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 very soon, or at least that is the conclusion I take from reading the current ‘Consultation on reducing ammonia emissions from solid urea fertilisers’. Out of the four options given, the stated preferred option is an outright ban on solid urea use. It appears the way forward has already been decided and this consultation is just an exercise. Does this meant that for those farmers using solid urea you will be forced to move to the only real alternative – ammonium nitrate (AN)? It seems to me that urea is being target not only as an easy victory but we are also chucking the baby out with the bath water. I suggest that urea brings some benefits over and above ammonium nitrate not only for the farmers but also in a wider 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? 

The UK has a legally binding commitment to reduce ammonia emissions to 8% below 2005 levels by 2020 and 16% below 2005 levels by 2030. Currently UK emissions stand just below the 2005 level, but projections indicate that the UK will overstep the 2020 target.

While there is little doubt that urea does contribute to net ammonia emissions its small fry compared to animal manures for instance, but it also brings some benefits too, not least where soil and plant health are considered. To understand this further let us 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 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 will mitigate losses considerably. So if we are applying solid urea to crops with a reasonable canopy typically from mid-Feb through to mid-April what is the likelihood of their being the conditions for significant ammonia loss through volatilisation?

The urease enzyme which activates hydrolysis is freely available in the soil, around 30% of the soil microbiome produces urease. When urea fertilisers are combined with a urease inhibitor it is this reaction that is controlled which can be seem as worthwhile when taken in isolation. If however your desire is to build a biologically active soil, there is some conjecture as to how much damage these urease inhibitors may do to the soil microbiome.

Ammonium within the soil is then nitrified to nitrite and nitrate principally by Nitrosommonas and Nitrobacter bacteria. This is an aerobic process that will slow down considerably in the absence of oxygen and/or low soil pH. This process happens very quickly in soils which have excess air in them, e.g. cultivated soils, but more recent work has shown that where the soil is less disturbed more ammonium tends to exist. Plants can take up and utilise nitrogen as nitrate or ammonium. Ammonium being a positively charged cation can be adsorbed onto the cation exchange sites and it therefore less vulnerable to leaching and the crop can take it up selectively and utilise it more efficiently than nitrate. Nitrate on the other hand being negatively charged is very prone to leaching and whilst soluble in the soil solution is taken up uncontrollably as the plant respires.

The crop therefore has little control over the amount of nitrogen is it taking up. This is evident in the differing growth habits of similar crops grown on cultivated and non-cultivated soils. Those grown on cultivated soils tend towards nitrate nutrition and consequently are much darker green (almost blue) and lush. This tends to make the straw weaker and more vulnerable to lodging; more susceptible to disease, particularly mildew. Whilst crops grown on no-till situations tend towards more ammonium nutrition which gives them a shorter more prostrate growth habit and gentler, more natural green. The plant it is able to assimilate ammonium into proteins and amino acids with relative ease, while it has to convert nitrate to ammonium before it can be utilised, this ‘costs’ the plant energy approximately 16% more energy than utilising ammonium.

So we believe that urea can be an effective fertiliser if applied correctly that can offer advantages over AN in terms of soil and plant health together with similar or better environmental attributes such as lower energy requirement for manufacturing per unit N, less bulk requiring less transport and lower spreading costs, easier storage for the farmer. To name but a few. It’s also clear that urea came contribute to lower nitrous oxide emissions from the soil compared to AN. Research published from five grassland trials sites across the UK in 2019 shows that nitrous oxide (a potent greenhouse gas) emissions from grassland (and presumably cropland too) are higher when using ammonium nitrate and CAN as opposed to urea. Nitrous oxide emissions from urea use were further reduced by using a stabilised urea with DCD.

So where could the increase ammonia emission come from? We know that animal manures contribute the largest percentage of agricultural emissions, but sheep numbers have been steady, while cattle numbers have been in slow decline for a few years now in the UK. So what else could be contributing to the increase in ammonia emissions? Could it be that green energy production is a significant contributing factor? In 2013 there were less than 100 AD plants operating in the UK, that figure now stands at around 600 producing around 300 MWe. AD plants produce digestate. Approximately 18,000 m3 of digestate per MWe per annum. Or 5.5 million cubic metres of digestate total per annum. The available nitrogen within digestate is predominantly in the ammonium form. Some recent digestate analysis from a crop fed digester showed it had ~ 2.5kg ammonium (readily available N) per cubic metre. The current RB209 suggest that ‘around 40% of the readily available nitrogen content of organic materials can be lost following surface application’ but this can be reduced by 30-70% depending upon timing and form of application.

So if emissions from fertilisers only account for 20% of agricultural emissions, within that 20% urea only accounts for half, why is urea being targeted? Could it be that solid urea is the lowest hanging fruit?