faba bean.
Next week. How to return to top winter legume pastures.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email robert.freebairn@bigpond.com or phone (0428) 752 149.
