ADVANCES M1 AGRONOMY. VOL. 40

UREA TRANSFORMATIONS AND FERTILIZER EFFICIENCY IN SOIL

W. D. Gould,’ C. Hagedorn,* and R. G. L. McCreadyl

’ Biotechnology Section, CANMET, Energy Mines and Resources Ottawa, Ontario, Canada K1 A OG1

Allied Corporation, Syracuse Research Laboratory, Solvay, New York 13209

I. INTRODUCTION

A. PREVIOUS REVIEWS CONCERNING UREA AND ITS USE

Although a number of review articles have dealt with the environmental, chemical, and physical factors affecting the hydrolysis of urea in soil (Bremner and Mulvaney, 1978; Kiss el a/., 1975; Ladd, 1978; Skujins, 1967, 1976), to date, none have discussed urea transformation in the field. Most of the studies relating to urease activity in soil have been conducted in the laboratory where a urea solution, with or without a buffer, has been added to a soil sample. These types of studies have improved our understanding of soil urease activity, but they do not simulate field conditions, and in order to improve the efficiency and use of urea as a fertilizer, it is necessary to understand the soil interactions of urea under field conditions. The environ- mental and soil factors affecting ammonia losses from fertilizer urea have been discussed by Terman ( 1979), Nelson ( 1982), and Gasser ( 1964a). Ure- ase inhibitors and methods of controlling urea transformations in soil were thoroughly reviewed by Sahrawat (1980) and by Mulvaney and Bremner ( 198 1). A number of papers have recently been published concerning a new class ofurease inhibitors, the phosphorodiamidates and phosphorotriamides which are the most effective inhibitors presently known for soil urease (Mar- tens and Bremner, 1984b).

The objectives of this review are to discuss (1) environmental, chemical, and physical soil conditions which must be considered in regard to control- ling urea transformations in the field; (2) studies that have directly measured urea transformations in the field or under simulated field conditions; and (3) new classes of urease inhibitors that may improve the efficiency of urea under field conditions.

209

CopvriehlO 1986 by Aadcmic Resq Inc. AU rights of reproduction in any forin mewed.

210 W. D. GOULD ET AL.

B. AGRONOMIC IMPORTANCE OF UREA

Although urea was first used as a fertilizer in 1935 it was not used exten- sively until the 1960s. A number of problems concerning the physical prop- erties and the low nitrogen efficiency of urea prevented its rapid acceptance as a fertilizer. Many of these problems have been overcome by new technol- ogy and proper management procedures. The production of urea has in- creased more rapidly than any of the other nitrogenous fertilizers. During 1980 - 198 1, urea accounted for 12% of the 12.7 X lo6 metric tons of fertil- izer utilized in North America and accounted for 70% of the nitrogen fertil- izer utilized in Asia. Due to its high N content, ease of handling, and recent technological improvements, urea is predicted to gain an even greater per- centage of the total N fertilizer sales in the future (Beaton, 1978). Presently, only anhydrous ammonia is a less expensive nitrogen source than urea.

c. FACTORS AFFECTING THE EFFICIENCY OF UREA AS A FERTILIZER

I. BeneJits

The manufacture of urea uses inexpensive starting materials, carbon dioxide and ammonia, the former a by-product, and the latter the final product of anhydrous ammonia manufacture. The chemical synthesis of urea has a higher yield efficiency than that achieved in the manufacture of ammonium nitrate (Beaton, 1978). Urea has less tendency to coalesce and compact than ammonium nitrate, is less corrosive than other nitrogen fertil- izers, and is suitable as a camer for a number of herbicides. Much ofthe urea in the United States is applied as urea ammonium nitrate solution prepared by combination of urea with ammonium nitrate, resulting in a solution of 28-32% N (Englestad and Hauck, 1974; Pesek et al., 1971), since their combination increases the solubility of both fertilizers (Hardesty, 1955).

2. Problems

Although urea is frequently equivalent to other nitrogenous fertilizers (Smith and Chalk, 1980c; Van Lierop and Tran, 1980) poor crop responses to urea have frequently been observed (Beaton, 1978). The rapid hydrolysis of urea in the soil can result in high soil pH values and high ammonium ion concentrations which are conducive to the accumulation of ammonia. The major problems observed in urea fertilization are the loss of volatile ammo- nia gas and ammonia toxicity to germinating seedlings (Court et al., 1964a).

UREA TRANSFORMATIONS AND FERTILIZER EFFICIENCY 2 1 1

Also, the accumulation of nitrite in the soil following the hydrolysis of urea can result in toxicity and nitrogen losses. Biuret, which is found in low concentrations in urea, is phytotoxic under certain conditions. The low density and small size of prilled urea makes it unsuitable for broadcasting and blending with other fertilizers. Prilled urea is also friable and tends to produce fines during handling. The disadvantages of prilled urea are elimi- nated by use of granular urea.

