Green manure crops a viable alternative to artificial nitrogen

Using green manure crops to nurture soil and crop health has been a longstanding and widely used method in traditional growing and horticulture practices. But now the benefits of green manure crops have been scientifically validated in New Zealand through a recently completed on-farm trial in Nelson.

Funded by Our Land and Water and carried out by market gardener Dominic Ferretti on his Brightwater property, the ninemonth trial showed green manure crops provided enough nitrogen to the soil to grow a bumper crop of high-quality potatoes.

Previously using large volumes of homegrown compost on his market garden, Dominic began looking into green manure crops as a less labour-intensive alternative.

“We were making compost on a big scale using sawdust and chicken manure, which does make great compost for growing vegetables, but there’s a lot of labour and a lot of machinery and costs. I was getting really tired of it. Soil tests were showing we were getting too high in phosphorus, coming from the chicken manure, so we thought this imbalance isn’t going to work long term.”

Although his horticultural consultant, Sjef Lamers, provided plenty of overseas studies to demonstrate the benefits of green manure crops, Dominic struggled to find any significant New Zealand-based research about the practice. “There was a whole lot of data, but it was hard to relate it to New Zealand,” Dominic said. “Some of the varieties they grow are different or the same thing with a different name, it’s all in pounds per acre instead of kilos per hectare, and it’s for growing corn and soya beans in America. It doesn’t really make sense to the average Kiwi farmer.”

Dominic and Sjef designed a simple trial on the Brightwater property where two green crops were grown and a third plot was left bare as a control. The first crop was legume only (broad beans – Vicia faba) and the second a legume/grain mix (50:50 broad beans and black oats – Avena strigosa).

Each treatment was assigned five plots of 48m x 2.5m and arranged randomly throughout the property.

“The legumes fix nitrogen out of the atmosphere. The oats, a grain, don’t fix nitrogen but they’re really good at mopping it up out of the soil, and they add more carbon but take longer to break down,” said Dominic.

After two months the captured biomass from the legume and mixed green crops was 9.7 tonnes per hectare and 9.9 tonnes per hectare respectively.

Captured nitrogen was 289kg per hectare for the legume green crop and 198kg per hectare for the mixed green crop. Measurements for the total captured nitrogen from the green crops were more than sufficient to grow vegetable crops.

The crops were chopped at the onset of the broad bean flowering, which is typically when the vegetation is at its densest. To speed decomposition, the biomass was reduced into smaller pieces by two passes with a slasher mower, then incorporated into the top 4cm of soil with a rotary hoe cultivator.

Dominic planted 185kg of agria seed potatoes in all 15 plots, and harvested them 112 days later.

The trial found that potato yields were 33.5 tonnes per hectare from the legume plots, 30.6 tonnes per hectare from the mixed plots and 30 tonnes per hectare from plots that didn’t grow any manure crops.

The legume plots also yielded the highest weight and number of high-quality sellable potatoes, as well as the largest mean tuber weight compared to the mixed and control plots.

Soil tests were conducted throughout the nine-month trial, measuring the levels of the different forms of mineral nitrogen, as well as Olsen P (phosphorous), K (potassium), Ca (calcium), Mg (magnesium), Na (sodium), total carbon and other elements.

To ensure accurate measurements, both the manure crops and potatoes were sent off for measuring. “We had to cut the plants at ground level, weigh that material and then pull out the roots. We sent it all to Hill Laboratories where they carefully washed the roots, and analysed them and the tops for nitrogen content,” says Dominic.

“It was what we expected – the trial went well and we were very pleased.”

Dominic said the findings have the potential to benefit any food producer who wants to reduce their use of costly, synthetic nitrogen inputs.

Nitrogen fertiliser costs doubled between 2020 and 2022 in New Zealand and are expected to keep increasing. Regulations are also on the rise, and new clean waterway requirements are likely to make the use of nitrogen fertilisers increasingly prohibitive.

“We just wanted to get people talking and maybe get them to think, ‘I might just try that down in the back paddock,’ or something. That’s all we want people to do at this stage, because as soon as you try it you’ll see the results and they’ll speak for themselves – and it’s a no-brainer from there.”

As well as effectively replacing artificial nitrogen, the research findings showed that the green crops improved soil structure. They can provide extensive benefits to soil and ecosystem health that can assist vegetable production, as well as vineyards and orchards.

Dominic and Sjef Lamers have secured additional funding to produce a best practice guide for other growers, building on the knowledge gained in the trial.

“We actually found that we didn’t need to do that much incorporating into the soil, mixing it in with a rotary hoe. It’s actually best left on the surface as residues, a mulch, so that’s even less fuel cost.

“I think Kiwi farmers are feeling the pressure. We’ve got heaps of regulations coming and I believe if we can just get the information to them to try this, even if it helps them to just use a bit less synthetic nitrogen to begin with, it would be a huge success.” Source: www.ourlandandwater.nz