By Tim Fulton
Photography by Victoria Rutherford

At Greenmeadows in Leeston, David Birkett is a third-generation mixed arable farmer with a lasting love of soil. He does a three-year soil testing programme as a rotation around the farm and is also checking for organic matter as part of a new testing programme with Ravensdown.

Through a family connection to the Wills family, David’s tie to Greenmeadows goes right back to early European settlement of Leeston in the 1860s.

Those long links drive some of his thinking, he says. “You want your soil to be as good as it can be and I guess that’s one of the key aims – to leave it in a state that’s better than when you got it.”

Alongside commodity crops like wheat and barley, the 2022 Arable Farmer of the Year grows ryegrass, white clover and hybrid radish for seed, and peas and beans for both seed and processing.

The farm also runs sheep in winter on the 200ha property – trading lambs and ewes on cover crops. It’s classic mixed cropping and David is happy to have livestock to complement the year-round cropping. It helps profitability and nutrient recycling, he says. “Our soil was never in a bad state – we’d always managed it using rotations, using grasses, clovers and legumes as a nutrient source for the following crops. We’ve always used that traditional approach and I guess before fertilisers, that’s exactly how we farmed. Early on we realised that soil is the key asset; that’s the thing that delivers us profitability and yield. So, I guess the first thing is going away from ploughing and incorporating residues, with a little bit less burning.”

Soil health has slowly improved under sustained residue management over the past 25 years, creating better workability and organic levels. Cover crops are helping too, selected carefully to avoid potential contamination with seed crops that follow.

The residues now going back in from cover crops are probably closer to a carbon: nitrogen ratio of 30:1, compared to 80:1 for cereal straws.

“The lightbulb moment was putting high carbon to nitrogen ratio residues into the ground and seeing slow gains. But with cover crops like wheat, oats, phacelia, buckwheat and perhaps beans, they were recycling nutrients and building the carbon in the soil at a far greater rate than that residue was, simply because they cycle carbon quicker.”

That carbon-enriched soil was holding more water and nutrient and releasing more N during the season. “And it’s free N – it’s N that up until now we haven’t really allocated in a mass budget.”

The key part of a mass budget for David is counting all the sources of N. In the past, rarely was the potential mineralisable N added into the equation.

Over the last seven years David has done his own calculations for nitrogen mineralisation – and they just happen to be similar to results from Ravensdown’s PMN (potential mineralisable N) test, which David does with the help of Senior Agri Manager Kate Higgins.

In his mass N budget, David worked on 100kg of nitrogen during the growing season. “When I do my mass budget across the whole farm, I’ve got the confidence to say there’s actually another 100kg coming on. And with the PMN test, it’s being able to say actually, it’s about 125kg for us. And the knowledge around that has allowed me to pull back on some of the nutrient inputs.”

David has cut his fertiliser use by about 30% in the last five to 10 years, mostly by reducing N inputs. To monitor N use as precisely as possible, he does a ‘No N’ strip in his paddocks every couple of years, recording the yield through yield monitoring. “That tells us that crop grew 12 tonne with no N application, therefore it must have had access to 300kg of N. So, where is that N – what are the components?” he asks.

Looking at the previous crop usually provides the answer to this. For white clover David works on the basis it had 150kg of applied N – and another 100kg of mineralisable N available from the soil. “At the end of the day we’ve ground-truthed this yield with yield data.”

That ground truthing is done leaving a strip with no N applied.

“There’s 50kg missing there … is it residue from the previous crop? Or it might be that that soil is in a better state with a higher carbon content, so the mineralisable content that came through was another 25kg … When you add those all up, you get your 300kg, which gives you 12 tonne.”

David applies a mass N budget to all crops and says it has been incredibly accurate. “I’ve been using it for quite a few years and I think it’s been more accurate than using Deep Mineralisable N [testing].”

He acknowledges he’s fortunate to have a heavy soil type with a clay base, which means little leaching. “If I was on Lismore [soil] I’d have to factor in leaching loss during the season. But I think we’ll [all] get to a point where we understand what this leaching looks like and this mass budgeting approach can be used.”

Cover crops also seem to be helping phosphate availability and Olsen P levels seem to rise following them. “There seems to be a process where that cover crop is converting phosphate into better plant availability,” he says. “One year we decided we’d go cold turkey and not do any base fertilisers on our wheats. The ones that had been in cover crops looked as good as the ones that had a base fertiliser on. And the ones that hadn’t, really struggled. Obviously, the availability of phosphate was quite crucial to that early development.”

David says it’s probably fair to suggest he’s “mining” his phosphate. “But if you’re mining it just a little bit and your profitability’s not that good [at the time] then sometimes that’s just what you’ve got to do. And we’re talking about very small percentages.”

He follows established science closely but also takes cues from what the soil shows him. “Soil is my biggest asset; it’s the medium my business grows in.”

David has also noticed that improving soil chemistry seems to have reduced susceptibility to disease. “We’ve seen a shift in the weed spectrum and that’s probably down to chemistry. We’ve probably gone higher on in our pHs than we would normally do. We used to go high fives and now I’m quite happy to see them at 6.2 – and I think that’s changed the weed spectrum that we’re seeing. If you look at the magnesium and calcium ratios, some people say there’s nothing in it, but our best performing paddocks always have a good magnesium-calcium ratio.”

Research might suggest no relationship here, but David says they are seeing something going on. “I think we’ve just got to keep being open minded about soil. There’s a lot of surprises in there and with some of the surprises that we’re witnessing … there’ll be an explanation. It’s just that we haven’t worked it out.”

Fact File

• Owners David and Justine Birkett, farming in a family partnership with David’s parents, Paul and Lois
• Temuka silt loam and Paparoa sandy loam (mostly heavy Temuka types), fully irrigated
• Most crops get a base fertiliser, determined by the previous crop, and soil testing (especially for
  the most valuable crops) and timings are based on growth stages
• Planting rate is relatively high, as David prefers to let the plants self-regulate. “I believe the plant knows better than me and it will regulate itself as to the number of tillers that it needs”
• Tile and open drains

What is the PMN test and calculator?

Ravensdown’s Nitrogen Mineralisation soil test option for measuring potentially mineralisable nitrogen (PMN) makes it easier for arable farmers to keep tabs on the amount of potential plant-available N in their soils for crop growth.

The Nitrogen Mineralisation Calculator predicts the amount of N that will be released from soil organic matter over a typical spring/summer growing season at different locations across New Zealand. The site-specific prediction represents the cumulative amount of mineral N (kg N/ha) that will be supplied to a crop over a four-month period following soil sampling. It’s important to recognise that the N is released gradually over that period and that the rate of release will be higher where the soils are warmer and wetter.

The calculator was developed based on generic soil descriptions and historic soil temperature and water content data for nine locations across New Zealand, representing different climatic zones in the major cropping regions. The N Mineralisation Calculator can be applied to cropping soils in other parts of the country where there is a reasonably close match with the soil type (soil order), temperature and rainfall at a soil test site and one of the nine original calculator calibration locations.