II. PROBLEMS ASSOCIATED WITH UREA FERTILIZERS

A. BIURET

The dimer of urea, biuret, is formed by the thermal decomposition of urea during the preparation of prilled and granular urea (Court et af., 1964a; Reynolds and Trimarke, 1964). Biuret is known to interfere with protein synthesis in plants ( Webster et al., 1957), and phytotoxic effects are generally observed when the biuret content of urea is greater than 1 % (Low and Piper, 1961; Smika and Smith, 1957; Wilkinson and Ohlrogge, 1960). There are some crops (i.e., citrus fruit, pineapple, and coffee) that are very sensitive to biuret; and urea containing more than 0.25% biuret cannot be used on these crops (Church, 1964). High concentrations (1 - 10%) of biuret in urea has been shown to inhibit nitrification of the urea in soil (Sahrawat, 1977). Foliarly applied urea containing 4% biuret has been shown to have no deleterious effects on soybeans (Poole ef al., 1983). However, the biuret content of most urea fertilizers currently being produced is less than 1% (Church, 1964; Reynolds and Trimarke, 1964); thus biuret contamination of urea is no longer considered a problem.

B. ACCUMULATION OF NITRITE IN SOIL

The accumulation of nitrite in the soil may be toxic to the plants and may result in losses of gaseous nitrogen by chemical denitrification. The alkaline pH and high ammonium ion concentrations resulting from urea hydrolysis in soils do not appreciably affect the oxidation of ammonia to nitrite by Nitrosomonas sp., but does inhibit the oxidation of nitrite to nitrate by Nitrobacter sp. (Chapman and Liebig, 1952; Hauck and Stephenson, 1965; Wetselaar ef al., 1972). Bezdicek ef af. (1 97 1) compared the behavior ofurea, diammonium phosphate, and ammonium sulfate in an alkaline soil. They

212 W. D. GOULD ET AL.

found that the highest accumulation of nitrite (260 ppm) occurred in the soil to which urea had been added.

In contrast to the biological reactions, three chemical reactions for the loss of nitrite are known.

1. The decomposition of nitrous acid (Allison, 1963; Nelson and Bremner, 1970b; Van Cleemput and Baert, 1976):

2HN0, - NOT + NO,f + H,O

In the soil, both the nitric oxide and nitrogen dioxide produced by this process may undergo further reactions in the soil (Nelson, 1982).

2. The Van Slyke reaction between the nitrite ion and a-amino acids (Christianson and Cho, 1983; Sabbe and Reed, 1964; Smith and Chalk, 1980b):

NH, 0 OH 0 I I1 I II

R-CH-C-OH + HNO, + R-CH-C-OH + H,O + NZf

3. The decomposition of ammonium nitrite (Allison, 1963) NH,NO, - N, + 2H,O

Volatile nitrogen losses by the decomposition of ammonium nitrite are unlikely since studies utilizing "N have shown that all of the nitrogen gas evolved from nitrite was a result of a Van Slyke type of reaction (Christian- son and Cho, 1983; Smith and Chalk, 1980b). Wullstein and Gilmour (1 964, 1966) have suggested that metal cations enhance nitrite decomposition in soil, but others have not confirmed this hypothesis (Moraghan and Buresh, 1977; Nelson and Bremner, 1970a). The reactions involved in the volatiliza- tion of nitrogen from nitrite are enhanced by low pH (Allison, 1963; Sabbe and Reed, 1964), but these conditions are not favorable for the accumulation of nitrite. However, appreciable chemical denitrification has been observed at alkaline pH values (Smith and Chalk, 1980a). Toxicity to plants from nitrite accumulation has been observed subsequent to the addition of urea fertilizer (Court et al., 1962, 1964b).

C. LEACHING

As urea is a nonionic compound, it is susceptible to leaching in soil, but at a slower rate than the nitrate ion; it can be weakly adsorbed by soil and is simultaneously hydrolyzed to ammonium bicarbonate by soil urease (Broadbent et al., 1958). Urea is retained within soil due to salt formation between urea and the carboxyl groups of soil organic matter. At acid pH

UREA TRANSFORMATIONS AND FERTILIZER EFFICIENCY 2 13

values urea can be protonated and behave as a cation (Broadbent and Lewis, 1964; Chin and Kroontje, 1962). Urea can also complex with clay minerals (Farmer and Ahlrichs, 1969; Mitsui and Takatoh, 1963; Mortland, 1966).

Leaching losses of urea may occur by two processes: ( 1) urea is leached per se from the soil, and (2) urea migrate below the rooting zone, is hydrolyzed, nitrified, and then leached as nitrate (Bauder and Montgomery, 1980). Split applications of urea have been shown to result in large losses of nitrate by leaching (Bauder and Schneider, 1979). Leaching losses of nitrogen from the application of urea to forest soils have been found to be quite low (Overrein, 1968, 1969).

D. Loss OF AMMONIA AND AMMONIA TOXICITY

Ammonia toxicity and the loss of nitrogen as volatile ammonia are the major problems encountered with fertilizer urea. An equilibrium between ammonium ions and ammonia gas occurs in aqueous solutions of ammo- nium salts. At pH values below 7, the ammonium ion predominates, whereas above pH 8.5 free ammonia is the predominant form (Bates and Pinching, 1950). Although the ammonium ion is relatively nontoxic, am- monia gas is extremely toxic (Warren, 1962). In studies on excised beet root disks and beet root mitochondria, ammonia was found to interfere with the NADH oxidase system, an integral part of the electron transport system (Vines and Wedding, 1960). The hydrolysis of urea with subsequent produc- tion of ammonia inhibits the germination of corn (Liege1 and Walsh, 1975a,b).

