Written by Rob Beaumont
Spring Nitrogen strategies: The thought process
As attention returns to spring field work after another long winter it is important to think about nitrogen strategies, timings and products. Many winter wheat crops have gone in late or in sub-optimal conditions and may need a bit more early encouraging than would normally be the case. Crops that were established well in September and have lots of tillers and forward growth should still be treated in the normal way. With the first hit of N at around growth stage 30. However late drilled crops with low tiller numbers will require an earlier feed and it will probably be sensible to give them the first 40kg/ha as soon as field conditions allow in late February or early March. This will ensure that crops are not deprived of N in the early spring growth which may cause them to abort tillers they can’t sustain, further reducing yield potential. If the winter and early spring continue to be wet and cold then soil biology will be slow to get going, further reducing the amount of nitrogen that will be available to the crop. Heavy soils can be particularly slow to warm up and can stay anaerobic for longer. This will be especially true where late autumn field work and cultivations have added further compaction and compromised soil structure.
Picking Your Nitrogen Source
Picking your N source will also be worth some consideration. Urea products would be the obvious choice for a first application of nitrogen. It leaches less quickly than ammonium nitrate and when soils are cool and wet volatilisation is less of a concern. Nitrate is an anion (negative charge) so doesn’t bind to soil particles (also negatively charged) this means it easily washes out of the soil profile. Hence why nitrates are such a water quality concern, it is very hard to stop them from leaving your field, particularly when it rains constantly. But we digres… Urea also has the advantage of being a much more plant available form of N and requires less energy to convert to ammino acids, the building block of proteins.
Urea vs. Nitrate for Tillering
However, there is evidence that products containing nitrate are actually better at stimulating tiller production during early growth. A 2020 article in Scientific Reports (available online) showed that nitrate had greater effect on increasing tiller production in cereals than other forms of N. This was due to the interaction between nitrate and the hormone within the plant that stimulated tiller formation. Therefore deciding what product you choose will depend on what you are trying to achieve with your crop. As always, there are compromises and trade-offs to be made when making these decisions.
Supporting Nutrients for Nitrogen Optimisation
It is important to remember that nitrogen does not function in isolation. Without the other elements to support growth, particularly protein production, your nitrogen use will not be optimised.
The Role of Sulphur in Protein Production
Sulphur is a key element for protein production. Like nitrate it is also an anion so is vulnerable to leaching during the wet winter months. There is also very little deposition from the atmosphere anymore as we have stopped burning coal and engines have become cleaner. Therefore, to get sufficient S into your crop, especially OSR, you will need to apply it as fertiliser. Nitrogen/sulphur products are common on farm and are highly soluble and are quickly available to the crop. However, if you have chosen to use a product like Polysulphate you will need to apply it as soon as conditions allow in the spring. Polysulphate is a from a natural mined rock mineral called polyhalite and has a much lower solubility compared to nitrogen/sulphur products. It therefore has a slower release so early application will ensure that the sulphur will be available when the crop demands it. The added benefit of Polysulphate is that it also contains potassium, calcium and magnesium which could be helpful, especially on lighter soils.
Phosphorus for Root Development and Energy Transfer
Phosphorus should also be considered during early spring growth. It is key for root development and energy transfer within cells. Managing your N to produce and retain tillers will only pay off if the plant has the rooting structure to support it. Phosphorus uptake can be slow when soils are cold and it might be worth planning a foliar application to support growth when that first dose of nitrogen arrives. The added advantage of applying some early foliar P is that it can be tied in with other micronutrient applications, such as zinc, copper, manganese and boron, as any herbicides or fungicides required. However, check with your agronomist as some SU graminicides should only applied on their own.
Assessing Soil Mineral Nitrogen (SMN) and Crop Needs
Finally, take the time to consider how much soil mineral nitrogen (SMN) may be available to your crop. This can be done by using information available in RB209 based on soil type, rainfall, previous cropping and organic manure applications. This will give you a Soil Nitrogen Supply (SNS) figure that can be used to adjust the amount of nitrogen the crop requires. The SNS figure is a “best guess” based on research work carried out for the RB209. To get further information you can get core samples taken for SMN, however experience has shown that these do not necessarily correlate to final N use or yield so should be taken with some caution.
Using Green Area Index (GAI) and Imaging Technology
The green area index (GAI) is a good way to estimate how much nitrogen is in a crop (OSR is good for this) and what it’s potential might be. There are now phone apps and other imaging technology that can be used for this task and they can provide useful information to plan your N strategy.
Accounting for Nitrogen Release from Cover Crops
For spring cropping, don’t forget to consider nitrogen release from your cover crops. You can estimate this by taking a representative sample, weighing the fresh weight and then sending sample off to be measured for nitrogen. Once you have the dry matter results you can work out how much N was in the cover crop per/ha and tailor your fertiliser requirements accordingly.
Let’s hope we have a warm and sunny spring to give everything the best possible chance for next harvest. We’ve certainly paid for it!
