Ammonia volatilization from urea nitricphosphate and urea applied to the soil surface

Urea nitricphosphate (UNP) is an N-P fertilizer prepared by solubilizing phosphate ore with nitric acid and conditioning the product with urea. The product is acidic, and its nutrient analysis is 23-12-0. Urea makes up 74% of the N component of this material and the remainder comes from the nitrate added as nitric acid. In volatilization trials, UNP lost significantly less N than did urea in a noncalcareous soil (13 and 31% respectively). In calcareous soils the urea-N component of UNP exhibited loss patterns similar to those of urea. Soil pH remained stable at the center of the granule placement site during UNP hydrolysis, thereby reducing NH₃ loss, whereas the pH of the same soil treated with urea rose almost 1.9 units. The urea component of UNP appeared to diffuse from the center of the acidic microsite allowing hydrolysis to take place and permitting limited NH₃ volatilization to occur. UNP appears to be an attractive NP fertilizer in terms of nutrient analysis and resistance of the N component to volatile N losses as NH₃.     

Ammonia volatilization losses from fertilizers applied to acid soil in the field

Losses of ammonia by volatilization from ammonium sulphate and urea applied to soil were studied in field conditions.Losses from ammonium sulphate generally were not large; ammonia volatilization is thus unlikely to be an important pathway of nitrogen loss from cropped soils, and does not explain the low responses to nitrogen fertilizer of wheat grown in the higher rainfall cropping areas of South-Eastern Australia.Losses of nitrogen from ammonium sulphate were not greatly affected by meteorological variables, rate of application, water applicaton or incorporation into soil.The above variables all affected losses of nitrogen from urea, by influencing the rates of solution and hydrolysis of urea, and volatilization of ammonia. Losses ranged from 4 to 50% of the applied urea-nitrogen. Losses of urea-nitrogen were large when evaporation rates were high, and large variations occurred in the rates at which urea could be hydrolyzed.Extrapolation of the results to grazing conditions suggests that ammonia volatilization may result in large losses of nitrogen from short pastures in dry conditions.     

Ammonia volatilization from fertilizers applied to irrigated wheat soils

A series of experiments using flow chambers was undertaken in the field to investigate the effects of stubble and fertilizer management, soil moisture and precipitation on ammonia volatilization following nitrogen application on chromic luvisols. In the first factorial experiment, urea at 100 kg N ha^–1 was applied to the soil surface one, three and six days following irrigation; there were four rice stubble management systems comprising stubble burnt, stubble burnt then rotary hoed, stubble rotary hoed into the soil and stubble retained on the surface. Cultivation almost halved ammonia loss. The higher loss from uncultivated plots was ascribed to an alkaline ash bed on burnt plots, and to higher soil moisture and some retention of urea prills in the crop residue above the soil surface of the stubble retention plots. Average volatilization over a 12 day period following urea application from plots fertilizer one, three or six days after irrigation was 16, 15 and 4 kg N ha^–1, respectively. Daily application of up to 1.7 mm of water did not reduce volatilization and 35 kg N ha^–1 was lost within five days of fertilization. Daily precipitation of 6.8 mm reduced loss to 14 kg N ha^–1. This quantity of rain is uncommon in the region and it was concluded that showery conditions are unlikely to reduce volatilization. The third experiment demonstrated that the quantity of stubble on the soil surface had no effect on volatilization, and all plots lost 25% of applied nitrogen. In the fourth experiment, 100 kg N ha^–1 as urea or ammonium nitrate was either broadcast onto the surface or stubble retention plots, or placed, and partly covered to simulate topdressing with a disc implement. Partial burial of urea reduced ammonia volatilization from 36 to 7 kg N ha^–1, while partial burial of ammonium nitrate reduced loss from 4 to 0 kg N ha^–1.     