The alkaline hydrolysis products of urea (ammonium bicarbonate and ammonium hydroxide) can increase the pH of soil microenvironments, resulting in the loss of soil ammonium by volatilization (Ernst and Massey, 1960; Terman, 1979). The greatest ammonia losses have generally been observed in soils where urea has been broadcast. The loss of ammonia from flooded rice soils that have been fertilized with urea has recently been recog- nized as a severe problem (Fillery ef al., 1984; Vlek and Craswell, 1981). However, when urea is applied to rice at an earlier stage of growth biological denitrification is the major mechanism of nitrogen loss (Simpson et al., 1984).

The loss of nitrogen from urea by ammonia volatilization is favored by a high initial soil pH (Overrein and Moe, 1967), low soil buffer capacity (Ferguson etal., 1984;MartinandChapman, 195l;Nelsonetal., 1980),and high temperatures (Lippold et al., 1975; Watkins et a]., 1972). Ammonia losses from agricultural soils vary from 0.4 to 80% of the applied urea nitro- gen (Fernando and Roberts, 1975; Gasser, 1964b; Hargrove and Kissel,

214 W. D. GOULD ET AL.

1979; Kresge and Satchell, 1960; Simpson, 1969; Torello et al., 1983; Volk, 1959), and from 3.5 to 24.9% in forest soils (Nommik, 1973a; Overrein, 1968). Volatilization loss measurements are dependent on experimental technique (Kissel et al., 1977; Watkins et al., 1972), but even though the absolute magnitude of nitrogen loss may be difficult to predict, comparative studies are still valid. In order to simulate field conditions Boumeester et al. (1985) measured ammonia losses from urea broadcast on soil placed in a wind tunnel. They measured a loss of 40% of the urea nitrogen when a simulated rainfall event of 1 cm was applied 7 days subsequent to fertiliza- tion. The ammonia loss was reduced to 13% of the added urea nitrogen when a simulated rainfall event of 4 cm was applied subsequent to fertilization. A urease inhibitor could have some use in maintaining the fertilizer as urea until sufficient rainfall occurred to wash it into the soil.

Hanawalt ( 1969) and Mahendrappa and Ogden ( 1973) have suggested that volatile ammonia lost from urea-fertilized soil may be reabsorbed by adjacent soils.

E. IMMOBILIZATION

Immobilization of urea nitrogen has been shown to be a problem particu- larly in some forest soils and under reduced tillage. Appreciable amounts of the urea applied to forest soils are rapidly immobilized in a nonexchangeable form which is not available to the trees ( Momson and Foster, 1977; Nom- mik, 1970; Overrein, 1970; Worsnop and Will, 1980). Several explanations have been advanced for the rapid incorporation of urea into the organic fraction of forest litter: ( 1) microbial immobilization of nitrogen (Overrein, 1970), (2) a chemical reaction between humus and urea (Ogner, 1972), and (3) a chemical reaction between ammonia and soil organic matter (Burge and Broadbent, 1961). Foster et al. (1985) added I5N-labeled urea to sterile and nonsterile forest soils and concluded that the immobilization of urea occurs via the chemical reaction of ammonia with soil organic matter at elevated pH values. Both elevated pH and high concentrations of gaseous ammonia would be present in the vicinity of a urea granule and likely make urea more prone to such reactions than other nitrogen fertilizers. Heilman et al. ( 1982a,b) reported that most of the fertilizer nitrogen was assimilated by Douglas fir within 168 days of fertilization and thus immobilized nitrogen would probably not be utilized. Also, the high C/N ratio ofthe surface mulch that accumulates on no-till soils is conducive to microbial immobilization of nitrogen (Allison, 1966; Frederickson et al., 1982; Rice and Smith, 1984).

UREA TRANSFORMATIONS AND FERTILIZER EFFICIENCY 2 15

F. EFFECT OF MANAGEMENT ON UREA RESPONSE

Intensive fertilization of grasses and forage crops is practiced in western Europe (Craven and Kilkenny, 1974), and nitrogen applications vary from 125 to 500 kg/ha (Widdowson et af., 1973). Although increased yields have been obtained by fertilization of fescue grassland in southern Alberta, fertil- ization of pastures is not considered economically feasible in western Can- ada (Smith et al., 1968). The current agronomic practices for nitrogen fertil- ization of forages and grasses are: (1) to broadcast solid fertilizer (Hill and Tucker, 1968; Penny et af., 1977; Widdowson et af., 1973), (2) surface banding of fertilizer solutions (Mengel, 1985), and (3) injection of anhy- drous ammonia or fertilizer solutions (Penny et af., 1977; Widdowson et af., 1973). However, anhydrous ammonia may temporarily kill grasses adjacent to the zone of injection (Hill and Tucker, 1968; Widdowson et al., 1973). Urea has not been as effective as other nitrogenous fertilizers when broadcast on grasses (Simpson, 1968; Power, 1974; Westerman et af., 1983).

Ammonia volatilization from urea broadcast on grasses has been well documented (Gasser, 1964a; McGarity and Hoult, 197 1; Simpson, 1969; Simpson and Melsted, 1962; Volk, 1959) and can be attributed to the high urease activity of the surface thatch layer (Torello and Wehner, 1983) and a low absorbtive capacity for ammonia (Nelson et af., 1980). Surface applica- tion of urea is equivalent to other fertilizers if it is incorporated into the soil by mixing or washed in by rainfall subsequent to application (Ansorge et af., 1973; Ernst and Massey, 1960).

