Efficient Use of Nitrogen - Tim Kerr - Agronomist & Arable Farmer

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Hutchinsons fertiliser manager Tim Kerr outlines how to alleviate losses from the soil.

The Government’s recently published 25-year Environment Plan includes a section on working with farmers to use fertilisers efficiently, stating how they will “encourage the use of low emissions fertiliser”.

However, you will be hard-pressed to find anyone promoting their own brand of 'high emission' fertiliser, so what do you need to consider?

A reasonable assumption is that all nitrogen fertilisers are 100% efficient before they are applied. For example, a tonne of 34.5% AN will contain 34kg of nitrogen - all of which could, theoretically, be utilised by a plant.  Inefficiencies start at application - although hopefully everything practical will be done to ensure fertiliser is applied accurately.

Losses through emissions can occur either as ammonia volatilisation or as nitrous oxide. Low emission fertilisers could certainly help reduce volatilisation. This loss mechanism is much more likely with urea but can be mitigated by the use of urease inhibitors.  Indeed, there is pending legislation in Germany that will mean all urea fertiliser that cannot be incorporated will have to be stabilised – and this may be an outcome in the UK.

Emissions of nitrous oxide are more a function of poor (anaerobic) soil conditions rather than fertiliser type – therefore it would be more likely influenced by the soil heath index.  Another feature of the 25-year plan.    Hutchinsons is already offering a ‘Healthy Soils’ assessment that provides such a soil health score.

Standard nitrogen use efficiency (NUE) of applied fertiliser is quoted in RB 209 (the nutrient management guide) between 55-70% - so are we losing the remaining percentage and is this another area that low emission fertilisers can improve upon?

It is not simple – the soil nitrogen cycle is a complex matrix of biological and chemical reactions – some of which could indeed lead to losses from the soil, while others maintain nitrogen in the soil in a variety of forms.

Mitigating the Losses

Maintaining a good soil structure, encouraging microbial activity with considered cultivations and increasing organic matter levels will help to increase the amount of nitrogen retained by the soil.  Consequently, less will be lost – either as emissions or via leaching.

A 10-tonne crop of wheat, as a rule of thumb, will have an uptake of 240kg/ha of nitrogen.  Oilseed rape and potatoes have a very similar requirement – an average of 5kg/ha per day through the growing season to meet crop demand.

At any given time, and without the addition of nitrogen fertiliser, soil components such as humus, crop residues and other organic matter and clay particles are likely to be holding more than the annual crop demand for any of these crops.  However, the ability of the soil to release nitrogen into plant available forms is the limiting factor.

The main principle of using nitrogen fertilisers is to bridge the gap between soil-supplied N and the crop’s requirement.

Slow Release Foliar Products

One valuable method of doing this is the judicial use of foliar applied N – something that the Yield Enhancement Network wheat quality award winner Sam Markillie is keen to point out.

We have been very pleased with the results from the use of slow release nitrogen foliar products – which are very safe to apply and demonstrate a high degree of NUE.

In Mr Markillie’s case this is a way of improving protein levels in milling wheat, but equally as an agronomically effective way of improving nitrogen uptake without inherent risks of losses.

Foliar applications are unlikely to completely replace soil-applied fertilisers. However, they do offer a very real possibility of replacing a proportion of ‘bagged’ fertiliser.

Switching the final dose of nitrogen from soil-applied to foliar - applied can improve both yield and quality without increasing overall input costs.  These products are compatible with most pesticides and trace elements, potentially saving a pass as well.

Farming in the UK will undoubtedly be affected by the 25-year Environment Plan – but if we put our best foot forward now, we can continue to develop and improve our use of crop nutrition to a point where we are able to demonstrate that we are using fertilisers more efficiently and hopefully already achieving the Government’s goals.