Use of urease inhibitors and urea fertilizers on winter wheat

Phosphoroamide urease inhibitors were evaluated for their ability to increase grain protein and yield of winter wheat (Triticum aestivum L.) when added to surfaceapplied urea-based fertilizers. Six urease inhibitors [trichloroethyl phosphorodiamidate, diethyl phosphoric triamide, dimethyl phosphoric triamide, N-(diaminophosphinyl)-cyclohexylamine, N-benzyl-N-methyl phosphoric triamide, and phenylphosphorodiamide] were evaluated. Nitrogen treatments were urea prills, urea solution, and ureaammonium nitrate (UAN) solution broadcast and UAN solution band applied. Ammonium sulfate and no N treatments were included as controls. Fertilizer treatments were applied in the fall and spring. Soils were Ryker silt loam (Typic Paleudalf), Rensselaer loam (Typic Argiaquoll), and Avonburg silt loam (Aeric Fragiaqualf).Grain yield was a more responsive indicator of N addition than was grain N content. Urea prills and ammonium sulfate were more effective fertilizers than was UAN solution. UAN was not more effective applied in a band than broadcast. Urease inhibitors did not improve the efficiency of urea fertilizers since NH₃ volatilization did not appear to be a problem following addition of urea fertilizers in spring or fall.Journal Paper No. 10528. This work was supported in part by a grant from Allied Chemical, Solvay, NY 13209.     

Dynamics of ammonia volatilization from simulated urine patches and aqueous urea applied to pasture. III. Field verification of a simplified model

Published field experimental data [11, 15, 19] were used to compare measured NH_3(g) losses following applications of urine or aqueous urea to pasture soils with values predicted by a simplified ammonia volatilization model [16]. Total measured losses were generally in close agreement with predictions. For example, predicted losses following applications of urine to a ryegrass-white clover pasture in Canterbury, New Zealand were 20.7% in summer and 22.4% in autumn and were highly correlated with measured losses of 21.5% and 24.4% respectively (r = 0.998).The model was also tested for instantaneous rate of ammonia gas loss at 33 discrete sampling times for the summer experiment. Correlations were again highly significant (r = 0.951 for urine and r = 0.885 for urea).The interception of urine solution by herbage and litter on the pasture surface is discussed and was shown to account for some of the discrepancies between measurements and predictions. Soil surface pH was confirmed as an important factor in determining the extent of ammonia gas loss, and the practicalities of measuring this parameter under field conditions are presented. It was concluded that the model offers the potential for predicting ammonia volatilization losses following urine or aqueous urea applications to short pasture in non-leaching, non-nitrifying environments.     

Dynamics of ammonia volatilization from simulated urine patches and aqueous urea applied to pasture I. Field experiments

Ammonia (NH₃) volatilization losses from simulated sheep urine patches in a perennial ryegrass (Lolium perenne L.)/white clover (Trifolium repens L.) pasture in New Zealand were measured in the field during the summer, autumn and winter periods. An enclosure technique was used with microplots (23 cm diameter) receiving either sheep urine or aqueous urea at rates equivalent to 500 kg N ha^–1 and monitored continuously until measured losses decreased to 0.5% per day. Mean volatilization losses for urine treated plots were 22.2% of the applied N in summer, 24.6% in autumn and 12.2% in winter. Corresponding losses for the urea treated plots were 17.9%, 28.9% and 8.5%. Differences between these two N sources were not significant although the seasonal differences were significant (P 0.05). Changes in NH₃ gas fluxes were found to be related to measured changes in soil pH and air temperature. Two repeated applications of urine or aqueous urea to the same microplot resulted in significantly greater subsequent volatilization losses averaging 29.6% from the second and 37.5% from the third application.Most of the applied N was accounted for as either soil mineral N (NH ₄ ⁺ + NO ₃ ^- + NO ₂ ^- ) or NH_3(g) . Urea hydrolysis was rapid and obeyed the first order kinetics during the 24 hours following application. Calculated half-lives of urea in urine and aqueous urea were significantly different and were 3.0 and 4.7 h respectively during the summer and 4.7 and 12.0 h during the autumn.Implications of the results obtained to practical field situation together with the efficacy of the enclosure technique for measuring volatilization losses are discussed.     