Urea has several advantages as a nitrogen source for aerial application on forests; it is relatively nontoxic, has a high N analysis, and may be directly absorbed by the foliage (Volk, 1970). Urea is generally equivalent to other nitrogenous fertilizers used in forests (Fisher and Pritchett, 1982; Weetman and Fournier, 1984), but in some studies has been shown to be an inferior N source (Brix, 198 1 ; Pang, 1985). The major problems observed in the urea fertilization of forests are ammonia volatilization (Acquaye and Cun- ningham, 1965; Craig and Wollum, 1982; Marshall and De Bell, 1980) and immobilization (Nommik, 1970).

Management systems employing zero and minimal tillage have become more popular, and by the year 2000,45% ofthe cropland in the United States could be under reduced tillage (Phillips et al., 1980). The presence of the surface residues on no-till soils increases soil moisture, affects biological activity in soil, and changes soil nitrogen transformations (Aulakh et af., 1984; Doran, 1980a,b; Rice and Smith, 1982, 1983).

The current nitrogen fertilization practices in reduced tillage cultivation are broadcasting, injection of anhydrous ammonia and liquid fertilizers,

216 W. D. GOULD ET AL.

spray application of urea- ammonium nitrate (UAN) solutions, and surface banding of UAN solutions (Mengel, 1985; Richey et al., 1977; Touchton and Hargrove, 1982). Broadcast urea has been shown to be less effective than ammonium nitrate or injected N sources on no-till soils (Bandel et al., 1980; Fox and Hoffman, 198 1; Touchton and Hargrove, 1982), primarily due to volalitization of ammonia (Fox and Hoffman, 198 1 ; Mengel, 1985).

Ill. UREA TRANSFORMATIONS

A. HYDROLYSIS

1. Laboratory Studies

The enzyme urease (urea amidohydrolase, EC 3.5.1.5) which catalyzes the hydrolysis of urea was first crystallized from the jack bean (Cunavaliu ensi- formis) by Sumner ( 1926). Urease is found in some higher plants and in most species of bacteria, yeast, and fungi (Sumner, 1953).

The ability to hydrolyze urea was found to vary from 17 to 7 1% for soil bacteria and from 78 to 98% for soil fungi (Lloyd and Sheaffe, 1973; Roberge and Knowles, 1967). Although soil urease is considered to be of microbial origin (Skujins, 1976) there is evidence that some soil urease activity may be derived from plants (Frankenberger and Tabatabai, 1982). However, there is no direct evidence for the production of urease by plant roots (Estermann and McLaren, 196 1).

Paulson and Kurtz (1969) altered soil urease activity by adding various amendments, and calculated, with regression analysis, that 79 - 89% of soil urease activity was extracellular and complexed by soil colloids. A significant amount of soil urease can be extracted as an organo-urease complex (Bums et al., 1972a,b; Ceccanti et al., 1978; McLaren et al., 1975; Nannipieri et al., 1974,1978). This extracted organo-urease complex was found to be resistant to digestion by pronase (Bums et al., 1972a). Soil urease and organo-urease complexes extracted from soil are more stable than jack bean urease under comparable conditions (O'Toole and Morgan, 1984; Pettit et al., 1976). Zantua and Bremner (1976) found that a number of Iowa soils had a stable level of urease activity that depended on the properties of the individual soil.

Although varying results have been obtained when urease activity has been compared with soil properties, the soil organic carbon content has been shown to correlate quite well with urease activity (Beri and Brar, 1978; Dalal, 1975a; Speir, 1977; Zantua et al., 1977). Urease activity is usually highest in the surface horizons of forest soils (Roberge and Knowles, 1966, 1968) and

UREA TRANSFORMATIONS AND FERTILIZER EFFICIENCY 2 17

lowest in alkaline and saline soils (Kumar and Wagenet, 1985; Skujins and McLaren, 1968). Very high urease activities have been observed in some tropical soils (Dalal, 1975a) while weak urease activity has been observed in the lower mineral horizons of a number of soils (Gould et al., 1973; Myers and McGarity, 1968).

A large number of studies have examined the effect of temperature, pH, water level, and urea concentration on the rate of urea hydrolysis in soil. Many of these measurements have been made with different assay tech- niques and comparisons between soils using data from the literature must be made with caution. The hydrolysis of urea in soil generally follows Michaelis- Menten kinetics; that is, the hydrolysis rate increases with in- creasing substrate concentration until the enzyme is saturated (Dalal, 1975a; Tabatabai, 1973; Tabatabai and Bremner, 1972). At very high urea concen- trations the hydrolysis rate decreases, probably due to substrate inhibition of the enzyme ( Rachhpal-Singh and Nye, 1984a). The hydrolysis of urea in soil increases with increasing temperature according to the Arrhenius equation (Dalal, 1975a; Gould et al., 1973) up to 60 to 70°C and then decreases rapidly above that temperature range (Pettit et al., 1976; Sahrawat, 1984; Zantua and Bremner, 1977). A pH optimum for soil urease of 6.5 -7.0 was found by Pettit et al. (1976) and pH optima of 8.8-9.0 have been found by others (May and Douglas, 1976; Tabatabai and Bremner, 1972).

