The Soil Wealth and Integrated Crop Protection (ICP) project works with growers to put soil management and plant health research into practice. This edition explores the use of legume cover crops to better manage nitrogen on-farm. Soil Wealth ICP Phase 2 (VG16078) is a strategic levy investment under the Hort Innovation Vegetable Fund.

The commercial nitrogen fertiliser market has been very volatile in recent times, with shipping issues and shortages for all of Australia. As Australia does not manufacture enough nitrogen to supply its own domestic market, the cost of nitrogen fertiliser has more than doubled.

Given the price rises and uncertainty of nitrogen fertiliser supply, it’s a good time to add a legume cover crop into your rotation and reduce your reliance on nitrogen fertiliser.

Legume cover crops can add lots of nitrogen, with a good cover crop adding 160-200 kilograms per hectare (kg/ha) of nitrogen. Based on recent nitrogen fertiliser costs, that is $500-$600 of added nitrogen per hectare, with seed and inoculant costing around $100-$200 per hectare.

The science of biomass

To add the most nitrogen, legume biomass is king.

For every tonne of shoot biomass grown, about 20 kg of nitrogen will be added. So, if you can grow a good legume cover crop with the right inoculant and produce 8-10 tonnes of shoot biomass, then that’s 160-200 kg of nitrogen added. Your cover crop roots can have a further 30-100 kg of nitrogen.

Less nitrogen will be added when cover crops are grown as part of a cover crop mix, as the legume growth and biomass are reduced due to competition from the other cover crop species in the mixture. However, cover crop mixes are a good option when you have high soil nitrate levels.

High soil nitrate levels will reduce nitrogen added by legumes. Basically, if the plant can get lots of ‘free’ nitrogen from the soil then it will not allow the Rhizobium bacteria to nodulate the roots and will not feed the bacteria. Instead, the plant will take the ‘free’ nitrogen from the soil.

It’s important to use soil tests before your cover crop is planted to find out how much soil nitrate is remaining. If it’s below 50 kg of nitrogen per ha (to 30 cm), the legumes will add lots of nitrogen. Above 200 kg of nitrogen per ha will limit your legume from adding much nitrogen.

Managing soil nitrogen levels

Typically, soil nitrogen levels will be low following nitrogen-hungry crops like corn and potatoes; higher soil nitrogen levels are likely following leafy vegetable crops. The levels of soil nitrogen before you sow your cover crop will also depend on how much in-crop nitrogen fertiliser was used.

If you have high soil nitrogen levels, it may be better to plant a mixed cover crop of a cereal/broadleaf and legume. The cereal and broadleaf will recover and store the nitrogen in the soil and force the legume to fix its own nitrogen.

A mix like a sunn hemp, sorghum and tillage radish for summer – or oats and vetch mix for winter – will do a good job of soaking up the nitrogen already in your soil left over from a previous crop and encourage the legumes in the mix to add some more nitrogen.

Nitrogen added by legume cover crops must be converted into nitrate and ammonia by the soil biology to be available to your vegetable crop. This typically means there is a nice, slow release of nitrogen over the life of the crop.

Additional resources

To understand how much and when nitrogen will be available for your vegetable crop, watch the Soil Biology Master Class 2021 – Nitrogen availability webinar.

To help choose the right legume and Rhizobium, see the Cover Crops for Australian Vegetable Growers poster.

You can also find out more about managing nitrogen on-farm and using legume cover crops in this recent webinar recording with Kelvin Montagu, Doris Blaesing and Marc Hinderager. Click here to watch.

The presentation also covers:

• Optimising fertiliser practices.
• Reducing nitrogen fertiliser losses (e.g. leaching and denitrification).
• Soil testing and crop monitoring.
• An agronomist panel that discusses the tried strategies to manage nitrogen on clients’ vegetable farms.

Tips to get the most nitrogen out of your legume cover crop

• Add the right Rhizobium inoculant at the right time.
• Select and grow a vigorous legume cover crop – the more biomass grown, the more nitrogen added.
• Grow a straight legume (i.e. not part of a cover crop mix).
• Legume cover crops are most effective with low to moderate soil nitrate levels – less than 50 kg/ha of soil nitrate will encourage your legumes to add the most nitrogen.

The Soil Wealth and Integrated Crop Protection team will be returning to Hort Connections this year. Pictured (from L-R) are team members Pieter Van Nieuwenhuyse, Kelvin Montagu, Doris Blaesing and Gordon Rogers.

Don’t miss the Soil Wealth ICP grower panel at Hort Connections!

The Soil Wealth ICP grower panel at Hort Connections 2022 will explore cutting-edge practices and technologies that are key to improving productivity, profitability and sustainability in the Australian vegetable industry.

On Monday 6 June during the Annual Vegetable Innovation Seminar, leading growers will provide first-hand experience and insights from hosting a demonstration site as part of the project over the past eight years.

The panel will tackle some of the big topics – from strip-till, cover crops, soil amendments and biological products – and your burning questions about putting research into practice, including:

– What are the benefits of hosting demonstration sites and undertaking trials?

– Why is it important to innovate?

– How do you start trialling a new technology or practice on your farm?

– Who do you look to for inspiration and research?

You can also meet the Soil Wealth ICP team at the Trade Show (Booth 233 – RMCG and Booths 255/271 – Applied Horticultural Research).

Find out more

For more information, please contact project leaders Dr Gordon Rogers on 02 8627 1040 or gordon@ahr.com.au and Dr Anne-Maree Boland on 03 9882 2670 or anne-mareeb@rmcg.com.au.

This project has been funded by Hort Innovation using the vegetable research and development levy and contributions from the Australian Government.

Project Number: VG16078

Cover image: Sunn hemp in a mixed cover crop will soak up nitrogen in the soil and encourage the legume to add more.