Pulses on acid like timely lime

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Farmer Chris Shady stands in his rana faba bean crop on his farm east of Skipton: new DPI-GRDC data confirms that the timely, effective application of lime to acidic soils is key to improving yields in acid-sensitive legumes such as faba beans..
Farmer Chris Shady stands in his rana faba bean crop on his farm east of Skipton: new DPI-GRDC data confirms that the timely, effective application of lime to acidic soils is key to improving yields in acid-sensitive legumes such as faba beans.
Emma Field

With winter crop planning a priority right now for farmers across the state and many considering planting pulse crops, New South Wales Department of Primary Industries (DPI) has some timely news.

New data from a joint DPI and Grains Research and Development Corporation (GRDC) project confirms that with effective applications of lime, you can substantially boost faba bean yields in acidic soils.                   

The operative word here is effective: according to DPI development officer Helen Burns, merely sprinkling lime around the paddock assuming it will neutralise soil pH may not be enough.

“Current practices of surface lime application with minimal incorporation may not adequately increase soil pH in the rooting zone to create the conditions acid-sensitive legumes need for root growth and rhizobia survival,” she says.

Legume root nodules - critical for plant growth, health, vigour and yield - are affected by soil pH.
Legume root nodules - critical for plant growth, health, vigour and yield - are affected by soil pH.
US Department of Agriculture (USDA), via Wiki Commons

Getting to the root of the problem

The DPI-GRDC project team set out to find an explanation for the poor faba bean yield many NSW growers have experienced - and found it in the crucial top 10 centimetres of soil.

If the topsoil layer, the ‘rooting zone’, is too acidic, faba beans’ nitrogen-fixing root nodules will be stunted, which means poorer plant health and productivity.

“Rhizobia play an important role in nodulation and nitrogen fixation in these legumes – feedback from farmers and agribusiness advisers identified poor nodulation as a possible cause of low faba beans yields,” Burns explains.

“Investigations of commercial crops in 2015 revealed soil acidity was the likely culprit, reducing nodulation, root growth, plant vigour and yield potential.

“Most paddocks we monitored were limed in recent years, yet soil tests from those with poor faba bean nodulation had a pH below 5.0 in the top 10 centimetres of soil,” she says.

The project team hypothesised that merely applying lime to the soil surface wasn’t enough to neutralise acidity in the all-important root zone.

Detailed sampling to a depth of 15 centimetres indicated that any lime not incorporated into the soil had “a limited effect” on sub-surface pH. “At one site, soil pH was 7.3 at the surface and dropped to 4.8 in the five-to-seven-centimetre sample,” she notes.

Moreover, Burns said, bulking surface soil samples may deliver misleading soil test results. “Lime moves very slowly into subsurface layers, and our 2015 observations suggest that faba bean root growth is severely limited beyond the depth of the lime effect.”

Faba bean crop: if the top 10 centimetres of soil are too acidic, the legume's all-important root nodules are likely to be stunted, affecting plant health and yield.
Faba bean crop: if the top 10 centimetres of soil are too acidic, the legume's all-important root nodules are likely to be stunted, affecting plant health and yield.
Stan Zebla, Flickr CC, wwwflickrcomphotosstanzebl

3 tips to take away

1. If you’re going to apply lime to acidic soils, ensure you incorporate it thoroughly.
Mix it in to a depth of at least 10 centimetres to give your faba bean crop a fighting chance.

2. If it’s not feasible to incorporate it, apply lime early and give it a chance to soak in.
On properties where incorporating lime is not an option, Burns suggests that farmers apply lime well before they sow acid-sensitive crops such as faba bean. “The timing for lime to impact on subsurface soil pH will vary according to soil type and rainfall,” she says.

3. Pay attention to what herbicides you apply when.
Burns recommends that growers manage their herbicide schedules carefully according to intended crop rotations and paddock selection.

“Soil pH affects the breakdown of Group B sulfonyl urea herbicides, and high pH levels in soil surface layers can extend the re-cropping interval for legume species,” Burns explains. “We advise growers to pay close attention to herbicide labels.”

For more information, visit the NSW DPI and GRDC websites.

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