Differing results for the effect of water content on the rate of urea hydroly- sis in soil have been found. In most studies urease activity was not appreci- ably affected by the soil water content (Delaune and Patrick, 1970; Gould et al., 1973; Skujins and McLaren, 1967, 1968, 1969), but others have found the hydrolysis rate to increase (Kumar and Wagenet, 1984; Rachinskiy and Pel’tser, 1965) or decrease (Dalal, 1975a; Simpson and Melsted, 1963) with increasing water content. Sahrawat (1 984) found the urease activity to be unaffected by the water contents above 20% (w/w) but to decrease rapidly below that level. Vlek and Carter (1983) found that urea hydrolysis rates decreased below the permanent wilting point.

2. Field and Simulated Field Studies

There have been very few studies in which the hydrolysis rate of urea has been determined in the field. In the soil surrounding a urea prill, granule, or band, gradients of pH, urea, and ammonium ion concentrations develop ( Rachhpal-Singh and Nye, 1984b). Malhi and Nyborg ( 1979) measured the hydrolysis rate of urea under field conditions and simulated field conditions. They found that the equivalent of 200 pg urea N/g of soil added as pellets to soil in the laboratory was completely hydrolyzed in 160 hr. The same con-

218 W. D. GOULD ET AL.

centration of urea added as a solution to the same soil in the laboratory hydrolyzed in 20 hr (Gould et al., 1973). However, prilled urea added to field plots required 192 hr to completely hydrolyze (Malhi and Nyborg, 1979). Mohammed et al. (1 984) applied 100 kg urea nitrogen/ha to the soil surface and determined that 86% of the urea was hydrolyzed after 1 week. Aulakh and Rennie (1984) added lSN-labeled urea at a rate of 100 kg N/ha to field microplots during the fall and estimated that only 60% of the urea hydro- lyzed in 3 weeks. They attributed the reduced hydrolysis rate to the low soil temperatures at that time of the year.

Campbell et al. ( 1984) added urea to soil columns in order to simulate field conditions and found urea hydrolysis to depend on the application method and soil moisture content. Urea incorporated in the soil hydrolyzed more rapidly than banded urea. The difference in hydrolysis rate between banded and incorporated urea was greater in the dry soil, probably due to less water contact with the banded urea which would limit dissolution and hydrolysis (Campbell et al., 1984). Savant and De Datta (1979) measured the amount of urea remaining at placement sites when urea was placed at lower depths in a wetland rice field. However they only recovered a portion of the urea, and thus an accurate hydrolysis rate could not be calculated.

A number of simulation models have been developed in order to under- stand some of the processes involved in urea transformations in soil (Arda- kani et al., 1975; McLaren, 1970; Parton et al., 1981; Scotter et al., 1984; Wagenet et al., 1977). The effects of a number of environmental variables on the spatial and temporal distributions of the various nitrogenous interme- diates can be easily simulated by various computer models.

B. SUBSEQUENT REACTIONS OF UREA IN THE SOIL

Urea is rapidly hydrolyzed under favorable conditions to ammonium bicarbonate, which is subsequently nitrified (Fig. 1). Nitrite and nitrate ions

0 II

H,N ; tc - N H ~ - NH~-NO;- NO;

N 2 0 I I 1- NO;

N2 f FIG. 1. Possible reactions of urea in the soil.

UREA TRANSFORMATIONS AND FERTILIZER EFFICIENCY 2 19

and urea can readily diffuse through the soil or be leached. However, ammo- nium ions are much less mobile. Nitrification is favored by neutral pH, adequate aeration and higher temperatures (Ayanaba and Kang, 1976; Pang et al., 1977). The nitrification of band-applied urea has been shown to be dependent on the diffusion of ammonium away from the band (Pang et al., 1973). Pang et al. (1 975a) studied the nitrification of band-applied urea in soil columns and found rapid nitrification in high pH soils and a very slow rate of nitrification in low pH soils. Liming the low pH soils decreased the rate of nitrification of urea-derived ammonium. In order to obtain rapid nitrification in the low pH soil it was necessary to both lime to neutral pH and either add an inoculum of the active soil or a pure culture of Nitroso- rnonas to the soil (Pang et al., 1975a). No nitrite was observed in a soil column when urea was applied at a rate equivalent to 100 kg N/ha, but significant quantities of nitrite accumulated when 200 and 800 kg urea N/ha were added to the soil columns (Pang et al., 1975b). Very high concentra- tions of banded urea could produce nitrite sufficient to cause gaseous losses of N2 and N20 (Christianson et al., 1979). In the immediate vicinity of a band, the high pH values and high ammonium ion concentrations are con- ducive to the accumulation of nitrite (see Section 11,B).

After 6 weeks, the distribution of inorganic nitrogen was symmetrical around the band of urea under isothermal conditions (Pang et al., 1977). However, when a temperature gradient (8.5 to 23 "C) was established in a soil column, a higher proportion of the added nitrogen was found at the warm end of the column after a 6-week incubation (Table I). Vaporization ofwater

Table I

Effect of Temperature Gradient on Distribution of Inorganic Nitrogen around a

Band of

Nitrogen concentration (Pg N/g soil)

Temperature conditions Above urea Below urea

Isothermal 600 74 1 Temperature 1032 329

gradient

a From Pang ef al. (1 977). bDistribution determined after 6 weeks

above and below banded urea under a temper- ature gradient of 8.5 to 23°C and under iso- thermal conditions.

