‘Evaluation of urea-based nitrogen fertilisers’ Report for Defra projects NT2601/02
Report for Defra Projects NT2601 and NT2602
Evaluation of urea-based nitrogen fertilisers
Edited by
Anne Bhogal, ADAS Gleadthorpe
Peter Dampney, ADAS Boxworth
Keith Goulding, Rothamsted Research
October 2003
Contents
Abbreviations
Glossary of terms
1. Executive summary 6
2. Introduction 12
3. Characteristics of urea 14
3.1 World consumption and production 14
3.2 Chemical and physical properties 14
3.3 Current use on UK farms 14
3.4 Conclusions 15
4. Behaviour and fate of urea-N applied to soil 16
4.1 Fate of urea applied to soil 16
4.2 Conclusions 19
5. Agronomic effectiveness 21
5.1 Arable cropping 21
5.2 Grassland 30
5.3 Horticultural crops 33
5.4 Conclusions 43
6. Environmental impacts 46
6.1 Ammonia emissions 46
6.2 Nitrous oxide emissions 51
6.3 Nitric oxide emissions 56
6.4 Leaching and surface runoff 56
6.5 Conclusions 58
7. Methods to mitigate ammonia emissions 61
7.1 Slow release formulations of urea 61
7.2 Chemical additives 61
7.3 Inorganic salts 62
7.4 Pellet size and soil incorporation 62
7.5 Urease inhibitors 63
7.6 Conclusions 78
8. Effects on soil processes 81
8.1 Factors affecting soil processes 81
8.2 Impacts of urea on biological processes 82
8.3 Impacts of urea on chemical processes 83
8.4 Conclusions 84
9. Modelling ammonia emissions 85
9.1 Modelling process stages 86
9.2 Scenario testing 90
9.3 Choice of models for future use 96
9.4 European approaches to modelling ammonia emissions 97
9.5 Conclusions 99
10. Implications for new research and other studies 101
11. References 103
Abbreviations
AC / Ammonium carbonateACl / Ammonium chloride
AN / Ammonium nitrate
APP / Ammonium polyphosphate
ATS / Ammonium thiosulphate
AS / Ammonium sulphate
ASN / Ammonium sulphate nitrate
AnA / Anhydrous ammonia
AqA / Aqueous ammonia
BSFP / British Survey of Fertiliser Practice
CAN / Calcium ammonium nitrate
CC / Calcium cyanamide
CEC / Cation Exchange Capacity
CN / Calcium nitrate
Chilean CN / Chilean potassic nitrate
CORINAIR / Core Inventory of Air Emissions in Europe
CV / Coefficient of Variation
CDU / Crotonylidenediurea
DAP / Di-ammonium phosphate
EF / Emission factor
EMEP / Cooperative programme for monitoring and evaluation of the long-range transmision of air pollutants in Europe
FMA / Fertiliser Manufacturers Association
FSU / Former Soviet Union
HSE / Health and Safety Executive
IBC / Intermediate Bulk Container
IBDU / Isobutylidene urea
MgAP / Magnesium ammonium phosphate
MgN / Magnesium nitrate
MU / Methylene urea
MAP / Mono-ammonium phosphate
MOP / Muriate of potash
N / Nitrogen
NARSES / National Ammonia Reduction Strategy Evaluation System
OSN / Other straight nitrogen
Ox / Oxamide
KN / Potassium nitrate
PSDA / Product Safety data Sheet
SMB / Soil microbial biomass
SSP / Single superphosphate
NaN / Sodium nitrate (nitrate of soda)
SCU / Sulphur coated urea
TAN / Total Ammonical Nitrogen
TSP / Triple superphosphate
U / Urea
UAN / Urea ammonium nitrate
UAS / Urea ammonium sulphate
UCN / Urea calcium nitrate
UKAEI / United Kingdom Ammonia Emissions Inventory
UNECE / United Nations Economic Commission for Europe
UP / Urea phosphate
Glossary of terms
Bulk density / Density of a mass of material, often expressed as kg/litre. The mass comprises the particles and the air spaces between them so bulk density is determined by the shape and size of particles as well as by the true density of the material from which the particles are formed. Particulate materials show differences in bulk density between loose and tamped or shaken states, in some materials as great as 15%. The bulk densities shown are intended to describe those of material in a spreader hopper. A value of 1.00 kg/l means that a 1000 litre hopper should hold 1tonne of material.
