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Can cover crops replace synthetic nitrogen?

RD&A News | March 2021
12 Mar 2021
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A Wine Australia-supported project is investigating whether cover crops can replace synthetic nitrogen fertiliser in vineyards.

Pippa French, a PhD student at the University of Tasmania and a Wine Australia scholarship recipient, is testing the hypothesis that sustainable management of nitrogen can maintain long-term vineyard productivity, encourage improved soil function and reduce winery costs without compromising wine quality.

‘It is already known that leguminous cover crops such as clovers and medics planted under vines can directly add nitrogen to the soil. They also have the potential to make the nitrogen already present in the soil more available to the vines’, Pippa explained.

‘My theory is that a cover crop planted in the normally bare under vine strip will improve the soil’s capacity to store and supply both water and nutrients to the vine.

‘The concept is that the cover crop improves the soil both physically and chemically, increasing the soil’s ability to store and release nutrients, increasing the storage and availability of water and stimulating faster biological turnover of nitrogen in the soil by supporting a diverse microbial community.’

Pippa said one of the perceived drawbacks of cover crops was that they competed with the vines for water.

However, she maintains that if growers select species that naturally die back before the peak of summer and give the vine roots time to adapt to the presence of cover crops, they might be able to minimise this competition.

Pippa is testing her hypothesis in a vineyard in the Coal River Valley in Tasmania with three under vine treatments: planted with legumes; with legumes and a grass; and a bare earth treatment maintained with herbicide. The treatments are repeated on two contrasting soil types – a cracking clay and a duplex soil with a sandy topsoil and clay subsoil.  

She will gather data on the plots for the three years of her PhD, monitoring the soil’s chemical, physical and microbial properties, soil moisture levels and plant stress – along with grape yield and quality.

‘I hope to quantify how much nitrogen cover crops can make available to vines through the combined effects of nitrogen fixation and improvements to soil function, and determine if they are viable as a complete nitrogen fertiliser replacement’, Pippa said.

‘I also hope to demonstrate a way that cover crops can be used in a vineyard without causing water stress in the vines. Finally, my research aims to show if any changes brought about by cover crops are likely to be constant across soil types, or if they will be different on soils of different textures.’

While the cover crops in Pippa’s project were only planted in the spring of 2020, some very preliminary results from the soil moisture sensors indicate that the cover crops stopped drawing moisture from the soil in December, which aligns with the end of the life cycle (for the annual species) or the summer dormant period (for the perennial species).

‘This is promising as it confirms that the cover crops I’m using will not compete with the vines for water over the warmest and driest part of the season.’

Pippa said finding holistic solutions that improved soil function could have a number of secondary benefits for growers – and the environment. These included carbon sequestration, better water infiltration, reduced need for herbicide passes and reduced fertiliser inputs. 

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This content is restricted to wine exporters and levy-payers. Some reports are available for purchase to non-levy payers/exporters.