220 W. D. GOULD ET AL.

at the warm end and condensation at the cold end would result in return liquid flow to the warm end, thus resulting in transport of nitrogen to the warm end. Since similar temperature gradients are observed in field soils during the spring, it is very likely that this phenomenon occurs in the field (Pang et al., 1977).

IV. METHODS TO ALTER THE EFFICIENCY OF UREA

A. UREASE INHIBITORS

I . Introduction

In recent years a number of approaches have been considered to prevent the accumulation of high concentrations of ammonium and the loss of ammonia by volatilization from soils when urea has been applied to the surface of the soil. The rapid biological hydrolysis of urea can be reduced by the use of urease inhibitors. In the past, many of the commonly used inhibi- tors were selected by a trial and error process. However, by understanding the reaction mechanism of the urease-catalyzed hydrolysis of urea and the chemistry of the active site of urease, it should be possible to choose potential urease inhibitors on a more rational basis. The currently accepted reaction mechanism for the enzymatic hydrolysis of urea is the enzymatic cleavage of urea to ammonia and carbamic acid (Fig. 2), followed by the chemical hydrolysis of carbamic acid to ammonia and carbon dioxide (Blakeley et al., 1969, 1982; Gorin, 1959).

The presence of sulihydryl groups within the urease structure was first

FIG. 2. Mechanism of the urease-catalyzed hydrolysis of urea.

UREA TRANSFORMATIONS AND FERTILIZER EFFICIENCY 22 1

demonstrated by Sumner and Poland (1933), and their essential role in the catalytic activity of urease is now well documented (Andrews and Reithel, 1970; Gorin and Chin, 1965, Gorin et al., 1962; Riddles et al., 1983). Recent findings indicate that urease is a metalloenzyme and that it contains nine atoms of nickel per urease molecule (Dixon et al., 1975, 1980). Barth and Michel ( 1972) suggested that the imidazole group of histidine was involved in the enzymatic hydrolysis of urea by urease. X-Ray fluorescence studies indicate that each nickel atom may be bound to one or more histidine residues (Hasnain and Piggott, 1983).

The ideal urease inhibitor (1) must be inhibitory at low concentrations in order to be an economically feasible additive in fertilizer formulations, (2) must have minimal detrimental effects on plants and soil microorganisms (May and Douglas, 1975), and (3) must not be inactivated by soil constitu- ents.

As Mulvaney and Bremner (198 1) published a comprehensive review of urease inhibitors in 1981, only representative compounds of the major classes of urease inhibitors and new inhibitor formulations will be discussed here.

2. Suljhydryl Reagents

Any organic or inorganic compound which will react with sulfhydryl (mercapto) groups will inhibit urease to various degrees. Metal ions inhibit urease by reacting with the sulfhydryl groups (Shaw and Raval, 1961b) and the observed inhibition is inversely proportional to the solubility product of the metal-sulfide complex (Hughes et al., 1969; Shaw, 1954; Toren and Burger, 1968).

pChloromercuribenzoate (Moe, 1967), dihydric phenols, aminocresols (Rodgers, 1984), and benzoquinones have also been used to inhibit soil urease (Fig. 3A) (Bremner and Douglas, 197 la; Mishra and Flaig, 1979; Mulvaney and Bremner, 1978; Tomar and MacKenzie, 1984). The dihydric phenols only inhibit urease when they are in the quinone form (Quastel, 1933). Thiol or sulfhydryl groups form addition products with some qui- nones (Cecil and McPhee, 1959), and they inhibit urease activity by blocking the essential group@) at the active site. Until recently, the quinone class of compounds were the most effective inhibitors of soil urease.

Several heterocyclic sulfur compounds are known to be inhibitors of both jack bean urease and soil urease (Gould et al., 1978). Gould et al. (1978) postulated that the mechanism of inhibition was via a thiol-disulfide ex- change reaction between a heterocyclic disulfide and the sulfhydryl group at the urease active site.

222 W. D. GOULD ET AL.

A

OH

HYDRoQUINONE

OH

CATECHOL

I I 0

p-BENZOQUINONE

183 4 - T H I A D I A Z O L E 5;AMINO-1 3 4 -2, ~-DITHIOL - T H I A D I A Z O L E - 2 - T H I O L

B

0 51 H2N- 8 - N HOH C H 3 ( CH2 Is- C- NHOH

II CH3 C-NHOH

ACETOHY DROXAMIC HYDROXYUREA C APRY LOHY DROXAMI C A C I D A C I D

FIG. 3. Various compounds that have been evaluated as urease inhibitors. (A) Sulfhydryl reagents; (B) hydroxamates.

