From understanding how much nitrogen a cover crop might release to hearing how technology and research could improve potato growing, CUPGRA’s annual conference delivered some valuable insights. CPM attended the event to understand more.
“You have to build in how much nitrogen has probably been taken up by the cover crops, and how much water is left in the soil.” MARC ALLISON
By Mike Abram
How much nitrogen can a cover crop realistically transfer to a following potato crop, and could it be used to reduce fertiliser inputs? Those seemingly straightforward questions are far from easy to answer accurately, stated Dr Marc Allison at the annual CUPGRA potato conference in Cambridge.
In a bid to find out more, Marc has co-authored a £4000 literature review for CUPGRA – Cambridge University Potato Growers Research Association – of which, the findings will feed into a separate ongoing review of fertiliser recommendations being undertaken by GB Potatoes.
“The more you go through this, the more complicated it gets,” he told a workshop at the conference. This is because nitrogen uptake into cover crops is controlled by a mixture of factors including species type, how much nitrogen residue is in the soil, soil conditions – better structured soils allowing greater root growth take up more nitrogen – and weather, continued Marc.
Cover crop biomass, which research has shown is directly linked to ground cover and solar radiation, is also an important factor, alongside emergence date – more than drilling date – and termination and incorporation dates.
C:N RATIO
However, the carbon to nitrogen ratio (C:N) of the incorporated cover crop mix is a key factor in how much nitrogen will be available to the following potato crop, stated Marc. Cover crops, such as those containing lots of legumes with low C:N ratios are almost instantly mineralised, while mature cover crops with high C:N initially lock up nitrogen. But, that isn’t the complete story, noted Marc.
“Eventually [high C:N material] will be processed [by microbes], build up soil organic matter and there’ll be more nitrogen.”
For this nitrogen release to be most beneficial, it has to coincide with when the potato crop requires it the most – typically up to 35-40 days after emergence, commented Marc. If the C:N ratio is too high, nitrogen release may be delayed past the time when the potato crop is actively taking up nitrogen.
For his contribution towards the review, independent agronomist Martyn Cox has evaluated three systems other countries are using to measure cover crop performance and nitrogen release.
He pointed out that perhaps the best of the three is MERCI (Méthode d’Estimation des Restitutions par les Cultures Intermediaries), developed by the Regional Chamber of Agriculture in Nouvelle-Aquitaine, INRA and other partners, which allows for an estimation of nitrogen and other nutrient release.
The method involves a measurement of cover crop biomass, ideally split by species, which is then used with an online platform to incorporate data from extensive experiments and simulation models to provide detailed N, P and K estimates – total amounts, what will be immediately available, or after 30, 60, 90 days or beyond.
“It’s very educational,” said Martyn, while sharing comparisons between the model and lab analysis of biomass weight within 2kg/ha for a two-species cover crop he’d looked at.
Another system he’s reviewed is an American cover crop nitrogen calculation system (CC-NCALC), which Martyn highlighted only simulates nitrogen release. “This worked reasonably well, although you have to use a field location in the US.”
For his work, Martyn used Washington State as closest to a comparable climate to the UK. “You use real data and it gives you a prediction for N release for your estimated following crop planting date.”
Its prediction for radish was similar to MERCI, he said, but there were significant differences for some cereals, with MERCI predicting immobilisation while CC-NCALC suggesting nitrogen release.
Martyn stressed that a third system used by the Dutch was too complex and included the input of assumptions about cover crop N content, partly because its farmers were encouraged to put nitrogen on ahead of cover crops. Martyn said he thought that would affect the calculation’s accuracy in the UK. “We wouldn’t recommend you to use it currently,” he urged.
RB209
Broadly, the review has unearthed some potential challenges regarding using AHDB’s Nutrient Management Guide (RB209) field assessment method for calculating a following crop’s fertiliser requirements, when growing cover crops.
“At the moment, how much nitrogen is taken up by the cover crop and the reduced amount of water flowing through the soil profile aren’t taken into account properly by RB209,” suggested Marc. “Using oilseed rape as a proxy for a cover crop, and comparing it to bare soil over winter, you can change the amount of water that’s moving through the profile by a substantial amount depending on the size of a cover crop.
“So if you start making recommendations simply based on excess winter rainfall [on bare soil] and the previous crop, you’ll be wrong. You have to start building in how much nitrogen has probably been taken up by the cover crops, and how much water is left in the soil.”
Conversely, in a dry winter where there was little nitrogen leaching, a study by Rothamsted Research suggests that growing a cover crop could be detrimental, as the released nitrogen from it might not coincide with when it’s required by the potato crop.
“The problem is, of course, you don’t know whether it’ll be a dry or wet winter, so on average, you’re probably better off having the cover crop. But just be aware of some of the problems,” stressed Marc.
The literature review also highlighted research that showed some evidence that in cover cropping every year, perhaps by altering the amount of water going through the soil profile or by the slow build-up of nitrogen residues, that you might just be delaying nitrogen leaching to another point in the rotation rather than preventing it.
SCENARIO PLANNING
Finishing the workshop, Martyn discussed different potential cover cropping scenarios to highlight how to consider potential nitrogen release. Variables to note include soil type, cover crop mix, rainfall in early autumn, drilling or emergence date, incorporation date, and the status of the cover crop when chopped.
For example, a mid-August-drilled radish mix on sand, incorporated when green in early February, would likely release nitrogen, he said. “Radishes are deep scavenging and effective at picking up nitrogen, so they’ll absorb nitrogen when drilled early,” he said.
But the same scenario drilled in early October with higher autumn rainfall would be too late for the cover crop to as effectively mop up nitrogen. “With rainfall before the cover crop emerges, leaching will have already happened.
“It’s complicated, but RB209 would have assumed high N-leaching that the cover crop would have prevented in the first situation, but wouldn’t have done in the second.”
In a second example, he evaluated a mid-August drilled cover crop of black oats terminated in late March; rainfall had been low in autumn. “RB209 would assume on a sandy clay loam in a dry winter, not much nitrogen has been lost. But the cover crop would have taken up nitrogen, and as it’s been chopped in late when it’s woodier, it’s going to take a little while to break down. The outcome will be that you won’t get that 30kgN/ha from the cover crop.”
Remember cover crops can be radically different in how much nitrogen they might release and when, he concluded.
This article was taken from the latest issue of CPM. Read the article in full here.
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