Farm Carbon Cutting Toolkit

20.03.2014 Fertiliser use - spreading efficiency and lowering emissions

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At this time of year, with the weather finally on the turn, and a spell of dry days, fields have been buzzing with tractors out spreading fertiliser.  Within conventional agriculture, fertiliser is a fundamental part of growing crops and early season applications will stimulate plant growth.  However there are associated environmental effects of fertiliser, whether it is applied from a bag, or as organic manures that impact on greenhouse gas emissions and climate change.  The most significant greenhouse gas emission from arable cropping in the UK is associated with the use of artificial Nitrogen fertiliser.  In fact 60-70% of GHG emissions from arable cropping are related to artificial Nitrogen production and application.  The effects of nutrient application on emissions are twofold.  Firstly the associated emissions that are involved in manufacturing inorganic fertilisers. As farmers by the time the fertiliser is unloaded off the lorry on-farm there is not much that we can do about this, however it is an important component of emissions and accounts for around 50% of emissions from fertiliser.  The other 50% of the emissions are more directly attributable to farm management practices and are dependent on the timing, method and conditions when the fertiliser is applied to the field.

Fertiliser manufacturers have strict targets that they have to adhere to in terms of emissions from fertiliser production, and are striving along with the chemists to find more efficient methods of producing fertiliser.  Throughout April we will be looking at the topic of fertilisers more closely and will have a guest blog from a fertiliser manufacturer who will be able to explain what is happening in more detail.

There are some easy things that every farm can do to reduce emissions from fertiliser application.  These include:

Apply Nitrogen fertiliser to meet and not exceed crop requirements.  If you are not already planning nutrient applications, start now.  Making sure that you are only applying what the crop needs will make sure that you are not leaving pools of nitrates sitting in the soil to be lost either through volatilisation as ammonia up into the atmosphere, or by leaching through the soil profile and into the watercourses.

Grow grass and other high Nitrogen demanding crops in your driest soils

Maintain drainage to minimise soil wetness – nitrous oxide emissions are highest in soils that are warm and wet.  If drains are maintained and the soil profile is well structured, then not only will you grown better crops but nitrous oxide emissions will be lower.

Optimise soil pH to encourage nutrient uptake – a basic fundamental of crop and soil management, but it is essential to maintain the right soil pH in order to ensure that you can get the highest proportion of available nutrient to the crop to grow.  Regular soil testing will enable you to have an accurate assessment of soil pH and remediate any acidic soils with the application of lime.  General targets for pH for grassland are between pH6 – 6.2 and for arable is between pH6.2-6.4 (Defra RB209).  Soils with pH lower than the recommended values will have significantly reduced nutrient availability to the crop, especially in the case of phosphate. Micro nutrients are also significantly affected by pH and decisions to apply additional nutrients should only be made after underlying pH issues are remedied.

Calibrate your fertiliser spreader for an even spread pattern - inaccurate spreaders can result in inconsistent application of fertiliser across the field. Over application can result in increased nitrate leaching losses as well as reduced crop yields through lodging.  It is possible to achieve a reduction in nitrate leaching losses of 5% as well as the associated reduction in nitrous oxide emissions.

Timing of applications – as well as applying the fertiliser when it is going to be taken up by the growing crop, weather and soil conditions will affect the nitrous oxide emissions as well.  The best conditions for spreading fertiliser is cool dry conditions, if it rains just before or after spreading emissions are much higher.  Warmer weather encourages bacterial activity, which increases the rate of denitrification (and associated emissions).  If your farm produces organic manures, consider the timing of applications of fertilisers and organic manures.  If you have applied fertiliser, make sure that you leave at least two weeks between the fertiliser application and applying slurry.

Type of fertiliser applied – if you are going to apply several times throughout the season (for example if you are taking silage cuts), look at applying urea in the spring as it is less susceptible to emissions from high soil moisture levels, and apply the nitrate fertiliser later on (for example ammonium Nitrate, or calcium nitrate).

Science is also developing compounds that can inhibit the nitrification process and as such allow for reduced emissions from applied nutrients.  This will be explored further as we look at options for reducing emissions further in April.

Those who are growing crops and applying fertilisers and organic manures, have a good opportunity to improve efficiency and reduce emissions at the same time, thus reducing costs.  Fertilisers represent a huge cost associated with growing crops and are a resource that as time goes on will only become more and more expensive.  Anything that we can do as farmers to optimise their use and make sure that the fertiliser that we apply works in the best way possible for us makes good business sense and makes us more sustainable.


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