Tech Talk – Nitrogen choices

 

New season nitrogen pricing often enters discussions when the current crop is still someway from harvest and the following years cropping isn’t finalised. Such a predicament can be a barrier to any plans to amend your crop nutrition strategy. This Tech Talk is designed to provoke some in depth thought about appropriate product choices – from chemical analysis to physical spreader properties.

Nitrogen needs

This month usually marks the release of new-season prices for fertiliser. CPM finds out how taking stock of the crop in the ground is the best way to determine future needs rather than ordering more of the same.

This season more nitrogen may be required to achieve potential yield where crops have a low biomass.

By Lucy de la Pasture

Nitrogen management is evolving. The introduction of in-crop management tools and sensors is enabling growers to take a more considered, precise approach to assessing the nutrient requirements of crops and applying fertilisers to achieve the best possible return on investment.

Under Brexit and the green agenda for agriculture currently being discussed, it’s likely there’ll be an increased focus on legislation. Evidence-based decision making will become increasingly important, especially where nitrogen and phosphate applications are concerned.

Yara’s Mark Tucker believes that now is the time to reflect on the successes or failures of the nutrient plans, using the crop in the ground as a reference so that fertiliser requirements can be thought through and adjusted if necessary before placing an order for 2019.

What’s changed that affects N?

This spring marks the first time growers can use yield targets to help work out their nitrogen requirements using the revised RB209 guidance. When taking this approach, it’s important to be critical about whether it’s possible to confidently predict the N-use efficiency on the farm. If this tends to be variable, then using yield is not a good basis to calculate N-requirement. Where predicted yield is used, it should be realistic and in-line with yields used for regulatory compliance, ie the same as in the N-max calculation.

After the cold, wet winter and a late spring, this season more nitrogen may be required to achieve potential yield where crops have a low biomass. The lessons learned from the Yield Enhancement Network (YEN) reinforce the importance of high crop biomass being the main factor contributing to consistently high yielding crops. Poor above ground growth in early spring will be reflected underground, with a similarly affected root biomass, making N-capture by the roots a less efficient process.

In a lower biomass year, ear numbers are likely to be lower so it’s important to make every grain count and maximise grain number/ear. This is where other nutrients such as copper, zinc, boron and potassium come into play.

How do you assess crop need?

Assessing crop need for nitrogen can be carried out using either destructive (lab testing) or non-destructive methods (sensors) and provides useful information to help determine whether the current nutrient management plan is working.

Taking flag leaf samples for tissue analysis is one way of doing this. Target figures to aim for are 4.5% N for a milling wheat crop and 4% N for a feed wheat. This level of N in the leaf needs to be maintained through to anthesis and can be used as a guide as to whether any extra N application is required.

When taking tissue samples, it’s important to take the youngest leaf that’s working and fully developed. So if the flag leaf isn’t fully emerged, take leaf two instead.

An alternative means of assessing crop need is to use the Yara N-Tester. This measures the levels of chlorophyll A and B which is highly correlated to the level of N in the leaf. A typical value to target is an N-Tester reading of 700, which can then be adjusted for each variety using correction factors which take into account the leaf wax and leaf colour typical of that variety.

In France the N-Tester and ImageIT app are regarded by the regulatory authorities as tools that can be used to validate and support N-usage in crops.

What other methods are there?

The Yara N-Sensor is a real time variable-rate nitrogen sensor that allows farmers to measure a crop’s nitrogen requirement as the fertiliser spreader passes across the field and to variably adjust the fertiliser application rate accordingly.

The N-Sensor determines a nitrogen demand by measuring the crop’s light reflectance, covering a total area of approximately 50m². Measurements are taken every second, with the system designed to operate at normal working speeds and all bout widths.

N-Sensor measures light reflectance at specific wave bands related to the crop’s chlorophyll content and biomass to calculate the actual N-uptake of the crop. Optimum application rates are derived from the N-uptake data and sent to the controller of the variable rate spreader or sprayer, which will adjust fertilizer rates accordingly, enabling real-time agronomy.

ImageIT is a downloadable smartphone app that evaluates biomass and the amount of nitrogen taken up by crops from a digital photograph. It determines the leaf area index, green area index and the fraction of brown leaves to calculate N-uptake.

What form of N works best?

The solid forms of N-fertiliser available are ammonium nitrate (AN) and urea. Both have strengths and weaknesses – AN is more leachable and urea volatilises to ammonia, especially during dry weather.

Urea must be converted to nitrate in the soil to become available to plants and this process is dependent on soil temperature. Consequently this season, urea will take longer than AN to become available and this needs to be taken into account when planning applications.

The higher concentration of N in urea (46% N) compared with AN (34.5% N) is negated by its lower specific weight, which means in practice both forms require the same number of spreader loads/ha.

Generally AN fertiliser is more predictable and consistently provides a 3-5% greater yield response in Yara trials. If management tools are being used to try to be more precise about N-management, then urea can undo this precision because of its unpredictable nature.

Liquid fertilisers contain urea-ammonium nitrate (UAN) and logistically have a good fit within some farming operations. If you’re considering moving from a solid form of N to a liquid form, the first step is to discuss the relative pros and cons with a grower already using liquid.

