- Faba beans are sensitive to soil acidity, in both the topsoil and subsoil
- Surface-applied lime takes time to move down the topsoil 0 to 10cm layer so incorporation through cultivation prior to sowing is generally good practice
- It is not only faba beans that are sensitive to acid soils but also their specific group F rhizobia
Results from a 2015 GRDC-supported research study have highlighted the impact of acid soils on growth and yield of faba beans, showing clear links between successful nodulation and soil pH in commercial crops grown throughout southern NSW, Victoria and South Australia.
Note: The older, less reliable pH water method, used by some laboratories, tends to record pH 0.5 to 0.8 units higher than the calcium chloride method. The calcium chloride pH method is used in this article.
Analysis of nodulation scores and soil pH highlighted the close link between pH of the top 10 centimetres of surface soil and poor nodulation (r2=0.89) of faba bean crops studied in the project. Monitored paddocks showed that faba bean nodulation is very sensitive to soil pH.
Faba bean and their specific group F rhizobia are especially sensitive to pH (calcium chloride method) below about 5.2. Group F rhizobia are more sensitive to low pH than group C (clover) rhizobia and more tolerant than group AL (lucerne) rhizobia.
Dr Mark Norton, from the NSW Department of Primary Industries, leads the joint NSW DPI/GRDC research aimed at improving the performance of legumes in the high-rainfall zones of southern NSW, Victoria and SA. NSW DPI development officer Helen Burns is also closely involved in the project.
Helen Burns, from NSW DPI.
PHOTO: Bob Freebairn
Ms Burns points out that pH from a bulked 0 to 10cm soil sample may be misleading as unincorporated lime moves slowly into the subsurface layers. Unincorporated lime increases pH of the soil surface but has limited effect on subsurface pH in the short to medium term.
For example, some growers in the study had tested 0 to 10cm topsoil samples to ensure pH levels were suitable for faba beans. One typical case was a 0 to 10cm sample from a site in Holbrook, southern NSW, which tested pH 5.2. Further investigation of the site, as well as others at Kybybolite (SA) and Lismore (Victoria), and other poorly nodulated crops, revealed that root development was concentrated near the soil surface.
Most of these poorly nodulated and poorly performing crops were sown into paddocks with a recent lime history, but surface-applied and not incorporated.
This shows that applied lime without incorporation through the top 0 to 10cm layer can result in an elevated pH in the top 5cm layer, with a band of low pH in the 5 to 10cm layer badly affecting faba bean nodulation, root growth and performance.
In a Holbrook paddock, for example, Helen Burns points out that the average pH of the surface 0 to 2cm was 6.5, but ranged from 5.2 to 7.5 and was only 4.2 at 8 to 10cm depth.
The combination of soil acidity and closely related aluminium toxicity (35 per cent exchangeable aluminium below 10cm) is an ‘acid throttle’, which severely restricted faba bean root growth and nodulation.
Ms Burns reports that soil pH tests from commercial paddocks show that surface-applied lime, when not incorporated, has very little effect below the top few centimetres in time for crops sown in the year of lime application.
This supports her view that the common practice of lime application with minimal incorporation generally does not adequately increase soil pH in the rooting zone to ensure good establishment and development of key sensitive species such as faba beans.
Faba beans are unlikely to nodulate satisfactorily in the subsoil if pH is below 5.2, with root growth less able to use subsoil moisture and nutrients or build soil nitrogen.
Based on NSW DPI research near Wagga Wagga in southern NSW, subsoil pH can be increased slowly over time by liming sufficiently to maintain a pH of 5.5 in the top 10cm.
pH the critical factor
The impact of acid soils on faba bean performance was similar across a range of soils in the NSW, SA and Victorian paddocks assessed.
All crops were scored for nodulation in late winter/early spring with close correlation between topsoil pH (0 to 10cm), nodulation and crop vigour. In all cases faba bean crops with poor nodulation and vigour were in soil with a pH below 5.2.
A sowing depth of about 4 to 5cm meant rhizobia-inoculated seeds 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 had an optimal pH above 6.0.
Helen Burns notes many 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 that lime is applied well before sowing acid-sensitive species such as faba beans. The time it takes for lime to alter the subsurface layers will depend on soil type and rainfall.
Although the 2015 results focused on faba beans, Ms Burns says effective liming is required for all acid-sensitive pulse crops, including chickpeas and lentils. These have been included in 2016 studies.
Ms Burns reminds growers that surface-applied lime will also affect the breakdown of Group B sulfonylurea (SU) residual herbicides. Liming may result in an alkaline surface layer, which extends the re-cropping interval for legume species (check herbicide labels).
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