With fertiliser prices skyrocketing, Alice Sin, who helps manage the AHDB Nutrient management guide (RB209), highlights the latest research results set to keep arable crops sufficiently nourished.
It’s been almost five years since we issued the first update of the AHDB Nutrient management guide (RB209).
Previously managed by Defra, the baton was passed to us to deliver robust guidance on the use of fertiliser, manure and slurry applications to cropped land. This has been made freely available, so it can be adapted for use in the wide range of cropping situations across the UK (eg tailored within the SRUC technical notes for Scotland).
We invest in cycles of nutrient management research to inform the regular updates to RB209. Several projects that concluded in 2021 will inform the 2022 update, due this spring.
High fertiliser prices make it essential to understand the point at which the value of extra grain produced is not worth the cost of the extra nitrogen applied.
To account for this ‘economic optimum’, AHDB commissioned ADAS to conduct a rapid review and extend RB209 price tables.
Published last autumn, these tables now rise to the equivalent to £863/t of ammonium nitrate. The tables have also been extended to £350/t for cereals and £700/t for rapeseed, to help account for stronger prices.
For example, for a grain price of £200/t, a rise in the ammonium nitrate fertiliser price from £345 to £863/t would necessitate a per-hectare reduction of 70 kg nitrogen (associated with a yield decrease of 0.6 t/ha).
The review also considered the impact on quality specifications and the value of nitrogen from non-fertiliser sources.
While some growers forward-purchased fertiliser when price lists were released in summer 2021, others bought nearer to the time it was needed.
Those buying later experienced higher prices, with some unable to source sufficient quantities. There are anecdotal reports of growers scaling back their nitrogen use on this year’s crop to gain maximum cost efficiencies.
Although this has the potential to impact this year’s yields, many other factors are pivotal in yield formation and there is still plenty of growing time left this season.
Our research shows that lower nitrogen rates may not reduce yield as much as anticipated, by some. However, rate reductions shift the nitrogen response to towards a steeper part of the response curve – where there is less room for error. Consequently, it is more important than ever to ensure that nitrogen rates are tailored carefully.
Based on the extended price tables, we recently issued a fertiliser adjustment calculator.
This spreadsheet-based tool helps determine how much to alter nitrogen rates from the farm’s typical standard rate see it at: ahdb.org.uk/nitrogen-calculator
This makes its calculation based on five figures:
4, Typical nitrogen rate (kg/ha)
For a specified cereals or oilseeds crop, the tool calculates:
• Cost of nitrogen fertiliser (£/kg);
• Break-even-ratio (kg of grain required to pay for a kg of nitrogen fertiliser);
• An estimated change to nitrogen application (kg/ha);
• An estimate of the effect on yield/income (£/ha);
• An estimated impact on nitrogen fertiliser costs (£/ha);
• Total nitrogen fertiliser product required for the specified crop area planted.
Before work on the HS2 rail route started, a detailed assessment of it unearthed prehistoric remnants and Roman battlefields – but also an agricultural treasure trove. The collection of over 1400 topsoil and upper-subsoil (<50 cm) samples along the track’s route provided a unique analysis opportunity.
AHDB funded an analysis of these samples by Reading Agricultural Consultants, which focused on the relationship between nutrients/pH in the topsoil and the more mysterious subsoil, which resulted in several changes to the recommendations in the 2022 edition of RB209.
Changes to support mechanisms, lime products and farming systems, mean a review of liming guidance was overdue – not only in relation to lime’s influence on soil pH, but also its role in delivering calcium, an essential nutrient, to crops.
During the production of RB209, we worked with partners across industry – and the Agricultural Lime Association (ALA) was best placed to conduct the initial review (at no cost to AHDB).
Once again, recommendations from the review have resulted in changes to the new RB209.
Our research results underwent peer review, then the recommendations are considered by the Arable Technical Working Group (TWG), which includes farmers, manufacturers and agronomists, before being signed off by the RB209 steering group.
The 2022 edition has extensive revisions, with section 1 – Principles of nutrient management and fertiliser use – having the most significant updates. These are:
• New subsoil recommendations, to improve estimates of soil nutrient supply and lime requirements;
• New clay classification (in relation to potash release and applications);
• Revised magnesium section, with enhanced guidance on:
o Soil types most at risk of low magnesium indices
o Potential risk of magnesium deficiency in arable crops
o How to increase magnesium indices (when required)
o Management of soil types with high magnesium indices
• Revised soil acidity and liming section, with enhanced guidance on:
o The role of soil pH in fertiliser-use (mineral and organic) efficiency, including nutrient availability, and the reduction of greenhouse gases (nitrogen lost as nitrous oxide)
o Improved calculations of lime requirements (according to soil type)
o How to make maintenance/top-up applications of lime that compensate for losses, crop offtake and other acidifying factors
o Consideration of the reactivity (fineness of grinding) of liming materials, which influences speed of action.
Nitrogen use in spring barley
A section which focuses on arable crops, will also be updated to reflect recently completed work on spring barley.