Ammonia loss from surface applied urea-acid products

Ammonia (NH₃) losses from soils occur only under alkaline conditions; therefore, adequate acidification could prevent NH₃ loss. In acid soils this alkaline condition will exist only as a micro-environment around the decomposing CO(NH₂)₂ granule. The objective of this experiment was to examine the degree of NH₃ loss reduction that occurs when acids are placed with surface applied CO(NH₂)₂. Phosphoric acid, H₂SO₄, HCl and HNO₃ were used with surface applied CO(NH₂)₂ in a laboratory experiment to examine resultant NH₃ loss under very extreme NH₃ loss conditions. Calcium and magnesium chloride salts were added to urea:phosphoric acid to compare the relative effectiveness of acid and Ca + Mg salts for control of NH₃ loss.Little depression of NH₃-N loss was found from CO(NH₂)₂ containing H₃PO₄ and H₂SO₄ when the sand contained free CaCO₃. However, when CO(NH₂)₂:H₃PO₄ (UP) mixtures were applied as 17-19-0 on neutral and acid sands, NH₃ losses were reduced. Molar ratios less than 1:1 (28-12-0, 35-7-0) resulted in NH₃ losses similar to those from CO(NH₂)₂ alone even in acid soils. The 110 g N m^–2 as 17-19-0 reduced relative NH₃-N loss and pH in acidified and neutral soils more effectively than 11 g N m^–2. Ammonia losses are determined by chemical reactions occurring under the individual CO(NH₂)₂ granules; therefore, the use of the high 110 g N m^–2 rates in this research. The 17-19-0 reduced soil pH and retarded the rate of CO(NH₂)₂ hydrolysis with consequent reduction in NH₃ loss. Ammonia loss was reduced only slightly at 11 g N m^–2 from 17-19-0 even in acid soils. Ammonia loss was reduced from 70 to 30% of applied N by applications of HNO₃ and HCl with the CO(NH₂)₂. The HNO₃ and HCl react with CaCO₃ in a calcareous soil to produce CaCl₂ and Ca(NO₃)₂ which are known to reduce NH₃ loss from surface applied CO(NH₂)₂. However, a dry product of HNO₃ · CO(NH₂)₂ is explosive and can not be used as a general fertilizer.Calcium chloride or MgCl₂ combined with CO(NH₂)₂:H₃PO₄ reduced NH₃ loss more at 110 g N m^–2 than at 11 g N m^–2. Calcium chloride reduced NH₃ loss more effectively than MgCl₂. The CaCl₂ and MgCl₂ salts were more effective than H₂SO₄ or H₃PO₄ in reducing NH₃ losses except when (e.g., 17-19-0) mixtures were added to neutral or acidic sands.Contribution from Texas Agric. Exp. Stn., Texas A & M University, College Station, TX 77843.     

A comparative study of urea hydrolysis rate and ammonia volatilization from urea and urea calcium nitrate