3. Hydroxamates

The hydroxamic acids are the most thoroughly studied of the urease inhibitors. The hydroxamates are specific, noncompetitive inhibitors of ure- ase (Gale and Atkins, 1969; Kobashi et al., 1962). The most potent hydroxa- mate type of inhibitor identified to date is caprylohydroxamic acid (Fig. 3B) (Hase and Kobashi, 1967). Inhibition of urease by hydroxamates is believed due to a complex formation with one of the nickel atoms at the active site of urease (Dixon et al., 1975). Acetohydroxamic acid has been shown to be less effective in soil than many of the other urease inhibitors (Bremner and

UREA TRANSFORMATIONS AND FERTILIZER EFFICIENCY 223

Douglas, 197 la; Gould et al., 1978). This inhibitor reduces the rate of am- monia volatilization with a urea-fertilized soil; however the total amount of ammonia volatilized over an extended period of time is not reduced by this compound (Moe, 1967; Pugh and Waid, 1969a,b).

4. Structural Analogs of Urea and Related Compounds

Compounds that have structural similarities to urea inhibit urease by competing for the same active site on the enzyme. Thiourea, methylurea (Kistiakowsky and Shaw, 1953; Shaw and Raval, 196 la), and the substituted phenylureas (Cervelli et al., 1975, 1976) are known inhibitors of jack bean urease (Fig. 4A). Ashworth et al. ( 1980) have measured urea hydrolysis and nitrification inhibition in soil by a number of xanthates. Neither the substi- tuted ureas nor the xanthates provide sufficient inhibition of soil urease to be of practical agricultural value.

Recently, a new class of urease inhibitors, the phosphorodiamidates (Heber et al., 1979; Matzel and Heber, 1979; Matzel et al., 1978, 1979; Muller and Forster, 1980), the phosphorotriamides (Millner et al., 1982), and the thiophosphoric acids (Liao and Raines, 1985) have been reported (Fig. 4B). Some of these compounds appear to meet all of the criteria for agronomically acceptible urease inhibitors. They are extremely effective inhibitors of soil urease activity at very low concentrations; e.g., phenyl- phosphorodiamidate (PPD) has been found to inhibit soil urease activity at concentrations as low as 0.2 pg/g of soil (Martens and Bremner, 1984b). Martens and Bremner (1984a) measured the inhibition of soil urease by PPD, phosphorodiamidinic acid, and 10 phosphorotriamides. They found that all but two of these compounds are better inhibitors of soil urease activity than hydroquinone, which had been reported to be the most effec- tive inhibitor of soil urease prior to 1976. PPD, co-applied with urea pellets to flooded rice at a concentration of 1 % w/w, delays urea hydrolysis, reduces volatile nitrogen loss, and increases the assimilation of nitrogen (Byrnes et al., 1983; Vlek et al., 1980b). Martens and Bremner (1984b) studied the effect of a number of environmental and soil factors on the ability of PPD to inhibit soil urease activity, and found that PPD was ineffective at higher temperatures. Inhibitors added to surface-applied urea solutions did not improve nitrogen recovery, but trichloroethylphosphorodiamidate and diethylphosphorotriamidate improved nitrogen efficiency when coated on broadcast urea (Broadbent et al., 1985). Since only a few of the phosphoroa- mide class of compounds have been examined as potential urease inhibitors it is likely that compounds superior to PPD will be discovered.

224

THIOUREA

W. D. GOULD ET AL.

A

PHENYLUREA

B

PHENYLPHOSPHORODIAMIDATE PHENYLPHOSPHOROTRIAMIDATE

THIOPHOSPHORY L T R I A M I D E

on

THIOPHOSPHORYL D I A M I D A T E

FIG. 4. Various compounds that have been evaluated as urease inhibitors. (A) Substrate analogs; (B) phosphoroamides.

B. COATINGS OR MIXTURES

1. Sulfur-Coated Urea

Urea has been coated with resin (Brown et al., 1966), plastic (Mahen- drappa and Salonius, 1974; Beaton et al., 1967), shellac (Reddy and Prasad,

UREA TRANSFORMATIONS AND FERTILIZER EFFICIENCY 225

1979, silica (Savant et al., 1983), and sulfur (Rindt et al., 1968; Savant and De Datta, 1979) and impregnated with petroleum wax (Skogley and King, 1968) in order to retard the dissolution of urea in the soil. Sulfur-coated urea (SCU) is prepared by spraying molten sulfur onto heated urea pellets, which in turn are sprayed with molten wax containing a biocide, and the pellets are subsequently coated with a conditioner which is added to prevent caking of the fertilizer (Blouin et ul., 197 1). Both the wax and the sulfur are necessary to produce a retardant coating; a biocide may be included in the wax coating to prevent degradation of this layer by soil microorganisms (Allen et ul., 197 1 ; Blouin et al., 197 1). However, the cost effectiveness of the biocide in delaying the release of urea from SCU has been questioned, and its use has been discontinued (Shirley and Meline, 1975).

The release of urea from SCU has been postulated to occur by three mechanisms: ( 1) the wax sealant is microbially degraded and absorbed by the adjacent soil; (2) the conversion of polymeric amorphous sulfur to a crystal- line structure results in cracks or fissures in the sulfur shell (McClellan and Scheib, 1975), thereby opening channels through the sulfur coat which allow the release of urea (Lunt, 1968); and (3) the biodegradation of both the wax sealant and the sulfur coating releases urea from SCU (Jarrell and Boersma, 1979,1980). Temperature has the most pronounced effect on the dissolution of SCU (Hashimoto and Mullins, 1979; Jarrell and Boersma, 1979, 1980). Hashimoto and Mullins ( 1979) observed a two- to threefold increase in the dissolution rate of various SCU preparations when the temperature was increased from 8 to 35°C. The dissolution of SCU has been shown to be more rapid when incorporated in soil, compared to either in solution (Lunt, 1969) or on the soil surface (Prasad, 1976b). The increased dissolution rate of SCU in soil is consistent with the postulated role of microorganisms in breaking down the external coat of SCU. Giordano and Mortvedt (1970) found that SCU released urea very slowly in flooded soil. They attributed the slow release of urea to the formation of a FeS coating around the granules. However, this phenomenon has not been observed by other investigators. The accepted method of determining the release characteristics of commer- cial SCU products is to measure the amount of urea released from SCU granules into water at 37.8"C after 7 days immersion (Hashimoto and Mul- lins, 1979).

