Variable rate nitrogen: a practical way to manage rising fertiliser costs

With carbon-related fertiliser cost pressure building through CBAM, many arable businesses are re-checking nitrogen strategy. Variable rate nitrogen can help you put fertiliser where it returns most, and reduce spend where it won’t.

The carbon border adjustment mechanism (CBAM) is expected to add £50–£75 per tonne to nitrogen prices within the next year. Even before that, the impact may be felt sooner because the EU has already implemented CBAM, and the EU accounts for most UK nitrogen fertiliser imports.

Ben Foster, Product Manager for RHIZA, says this could affect spring pricing.

“We could see £40/t added to fertiliser prices this spring because of CBAM’s implementation in the EU, which isn’t ideal considering the current grain prices and that nitrogen is already at a high price.”

Against that backdrop, nitrogen decisions become less forgiving. The cost of over-applying is higher, and the return on the last part of yield is harder to justify in some situations.

What variable rate nitrogen does in practice

Variable rate nitrogen is not new, but the tools around it have moved on. Some farms have tried it before and stepped away. Others already have the spreading or application capability but have never turned it on.

Ben’s view is that the current price environment makes it worth revisiting, even if you start small.

“There are now new tools available to farmers that make variable rate nitrogen cheaper, easier and more accurate. I’d encourage all farmers with the capability of variable rate spreading to look at the technology this season, at least in a trial area, to examine its results on their bottom line.”

The principle is simple: field potential is rarely uniform. Variable rate nitrogen aims to match rate to performance potential, rather than applying one rate everywhere and hoping the crop responds evenly.

How to decide if your fields are good candidates

Start with data you may already have. Combine yield mapping is one of the most useful places to begin, and more farms now have access to yield maps as older machines enter the second-hand market.

Ben suggests pairing historic yield data with in-season satellite imagery.

“I’d recommend looking at the variation in satellite imagery from a platform like Contour in March or April, and overlaying it with historical yield maps. In my experience, the variation in spring satellite images will likely correlate very closely with yield data from the summer.”

If the same zones repeatedly under- or over-perform, that is often a strong signal that fixed-rate nitrogen is unlikely to be efficient.

Case example: Revesby Estate and variable rate nitrogen in wheat

Peter Cartwright, Farm Manager at Revesby Estate in Lincolnshire, moved to variable rate nitrogen on wheat last year after seeing the approach tested through an Agrii Digital Technology Farm trial. The estate had used variable rate nitrogen previously but stopped because confidence in how to adjust rates was limited.

“In oilseed rape, it’s straightforward because it’s linked to the green area index. With wheat, we didn’t know whether to push a backwards crop or hold back on it. We are still asking similar questions, but we have a better grasp of it with the information available to us.”

Peter says the economics have shifted. When nitrogen was cheaper relative to grain values, higher rates to chase yield often stacked up. With nitrogen higher and grain lower, the risk of spending too much is more acute.

Last season the estate did not reduce nitrogen use overall, but the distribution changed. They pushed the stronger areas and held back on others. Looking ahead, Peter is now actively looking for savings.

“The economic viability of that last 10% of yield has changed.”

The practical point is that variable rate nitrogen does not have to mean “less nitrogen”. It can mean better placed nitrogen, with tighter control over where spend is likely to return margin.

Two approaches: optimum yield or canopy levelling

Ben explains that variable rate planning in Contour supports different objectives, and the right choice depends on the season and how well you know the field.

• Optimum yield: the plan favours stronger areas with more nitrogen to maximise yield potential where the crop is already performing well.
• Canopy levelling: the plan increases nitrogen in weaker areas to try to even up crop development and reduce variability.

“There is no definitive answer to this. It is dependent on the season and the farmer’s knowledge of the field.”

Agrii’s Digital Technology Farm work at Revesby is building evidence around these choices. Last season, results suggested that holding back nitrogen on sandier areas of a trial field made sense in dry conditions.

Why soil type matters more in dry springs

Spring weather patterns increasingly include longer dry spells. In those seasons, differences in soil texture and underlying geology can drive yield response to nitrogen just as much as crop canopy colour on a map.

Ben notes that heavier soils and chalk can show better drought tolerance and therefore support higher nitrogen applications in drier years, while lighter sands can be higher risk.

This is where combining satellite imagery, yield history and a sound understanding of soil variability can sharpen decisions.

How Contour pricing works for variable rate nitrogen plans

If you want to try variable rate nitrogen this season, Ben says the Contour tool charges only for the areas where it is used.

“If you want to use it, we will unlock the tool for your whole farm. You’ll only be charged for the hectares that the system produces a plan for.”

That makes it easier to test the approach in a trial area first, then scale up where results justify it.

What to do next

• Pick a sensible trial area where you already see consistent variation across seasons.
• Overlay yield maps and spring imagery (March/April is a useful window for wheat).
• Decide your aim (optimum yield or canopy levelling) based on field knowledge and the season.
• Ground-truth the maps before committing to rate changes.
• Review after harvest using yield data and margin, not yield alone.

If you want support building a trial plan or interpreting field zones, speak to your Agrii agronomist. We can also share learning from the Digital Technology Farm work at Revesby and other sites.