Recent cold weather means crops are likely to require early season nitrogen but it is important to consider both nitrogen source and timing when thinking about spring fertiliser applications to get the best use efficiency and avoid lost yield potential.
Lower mineralisation in cold conditions
With recent cold weather it is likely there will not have been the same amount of mineralisation as the same period last year. This means that crops are likely to require nitrogen applications early-on to ensure there is no loss in yield potential.
Thinking ahead to spring nitrogen applications it is important to think of both source and timing in order to get the best use efficiency. The first nitrogen application should go on at the end of February as soon as the crop starts to grow; however this is obviously dependent on soil conditions.
Applying fertiliser to waterlogged soils is a pointless exercise as this will usually mean that denitrification will occur. This is when nitrates are converted to nitrogen-containing gases by soil bacteria who favour anaerobic conditions and nitrogen is therefore lost into the atmosphere. This means it will be necessary to wait for soil conditions to improve before making this first application so it will be even more important that the nitrogen source is immediately available to plant roots for uptake.
Nitrates immediately plant available
The source of nitrogen is also important particularly when choosing ammonium nitrate or urea. Nitrate nitrogen is immediately plant-available where as urea needs to be broken down to ammonium and then nitrate before the plant can utilise it. Under cold conditions this can take several days or even weeks.
In particular, slow-release nitrogen fertilisers such as urea with inhibitors that add another week (or more) to convert nitrogen to nitrate form compared to pure urea where the conversion of nitrogen in cold weather is already slow.
If considering straight nitrogen for the first application then you need to think how you will apply your sulphur – applying sulphur all in one application isn’t a good idea due to it readily leaching. The best advice for sulphur is to apply this 'little and often' in combination with nitrogen using NS compounds.
If using a compound fertiliser, rather than a blend, it will apply the nutrients evenly over the area meaning the crop has a uniform supply of all the nutrients in the analysis.
The following compound fertilisers are recommended for all crops and supply a combination of nitrogen and sulphur (N+S) or nitrogen, phosphate, potash and sulphur (NPKS) in appropriate ratios to allow the ideal timing for application of all nutrients.