A comparing of urea hydrolysis and NH₃ volatilization from urea supergranules and urea calcium nitrate (UCN, a new fertilizer produced by Norsk Hydro A/S, Norway) was made on two different flooded soil types, a high-CEC clay loam (Ås) and an intermediate-CEC clay loam (Kinn).Nitrogen loss by ammonia volatilization was reduced from 17% by surface application of urea supergranules (USG) on flooded Ås soil to 3% and 6% by UCN briquettes at either the same urea or nitrogen concentration as USG. A significant reduction was even found with the surface application of prilled UCN, 12% and 18% N-loss for prilled UCN and urea, respectively. The floodwater pH and NH ₄ ⁺ content was lower with UCN than urea, which reduced the potential for ammonia volatilization.NH₃-loss (5%) was significantly less when USG was surface applied on Kinn soil, while NH₃-loss from UCN briquettes was independent of soil type. The reduction in NH₃-loss from USG on Kinn soil was due to a decrease in the pH and NH ₄ ⁺ content of the floodwater caused by a reduced rate of urea hydrolysis.The rate of urea hydrolysis was lower with UCN than USG in both soils, but the difference between UCN and USG was greater in the Ås soil than in the Kinn soil. Three days after deep placement (10 cm), 18% of UCN urea and 52% of USG urea were hydrolyzed in Ås soil, while only 12% UCN and 17% USG were hydrolyzed in the Kinn soil.The surface application of USG on flooded soil reduced the rate of urea hydrolysis as compared to deep placement. 30% and 17% of USG urea was hydrolyzed after four days on Ås and Kinn soil, respectively. During the first few days the rate of hydrolysis of UCN was more affected by the soil type than the application method. Four days after surface application 32% and 13% UCN urea was hydrolyzed on Ås and Kinn soil, respectively. The rate of urea hydrolysis exhibited a zero-order reaction when USG and UCN-briquettes were point placed in flooded soils.     

Dynamics of ammonia volatilization from simulated urine patches and aqueous urea applied to pasture. II. Theoretical derivation of a simplified model

Theoretical considerations for the development of a simplified model for predicting volatilization losses of ammonia gas (NH_3(g)) from the urine patches of grazing herbivores in a pasture ecosystem are presented. The volatilization of NH_3(g) is treated as a physico-chemical phenomenon based on the soil solution chemistry of urine patches to develop a general equation to describe the rate of volatilization from a pasture surface. A semi-empirical approach was then used in which published data define typical limits for the parameters appearing in the volatilization equation. This led to the simplification of the general volatilization equation into a more useable and more readily verifiable form.The dominant factor in determining the rate of volatilization of NH_3(g) was shown to be the soil surface pH. To better understand the dynamics of pH changes within urine patches, the more extensive literature dealing with volatilization losses from flooded soils was reviewed. From the apparent similarities between the two systems a procedure was described by which a careful monitoring of soil surface pH as a function of time could be used to solve the simplified equation.To calculate NH_3(g) fluxes this model requires the following as input data: a knowledge of the disposition of the applied-N within the soil profile; the rate of urea hydrolysis in the topsoil; and soil surface pH and temperature measurements throughout the duration of a volatilization event.     

Relative ammonia loss from urea-based fertilizers applied to rice under different hydrological situations

Relative ammonia volatilization loss from prilled urea, urea supergranule (USG), neem cake-coated urea (NCU), rock phosphate-coated urea (RPCU), gypsum-coated urea (GCU), and prilled urea supplemented with dhaincha (Sesbania aculeata) green manure (Dh + PU) was measured in the fields under different hydrological situations of rice growing. Ammoniacal-N and pH of flood water were less with point placement of USG and Dh + PU treatments than with single basal broadcast applications of urea-based fertilizers. Ammonia collected with an acid trap in an enclosed chamber ranged from 1.47–3.07, 0.24–3.74, 0.80–3.50 and 0.50–1.20% of the applied N in upland, alternate wetting and drying, shallow submergence and intermediate deep water situations, respectively. The collected ammonia was less with point placement of USG at 5 cm depth in all situations and with Dh + PU treatment in shallow submergence than with other sources of N. Single basal broadcast applications of RPCU or NCU resulted in relatively higher loss. The loss from top-dressed urea was less than that from basally applied urea because of larger crop canopy at later stages of crop growth.     