Caking / Formation of large hard agglomerations of fertiliser particles due to chemical properties of the materials or to absorption of water. This phenomenon occurs when fertiliser granules adhere to one another through crystal bridges or plastic deformation.
Complex fertiliser / Compound fertiliser where all particles have the same composition.
Compound fertiliser / Product containing more than one of the major nutrients.
Deliquesce / Absorption of atmospheric water vapour resulting in the loss of physical structure of particles.
Fluid fertiliser / Products supplied in liquid form, either as solution or suspension.
Granulation / Methods of forming fertiliser particles, mainly in the range 2 to 4mm diameter. There are two main classes of granulation: slurry and non-slurry processes. In slurry processes, solid particles of the fertiliser (obtained through recycling of undersize particles) are coated with a slurry of the fertiliser in successive layers. In non-slurry processes, a liquid component is added to finely divided particles causing them to agglomerate. Most granular products are slightly irregular in shape but some, those made by fluidised bed processes for example, are nearly spherical.
Granular fertiliser / Solid fertiliser where particles are all produced by granulation. May be complex or blended though the term is sometimes erroneously used as an alternative to complex.
Hygroscopic / Material absorbs moisture from the air.
IBC / Intermediate bulk container or big bag, usually containing 500, 600 or 1000kg of fertiliser. IBC also can refer to 1m3 containers of solution fertiliser.
Median size / The particle size at which 50% of the material by weight is smaller and 50% larger. The median size can vary in some materials and the values shown should be treated as guides. The particle size for most manufactured granular and prilled fertilisers is in the range 2 to 4mm range.
Particle crushing strength / Force that must be applied to cause a particle to shatter or break. Measured in newtons (N).
Particle or true density / Density of the solid material from which the particles are formed. Particle density therefore is independent of particle size and shape. The weight of a particle is determined by it’s size and density and is an important factor in spreading properties.
Prilling / Method of particle formation in which the molten fertiliser is forced through holes in a metal disc or spinning bucket and allowed to fall as droplets in a tower. The particles solidify as they fall. Prills tend to be more spherical and slightly smaller than granules
Solution fertiliser / Products where the nutrients are present in true solution.
Straight fertiliser / Product containing only one of the major nutrients (nitrogen, phosphate or potash)
Suspension fertiliser / Products where the nutrients are present partly in solution and partly as finely divided particles in suspension.
1. Executive summary
1. This report forms part of the NT2601 and NT2602 projects for Defra. It describes and discusses existing knowledge on the effects of using urea-based nitrogen (N) fertilisers on the performance of arable, grassland and horticultural crops, and likely impacts on the air, water and soil environments. The report discusses possible mitigation options to minimise or avoid adverse effects. The information sources comprised published international literature, as well as information provided by representatives of the UK and international fertiliser industries. Other reports from the NT2601 and 2602 projects cover ‘Nitrogen fertilising materials’ and ‘Production and use of nitrogen fertilisers’.
Characteristics of urea
2. Urea is the predominant source of fertiliser nitrogen used in agriculture throughout the world (50% of total N use). However, in the EU-15, the predominant source (40%) of N is ammonium nitrate (AN) or calcium ammonium nitrate (CAN), and AN is used predominantly on UK farms. There is no production capacity for urea in the UK. All current supplies are imported from within or outside Europe.
3. Urea can be manufactured as prills or granules. However, because urea is very hygroscopic (i.e. absorbs water), its use as a raw material in the production of compound fertilisers is much less flexible and more limited than for AN or CAN.
4. Urea is primarily used as a solid straight N fertiliser (46% N) or in the production of urea ammonium nitrate (UAN) solution (28-30% N w/w). Solid urea represents 9% and UAN 10% of the total UK consumption of N-containing fertilisers. Most (95%) is applied as a topdressing to winter cereals (63%), oilseed rape (16%) or grass (17%), largely in the February to April period; only 12% is applied in the warm and dry months of May to August. Very little urea (2% of total) is used on spring cereals and virtually none on potatoes, sugar beet or horticultural crops.
5. Urea has a lower bulk density than AN which makes accurate spreading by spining disc more difficult. A small particle size, as usually found with urea prills, aggravates this problem. Urea granules are larger (up to 3.5mm diameter), which should improve the spreading accuracy.