Performance of ammonium nitrate and urea

 

What about sulphur?

More than 90% of soils are deficient in sulphur, according to the results of soil sample analyses by Yara’s Analytical Services. Broad spectrum tissue analyses provide a snapshot of the crop’s nutrient status, which can be a useful aid to future decision making as well as for checking your current sulphur strategy is working.

Sulphur needs to be applied during the season so choice of form and the selection of grades within that form are important considerations. If the aim is to variably apply nitrogen then the sulphur needs of the crop need to be addressed in the first application so the second and third N dressings can be made variably. This strategy would mean a fertiliser with a high percentage of sulphur is the best choice early.

If you’re not on a variable rate system then a lower percentage of sulphur inclusion is better, in the knowledge the crop will be receiving little and often. This is a preferable approach from an agronomic point of view as sulphur isn’t very mobile in the plant, so several applications maintain a readily available supply.

Testing grain from the previous season is another way of determining the sulphur status of soils. The N:S ratio should be 13:1 or above. As the ratio increases, this indicates a deficiency in S.

Sulphur is available as elemental sulphur, sulphate (SO₃) or thiosulphate (in foliar application products).

Which crops should receive a late N dose

When a milling wheat is grown, in order to achieve a protein premium late N is a must. This can be applied as 40KgN/ha of a solid N product from GS39-booting or as a liquid at GS70 (early post-anthesis).

Typically this application will give a 0.6% protein lift where a solid fertiliser has been applied and 0.8-1% where a liquid

For OSR, a late application (during yellow bud to petal fall) consistently produces a yield response of 0.3t/ha. It’s important the crop isn’t stressed at the time of application, so the decision to apply has to be a reactive one according to conditions at the time.

When is the best time to make an order?

Generally early season offers are good value and ordering a proportion of your fertiliser requirement is a way of managing the risk of volatility in the fertiliser market. But the most important thing is to order the right agronomic product for your farming system. Possibly a NPKS fertiliser may suit the early application slot in spring where spring P and K applications are planned.

Also consider not only the grades required but whether granules, prills or liquid are the best fit. As you move above 24m bout widths, then granules or liquid nitrogen are the only option to maintain accuracy of application.

No two years the same

Norfolk grower Mark Means knows a thing or two about growing wheat, and a bumper yield of 13.41t/ha in his 2016 YEN crop of Cordiale bears testimony to the fact.

The thing about farming is that no two seasons are the same, a fact highlighted by the difference in crop biomass this spring compared with last, he says.

“In 2017 it was a warm autumn and crops had a large leaf area index (LAI) by spring. This meant they were very able to scavenge residual nitrogen from the soil. This spring LAI is down considerably, roots have been sitting in anaerobic conditions for a prolonged period and a lot of the nitrogen in the soil has leached down the profile.”

It’s in years like this that the N-Sensor really comes into its own, he says. “We use the N-Sensor on the sprayer to determine our first three applications of UAN and find it balances the N over the field, giving us a much more even crop.”

He uses the N-Sensor map to work out his final nitrogen application, which he applies as ammonium nitrate (Extran, 33.5% N).

“We apply Extran in mid-late May to top-up N where needed and target the areas of crop with the most yield potential. It’s the opposite to the principles of Robin Hood, we’re stealing from the poor and giving to the rich,” he explains.

Mark uses tissue testing at just before the important fungicide timings, T1 and T2, with a further test in between (T1.5). These provide a snapshot of the crops needs at that time, and he uses the information to determine whether any trace elements are required.

Mark has 460ha of winter wheat currently in the ground and is growing Group 1 varieties, KWS Zyatt and Gallant; Cordiale (Group 2) and KWS Barrel (Group 3). All his crop is marketed through grain cooperative, Fengrain, and he follows their marketing advice in order to meet customer needs and achieve the best premiums.

The N-tester provides the guidance to any late-N required to boost grain protein. “The N-tester shows us what we need to do. We find that if we use it over a week or so, we have a physical way of identifying trends and can tell whether or not the crop is picking up late nitrogen.”

In his area of Norfolk, the marine, alluvial silts are rich in sulphates, so Mark has very little sulphur to apply. “We use muck and apply Korn Kali (40% K₂O, 6% MgO, 4% Na₂O and 12.5% SO₃) in late March-early April to apply 10-12.5Kg SO₃/ha.”

Nitrogen choices: top tips

• Optimise N and S management – keep it simple.
• Application accuracy – optimise through choice of product (granule, prill or liquid).
• Use latest technology – to help achieve optimum N application for each crop.

Sponsor message

Data available from many sources continue to support the need for sulphur to be included in the crop nutrition strategy, making straight nitrogen products less relevant for today’s arable and grassland farmers. There are also two other themes at the centre of many discussion. Firstly, there’s the growing demand for excellence when it comes to application as farmers adopt precision farming systems and look to apply fertiliser to 54m. Secondly, enterprises are increasing in size and complexity, meaning simplicity in management becomes ever more important. With YaraBela Axan, nitrogen and sulphur management can be made very simple spanning a range of crops, with the reassurance that accuracy in application due to its physical properties will be ‘best in class’.