Predicting the effect of soil-water-air dynamics on ammonia volatilization from applied urea with a mechanistic model

To assess the influence of varying soil water and soil air contents on ammonia volatilization from surface applied urea, a mechanistic model is used to simulate the system. The results are discussed in terms of the effects of soil-water-air dynamics on the movement of urea, ammoniacal-nitrogen and soil base, and on the rate of urea hydrolysis, and their influence on ammonia volatilization. Changing the soil moisture between 90% and 125% of field capacity did not have a marked influence on ammonia volatilization. The predicted losses were at their minimum with a moisture content slightly above field capacity, and increased sharply as the soil moisture fell below 90% of the field capacity. Ammonia volatilization losses measured by experiment at differentf values agreed very well with those predicted by the model. The relative contribution of the liquid pathway over the gaseous pathway of movement of NH₃ through soil increased with increase inf, and, at a givenf, decreased with increase in the pH.     

Lowering ammonia volatilization losses from urea application by activation of nitrification process

The aim of this work was to lower ammonia volatilization losses by increasing the rate of nitrification. This was achieved by eliminating the gap in timing between urea hydrolysis and ammonium nitrification. Soils were pretreated with a small amount of ammonium salt which led to the activation of the nitrification process. When nitrification passed its lag period, urea was applied to the soils. Ammonium produced by urea hydrolysis was quickly oxidized into nitrate and did not accumulate in the soil. This resulted in decreased ammonium concentrations in soil, and consequently, in decreased ammonia volatilization losses.This report is part of a doctoral thesis by the first author.     

Ammonia volatilization from a flooded rice field fertilized with amended urea materials

The extent of ammonia volatilization from prilled urea, urea supergranule and urea amended with neem seed cake, shell-lac and dicyandiamide was studied in a field experiment on flooded rice. The ammonia loss was measured by the closed acid trap method. The collected ammonia was highest from unamended prilled urea, accounting for 19 to 20 per cent of the applied N in 1983 and 20 to 24 per cent of the applied N in 1984. Coating of urea prills was either coaltar soaked neem seed cake or shell-lac was more effective than coating with dicyandiamide in reducing ammonia loss. Deep placement of urea as a supergranule was the most effective method of reducing ammonia volatilization. A diurnal variation in the pH and temperature of floodwater was observed. The quantity of ammonia collected in the acid trap was closely related to ammoniacal-N concentration and pH of the floodwater.     

Effect of urea granule size on ammonia volatilization from surface-applied urea

Urea powder and granules of varying size (1 to 8 mm diameter) were surface applied to a ryegrass/white clover pasture. Evolution of NH₃ was measured using a continuous air flow enclosure method. At 30 kg N ha^–1, the percentage of urea-N lost as NH₃ from powder or granules of 1–2, 3–4, 5.6 and 8 mm diameter was 18, 17, 20, 22 and 32 respectively. As the particle size increased, the rate of urea hydrolysis decreased and delayed the time at which the maximum rate of volatilization occurred. Mineral-N and soil surface pH measurements confirmed that during the period of volatilization, urea moved less than 30 mm from the application point.For the powder and 3–4 mm granule treatments, when the application rate was increased from 30 to 300 kg N ha^–1, the percentage of urea-N volatilized increased, but at any particular rate there was no significant difference in percentage loss between the powder and 3–4 mm granules.     

Effect of magnesium sulphate addition to urea on nitrogen loss due to ammonia volatilization

Fertilizer was applied as urea alone or as a mixture of urea and magnesium sulphate (MgSO₄·1H₂O) to study the effect on ammonia volatilization under laboratory conditions in relation to soil texture, N:Mg ratio, air flow rate, fertilizer form (solid or liquid) and organic material. When the mixture of urea and magnesium sulphate (UMM) was applied at a ratio of 1:0.21, significantly lower NH₃-N losses than from urea were found in 2 of 6 soils, and 4 soils showed a similar tendency. Increasing the N:Mg ratio to 1:0.5 resulted in significantly lower NH₃-N loss. Lower air flow rates reduced ammonia loss from UMM more than from urea alone. The effectiveness of UMM over urea was not improved in the liquid form. Increase of organic material had no influence on NH₃-N loss from urea alone or UMM.     

