Comparison of the efficiencies of urea, protected urea and ammonium nitrate fertilizers in cereal production under Scottish conditions

Conducting field experiments to quantify and compare the relative efficiencies of urea, protected urea and ammonium nitrate forms of nitrogen (N) fertilizer in cereal production. Nitrogen is a major nutrient that must be supplied to crops in large quantities to achieve acceptable yields and to meet grain quality targets. Between 2020 and 2022, N-fertilizer prices rose almost four-fold reflecting rising energy costs and disruption to the global supply chain from the war in Ukraine. Although prices have begun to decline, they are still more than double those in May 2020. Various forms of N-fertilizer are available to growers. Historically ammonium nitrate has been the form most widely used by Scottish farmers because of its lower cost compared to urea-based fertilizer. More recently the price of urea has fallen relative to ammonium nitrate making it now a more attractive option. However, the costs of fertilizer must be balanced against the risks of N losses than can occur and the impact these may have on efficiency of N capture by the crop and damage to the environment.

When applied to the soil, N-fertilizer undergoes several transformations mediated by soil microbes that can alter the availability of N to the crop and lead to gaseous emissions of N including ammonia and nitrous oxide (a potent greenhouse gas). Urea is prone to volatilization of ammonia as urease enzymes rapidly covert the urea to ammonium. There is evidence from experiments in the SE of England that these losses are large enough to reduce N uptake by wheat crops. Protected urea formulations are available that contain urease inhibitors to slow down the rate of conversion of urea thereby reducing ammonia losses. In addition, some products contain both urease inhibitors and nitrification inhibitors with the latter designed to minimize nitrous oxide emissions. However, these protected urea products are more expensive than straight urea. Currently there is a lack of information available for farmers to make informed decisions about the relative merits of the different N-fertilizer forms under Scottish conditions. Volatilization is favoured by warm temperature and alkaline soils. It is unclear what the efficiency of fertilizer recovery is from urea under the cooler conditions and higher organic matter soils typical of Scottish farms and how it compares with protected urea products and ammonium nitrate.

The project will compare crop N-fertilizer recoveries, yields and grain quality from applications of straight urea, protected urea and ammonium nitrate fertilizers to quantify the cost-benefits of the different N forms. The funding will be used to conduct experiments on winter wheat and spring barley crops at two sites in Scotland over two years. The sites will be in Aberdeenshire and Mid-Lothian. Fertilizer applications will be made according to SRUC recommended rates for soft wheat and malting barley crops and at timings of commercial interest. These include standard split application timings plus single early season applications to test whether protected urea products can be used as a slow-release fertilizer thereby reducing the number of tractor passes required over the crop. In the current year (2023) use will be made of existing plots in Mid-Lothian sown with wheat and spring barley and in Aberdeenshire sown with spring barley. In the second year (2024) trials with wheat and spring barley will be established at both the Mid-Lothian and Aberdeenshire sites. Measurements will be made of grain yield, specific weights, average grain weights, straw and grain N contents.