Behaviour and fate of urea applied to soil
6. Following application to soil, urea undergoes hydrolysis to ammonium (NH4) which is then subject to the same chemical and biological transformations as AN and other N fertilisers. The hydrolysis process is controlled by the urease enzyme (which is ubiquitous in soil, on vegetation and in surface litter), urea concentration, soil temperature and moisture. Grassland soils have more urease enzyme activity than arable soils. Rates of hydrolysis are generally rapid in most UK soils, but could be slower in arable soils low in organic matter, or in very dry, very wet or very cold weather.
7. Hydrolysis of urea results in a localised very high soil pH which can result in large emissions of ammonia to the atmosphere (see also paras 20-41). This is a well documented major loss process and is the main reason why urea has often been shown to be less effective for crop uptake compared to nitrate based fertilisers (see also paras 10-19). More research is needed to quantify ammonia emissions under different UK soil and agricultural conditions.
8. Unhydrolysed urea is soluble in soils and there is a risk that heavy rain immediately after urea application could wash urea and/or ammonium into surface or groundwaters, but there is little existing data. In soils above neutral pH, nitrite (NO2) could accumulate with risk of plant damage and leaching to waters. Both nitrate (NO3) and nitrite are at risk of loss as nitrous oxide (N2O) gaseous emissions. These transformations and processes make the efficiency of use of urea more difficult to predict and manage compared to AN.
9. There is no evidence that continued use of urea will have any long-term adverse effects on the national soil resource. Compared with the other major factors controlling biological and chemical processes in soils (e.g. pH, organic matter content), impacts arising from nitrogen fertilisers have always been observed to be small. No impact of urea (or the urease inhibitor Agrotain – see paras 29-33) on the soil microbial biomass (SMB) was observed in a 1-year trial. The spatial and temporal variations in microbial, chemical and biochemical properties were found to be much larger than any changes resulting from urea or Agrotain use.
Agronomic effectiveness of urea
Arable crops
10. There have been many studies comparing the agronomic efficiency of solid urea with other N fertilisers, largely from trials between 1960-1980 on winter cereals. The general conclusion was that urea gives more variable results and sometimes only 80-90% of the yield produced from other solid N fertilisers. Yield reductions have largely been attributed to a poorer efficiency of use of urea-N by the crop due to ammonia volatilisation losses post-application. A few studies comparing UAN solution with urea and AN have shown that UAN can give similar yields to urea (and sometimes less), and lower yields than AN.
11. Some trials have reported higher optimum rates of N (Nopt) from use of urea compared to AN, but lower yields at these N rates; however, in most cases the statistical significance of any differences was not reported. Where errors could be estimated there was no significant difference in the mean Nopt. Agronomic studies have shown no clear benefits from splitting urea applications. However, splitting of urea applications might be a strategy to consider to reduce ammonia emissions and increase the effectiveness of urea.
12. Statistically significant decreases in wheat grain N (protein) content have been reported from the use of urea compared to AN, typically ranging between 0.05-0.15% N in wheat (0.3-0.9% protein at 100% DM). Protein content is important for wheat grain marketing. Many reports have shown that foliar applications of straight urea solution are effective for increasing grain protein content, but these have a poor N use efficiency.
13. A few studies have reported that the effectiveness of urea appeared lower on calcareous soils, perhaps because of slower hydrolysis limiting the availability of N at a critical growth stage or due to higher ammonia losses. There appeared to be no effect of soil texture on the effectiveness of urea-N. Some trials have indicated a positive relationship between the effectiveness of urea and rainfall, but most reports gave little or no information on the prevailing weather, soil moisture or wind conditions.
14. A few studies have been done on the effectiveness of urea incorporated into crop seedbeds, but these did not record any adverse effects on germination or establishment of spring cereals where up to 90kg N/ha had been applied. However, higher rates of seedbed N for spring cereals (and oilseed rape) are currently used in the UK. Combine-drilled urea can result in reduced establishment and crop yields, but this practice is not recommended in the UK.
15. For sugar beet, no significant differences in sugar yield between urea and other nitrogen fertilisers were recorded. Reductions in plant population occurred at 3 trials, where 60 or 120kg N/ha as urea was applied in the seedbed, but this was unlikely at the normal seedbed recommendation of 40kg N/ha.
Grassland
16. Most grassland studies comparing urea with other N materials were carried out before 1985. The general conclusion was that urea was as effective as CAN or AN for spring grass production, but can result in 5 to 15% lower yields when used in the summer. Large yield reductions were observed on light textured soils and in dry weather periods. Rainfall in the 3 days after fertiliser application has been shown to increase the effectiveness of urea. Little research has been carried out under grazing conditions.