Obtaining granular NPK fertilizers from single superphosphate and urea

A laboratory study was conducted to elaborate methods for obtaining granular NPK fertilizers from mixtures of single superphosphate (SSP) with urea and potassium salts. Samples of products of various grades containing 32–39% fertilizer nutrients and some micronutrients (B, Cu, Co, Mo, Mn) were obtained and their characteristics determined. Instead of cured SSP the usage of a fresh den product was recommended to reduce environmental pollution. The best results were obtained by drying den superphosphate and neutralizing it with limestone before mixing it with other components. Granulation of mixtures should be carried out by the thermic method at 80–95°C, on account of the liquid phase from melting urea. The hardness of the granules obtained by this method, when stored in a dry room, remained satisfactory for 3 months.This paper is dedicated to the memory of Edgar Arumeel (1911–1993)     

Ammonia volatilization from point-placed urea in upland,sandy soils

The upland fertilization practice in Africa of placing N fertilizer below the soil surface near the plant might be facilitated through use of urea supergranules (USG). Since little is known about N losses from point-placed urea on light-textured African soils, laboratory studies were conducted in a forced-draft system to determine (a) the influence of soil properties on ammonia loss from USG and (b) to compare N loss from USG with that from broadcast N sources. Ammonia loss from 1.1 g USG placed at a 4-cm soil depth ranged from 2.9 to 62% of the added N on six light-textured soils. Ammonia loss was correlated with soil clay content (r = –0.93^**) but not with pH. A more detailed study on a soil from Niger revealed significantly less ammonia loss from either surfaced applied urea (18%) or surface-applied calcium ammonium nitrate (7%) than from USG placed at a 4-cm depth (67%). Amendment of surface-applied urea with 1.7% phenyl phosphorodiamidate (PPD), a urease inhibitor, essentially eliminated ammonia loss (1.9%). An¹⁵N balance confirmed that ammonia volatilization was the major loss mechanism for all N sources. The results suggest that point-placed urea may be prone to ammonia volatilization loss on light-textured African soils moistened by frequent light rainfall. In such cases, broadcast application of urea, CAN, or urea amended with PPD may be less prone to N loss.     

Ammonia volatilization from grassland receiving nitrogen fertilizer and rotationally grazed by dairy cattle

The micrometeorological mass balance method was used to measure ammonia (NH₃) volatilization from rotationally grazed swards throughout the 1987 and 1988 growing seasons. In both years the swards were dressed with calcium ammonium nitrate (CAN) split over 7 dressings. In 1987 the sward received a total of 550 kg N ha^–1, in 1988 a total of 550 or 250 kg N ha^–1. For the 550 kg N ha^–1 treatments there were 8 and 9 grazing cycles, respectively, in 1987 and 1988 and 7 for the 250 kg N ha^–1 treatment. Losses from the 550 N sward were 42.2 and 39.2 kg N ha^–1 in 1987 and 1988, respectively; this was equivalent to 8.5 and 7.7% of the N returned to the sward in the excreta of the grazing cattle. The NH₃ loss from the 250N sward was 8.1 kg N ha^–1 in 1988, which was equivalent to 3.1% of the N returned to the sward in excreta during the growing season. There was a wide variation in NH₃ volatilization between the individual grazing periods. This indicates the necessity of continued measurements throughout the growing season to obtain reliable data on NH₃ volatilization. Soil humidity is suggested to be a key factor, because emissions were high from wet soil, and low from drier soil. Results of a Monte Carlo simulation study showed that the measured NH₃ loss from the 250 and 550 N swards had a standard deviation of 13 and 5% of the mean, respectively.     

