THE FUTURE OF SOLID UREA FERTILISERS IS ON TRIAL.
James Warne from Soil First Farming defends urea and shows it may well be an easy scapegoat for the rise in atmospheric ammonia levels It seems as though we are due to loose solid urea fertilisers 16% below 2005 levels by 2030. Currently UK emissions very soon, or at least that is the conclusion I take from reading stand just below the 2005 level, but projections indicate that the current ‘Consultation on reducing ammonia emissions the UK will overstep the 2020 target. from solid urea fertilisers’. Out of the four options given, the stated preferred option is an outright ban on solid urea use. While there is little doubt that urea does It appears the way forward has already been decided and this consultation is just an exercise. Does this meant that for contribute to net ammonia emissions its those farmers using solid urea you will be forced to move to small fry compared to animal manures for the only real alternative – ammonium nitrate (AN)? It seems to me that urea is being target not only as an easy victory instance, but it also brings some benefits but we are also chucking the baby out with the bath water. I suggest that urea brings some benefits over and above too, not least where soil and plant health are ammonium nitrate not only for the farmers but also in a wider considered. To understand this further let us environmental context over AN. Ammonia emissions from agriculture fell by 21% between 1993 and 2013, but has risen subsequently by around 11%. Agriculture accounts for 87% of total ammonia emissions in the UK. Of this around 20% is attributable to inorganic fertilisers, the largest contributor of agricultural ammonia emissions are animal manures, particularly whilst in storage and spreading. ‘The British survey of fertiliser practice’ 2019 shows that total Nitrogen fertiliser use, averaged across grassland and cropping, is declining slowly and has been for several years. This seems to correlate to AIC data showing that fertiliser N consumption has been steady for the last decade at around 1030 kilotonnes (Kt) N per annum, which is well down on its peak of 1674 Kt N in 1987. While N use in cropping systems has remained steady, N fertiliser use on grassland has been slowly declining. If we analysis the data it is clear that since 2000 solid urea use in the UK has seen a significant rise (see the chart below) but has been dropping for the last few years. If urea were the cause of the increase in ammonia emissions within the last decade why were ammonia emissions decreasing in the first decade of the 21st century when there was a significant rise in urea use?
take a closer look at the interaction of urea in the soil and plant.
Once urea hits the soil if there is moisture and biological activity, hydrolysis (breakdown of urea into ammonium and carbonate), controlled by the urease enzyme, will begin. This is where the main loss of ammonia gas can come from. Ammonium and ammonia will sit in equilibrium with each other, dependent upon the pH of the solution. See the chart below.
The amount of nitrogen lost as ammonia volatilisation depends upon a range of other factors such as, soil & air temperature, air movement, soil CEC, crop canopy and a wetting or drying soil.
The UK has a legally binding commitment to reduce ammonia emissions to 8% below 2005 levels by 2020 and 46 DIRECT DRILLER MAGAZINE
The biggest opportunity for loss is likely to come from spreading urea onto a drying high pH soil. In order to minimise this loss urea should not be spread onto damp high pH (>pH 7.0) soils unless significant rainfall is predicted soon after spreading. Application to bare soil in direct sunlight without incorporation can increase volatilisation significantly. Similarly application into a crop with a good canopy followed by rainfall
ISSUE 12 | JANUARY 2021