In recent years, numerous pressures on winter crops led to an explosion in spring barley – rising from just under 700,000 to 1m ha between 2016 and 2020 (although contracting in 2021).
Behind this statistic is a raft of new (or returning) farmers to the market. This expansion also saw the crop march into new territories beyond the traditional light-textured soils, including those with a heavier texture.
With new spring barley varieties also hitting the AHDB Recommended Lists (RL) each year – altering nutrient-use efficiency dynamics – we commissioned ADAS to lead research to help RB209 keep pace with modern production systems.
The work featured an impressive array of trials, building on bespoke research experiments conducted as part of the project (2018–20), and a review of other recent experiments (2005–17). With rates, timings, yield and quality in the spotlight, the work represents one of the most significant investments in nutrient management for spring barley for quite some time.
Adjusting N for spring barley yield
When it comes to yield, RB209 recommendations are based on a ‘typical’ yield benchmark of 5.5 t/ha (spring feed barley).
Going into the project, it was felt this yield might be a little conservative. Certainly, when yields are pushed, the crop is capable of going far higher.
For example, the five-year (2016–20) yield average in RL trials is 7.5 t/ha. This project confirmed these suspicions.
Read more: Including legumes in an arable rotation can have many benefits
It followed RB209 recommendations and landed an impressive average yield (selected experiments) of 7.4 t/ha. Although many commercial yields will fall shy of this, it is clear that modern spring barley varieties use nitrogen more efficiently than their predecessors.
As RB209 suggested that the recommended N rate is increased by 20 kg for each additional expected tonne (up to 9 t/ha) above the 5.5 t/ha benchmark, it may actually overestimate fertiliser N requirements.
In fact, across all experimental datasets, the average recommended rate was 165 kg N/ha. However, the measured average economic optimum rate was far lower – just 118 kg N/ha.
Researchers proposed two solutions. Firstly, to increase the yield adjustment and secondly, to adopt a method that calculated fertiliser N requirement based on crop demand and fertiliser recovery. Each option resulted in similar N recommendations at expected yields of 7-8 t/ha.
Readjusting N for spring barley quality
Attention to detail is essential, especially when it comes to hitting the typical grain N content specifications required by malting markets – malt distilling (below 1.65%), brewing (1.60-1.85%) and grain distilling (above 1.85%). Hitting spec's requires a mix of quality guidance and local experience to strike a balance between pushing yields and keeping grain N content within tight N% bands.
Across all experimental datasets, it was found that a reduction in the N rate of 29 kg N/ha brought down grain N% by 0.1%. These results are broadly in line with the current RB209 recommendation (reduction of 30 kg N/ha).
The average grain N% at the N optimum was 1.7% (currently, RB209 is based on 1.9%), and around two-thirds of crops achieved a grain N% of under 1.8%. Ultimately, historic field grain N% and yield data will help guide decisions on the potential to reduce fertiliser rates.
N timing for spring barley
Results from 11 N-timing experiments largely confirmed RB209’s recommendations, with all N applied between drilling and early stem extension (with large timing flexibility in this window).
The work also concluded that application of at least 40 kg N/ha in the seedbed is often beneficial – but should be capped at 40 kg N/ha, where nitrate-leaching risks are high.
The research also investigated sulphur rates and found that current RB209 recommendations were sufficiently accurate.
Value of nutrients from organic materials
Although RB209 also covered the use of organic materials, the situation is relatively complex compared to synthetic fertilisers.
The published figures are averages, based on many samples. The following are just some of the factors that impact nutrient content of manures and slurries:
As a result, averages need to be used cautiously. Speaking at a ‘Make the most of your muck’ webinar, Lizzie Sagoo, ADAS, said: “RB209 gives average figures. While there are lots of data points behind these numbers, laboratory testing is the only way to get an accurate assessment of the nutrient value of the material you’re applying.”
Lizzie also stressed the importance of testing representative samples, based on several mixed sub-samples.
In the case of slurry this meant stirring, because dry matter content settles during storage. It is important to remember this at application too – later loads (from the bottom of stores), often contain higher levels of dry matter and nutrients.
When it comes to application, timing and technique are key to reducing losses.
Nitrate leaching is higher when applications are made during the autumn and the early winter period. This is because of the greater rainfall between application and the end of soil drainage in the spring. It is the main reason for ‘rule one’ in Defra’s Farming Rules for Water, which demand a shift away (in some situations) from the autumn spreading of organic materials.
Up to a third of readily available N can be lost through volatilisation. Applying slurries using precision application kit – such as a trailing hose, a trailing shoe, or shallow injection – will reduce ammonia loss from slurry applications. Incorporating material will lower losses from solid manures.
Input-cost and commodity-price volatility means a constant eye needs to be kept on the nutrient management equation.
With climate change mitigation firmly back on the world’s radar, in addition to numerous other environmental concerns, RB209 is needed more than ever to ensure nutrient calculations are based on best available evidence.
° The 2022 edition of the AHDB Nutrient management guide (RB209) will launch in the spring at ahdb.org.uk/rb209
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