Ammonia volatilization from dairy farming systems in temperate areas: a review

Ammonia (NH₃) emissions from dairy farm systems cause environmental problems. This paper reviews and quantifies the major loss routes of NH₃ in dairy farms. Furthermore, management options are discussed that reduce NH₃ losses.Losses of NH₃ occur during slurry application, housing, slurry storage, grazing, fertilizer application and from crops, in descending order of importance. Animal waste is the major source in four of the six cases. This ranking varies between farms and between countries, depending on environmental conditions and management practices. Total NH₃ losses range from 17 to 46 kg N cow yr^-1, reflecting the variability in amount and composition of animal excreta (urine + faeces), management of the slurry and soil and environmental conditions. The amount and composition of urine and faeces depend on N tranformations in the digestive track of the cow. Of the major nitrogen compounds excreted urea has the highest potential for NH₃ volatilization followed by allantoin, uric acid and creatinine in decreasing order. Creatine, xanthine and hypoxanthine have a low NH₃ volatilization potential.Reducing the excretion of urea and urea like products by optimizing N Intake (NI) and N Retention (NR) is one way of decreasing NH₃ losses. Improvement is possible since NR is about 20% of NI in practice, whereas 43% is theoretically possible. The second solution is to reduce the rate of NH₃ loss by technical means like direct incorporation of slurry into the soil, dilution or acidification of slurry, covering of the slurry storage and/or acidification or dilution of slurry in the storage. These techniques have been known for a long time and now become available on a large scale in practice. Reducing the surface area per cow in the shed and sprinkling floors with water to remove and to dilute urine also decreases NH₃ loss.Reducing NH₃ loss requires a whole farm system approach, because it shows how intervening in one part may affect NH₃ losses in other parts of the system. Reducing NH₃ loss may increase nitrate leaching and denitrification. To prevent this, the achieved reduction in NH₃ loss should lead to a reduction of total N input of fertilizers, concentrates and forage on the N budget of the farm, which is possible as a reduction of NH₃ loss improves the N fertilizing value of slurry. Model calculations showed great scope for reducing NH₃ losses on dairy farms by improved management. Up to three fold reductions in NH₃ loss are possible together with marked reductions in mineral fertilizer usage. The rate at which improved management techniques, will be introduced in practice depends on legislation, the applicability of new techniques and the expected increase in net production costs. To comply with environmental targets requires a huge effort of farmers with associated high costs.     

Soil properties related to the dynamics of ammonia volatilization from urea applied to the surface of acidic soils

Ammonia volatilization after urea application is the most likely mechanism responsible for variation in urea performance even on acidic soils. The rate and the cumulative amount of NH₃ loss are important features for any soil. Field rates of NH₃ volatilization were simulated by using ventilated enclosures in the laboratory. The daily rates of NH₃ volatilization from 36 soils of widely differing properties from Northern Ireland were measured for 14 days. Cumulative loss rates for each soil were fitted to a logistic model from which the total cumulative loss (A_max) and the time for maximum rate of volatilization (T_max) were calculated. Stepwise multiple linear regression analysis was used to relate A_max and T_max to soil properties.A_max values ranged from 1.6 to 26.1% and averaged 16.8% of the urea-N applied. Titratable acidity (TA) was the single soil property most related to A_max accounting for 65% of the variance. Including moisture loss and pH_KCl in the multiple linear regression equation explained 74% of the variance. T_max values ranged from 0.0 to 10.6 days after urea application. Non-buffered urease activity and loss-on-ignition accounted for 70% of the variance in T_max. Including clay and CaCO₃ in the multiple linear regression equation explained 80% of the variance.In the absence of rainfall, the most important characteristic of the NH₃ volatilization process will be A_max. Under field conditions, rainfall will alter the dynamics of the NH₃ volatilization process depending on when it falls and on the T_max value for the soil. Since sufficient rainfall to prevent NH₃ volatilization is unlikely to occur in Ireland on many of the occasions when urea is applied, less NH₃ volatilization will occur from soils with high T_max values. Lowest rates of NH₃ volatilization will occur from soils with high values of TA, low values of non-buffered urease activity and high values of loss-on-ignition.