SCU has a number of advantages over soluble fertilizers. (1) By releasing urea slowly into the soil solution, leaching and volatilization losses are re- duced. (2) In contrast to ammonium salt fertilizers, SCU does not result in high rates of N assimilation by the plants following fertilizer application and a subsequent N deficiency in the soil for the later stages of plant growth; rather it results in the gradual release of N throughout the plant growth cycle (Lunt, 1971). The disadvantages of SCU are (1) higher cost than soluble

226 W. D. GOULD ET AL.

nitrogen fertilizers, and (2) soil acidification due to the formation of H,SO4 resulting from the metabolism of sulfur by the sulfur-oxidizing organisms.

SCU has been shown to be an effective nitrogen source when applied to turfgrass and forage grass (Dunavin, 1975; Hummel and Waddington, 1984; Mays and Terman, 1969; Volk and Horn, 1975). As fertilizers are generally surface applied in such systems the potential for volatile nitrogen losses are much greater than the losses observed when fertilizers can be incorporated into the soil (Terman, 1979). In laboratory studies, lower ammonia losses from SCU placed on the soil surface (0 to 7% of applied N) have been measured compared to ammonium sulfate (6.6 to 16.7%) and urea (8.4 to 20%) (Prasad, 1976a). SCU was superior to a single ammonium nitrate application, and was equivalent to three split ammonium nitrate applica- tions for forage grasses (Dunavin, 1975; Mays and Terman, 1969). Single applications of SCU have been used in place of three split applications of other nitrogen fertilizers for corn (Dalal, 1974, 1975b; Lunt, 1969), sugar- cane (Dalal and Prasad, 1975), and citrus trees (Puchades et al., 1984). Sanchez et al. (1973) found SCU to be superior to urea and ammonium nitrate for fertilization of rice. They assumed the major nitrogen losses with urea and NH4N03 resulted from leaching and denitrification. Flinn et al. ( 1984) compared the responses of irrigated rice in the tropics at 17 sites to prilled urea, urea supergranules, and SCU. They found urea supergranules and SCU to be economically viable alternatives to prilled urea.

Addiscott and Cox (1976) found SCU and urea to be inferior to ammo- nium sulfate when they were applied during the fall, and Sander and Moline (1980) found SCU to be inferior to urea for imgated corn on a sandy soil. Other workers have found SCU to be equivalent to other nitrogen sources for forage production (Lamond et al., 1979 Ludwick et al., 1978). In a green- house experiment Oertli (1975) determined nitrogen uptake for sunflowers and tomatoes in sand under conditions of intense leaching, and found that 50% of the nitrogen in SCU was assimilated by the plants. Under comparable conditions only 1-4% of the nitrogen in prilled urea was taken up by the plants.

2. Urea -Aldehyde Polymers

Another slow-release urea fertilizer is produced by polymerization or con- densation of urea with an aldehyde. Crotonylidenediurea and isobutyliden- ediurea are two slow-release nitrogen sources produced by the condensation of two urea molecules and two aldehyde molecules (Fig. 5) . The most com- mon aldehyde used to produce this class of slow-release fertilizer is formal- dehyde, which is polymerized with urea to form ureaformaldehyde. Com-

UREA TRANSFORMATIONS AND FERTILIZER EFFICIENCY 227

( H2N-C-NH-CH2-NH-C-NH2), !? f?

UREAFORMALDEHYDE POLYMER

CROTONYLIDENEDIUREA

0

0

ISOBUTY L I DENEDIUREA

FIG. 5. Urea-aldehyde condensation products that have been used as slow-release nitrogen fertilizers.

mercial ureaformaldehydes consist of a mixture of polymers and can be classified into three fractions based on their solubility in water (Hays and Hayden, 1966; Kaempffe and Lunt, 1967). Fraction I is cold-water-soluble nitrogen which is a mixture of urea, methylenediurea, and dimethylene- triurea. Fraction I1 is hot-water-soluble nitrogen (excluding the cold-water- soluble nitrogen) and is a mixture of trimethylenetetraurea and tetramethy- lenepentaurea. Fraction 111 is composed of the water-insoluble nitrogen and is a mixture of pentamethylenehexaurea and other long-chain polymers. An activity index has been proposed as a measure of the amount of slow-release nitrogen in ureaformaldehyde products (Hays et al., 1965; Kaempffe and Lunt, 1967. The activity index (AI) is defined by the following relationship:

Cold-water-insoluble N - Hot-water-insoluble N Cold-water-insoluble N

Plant assimilation of nitrogen has correlated well with the activity indexes of various ureaformaldehyde preparations (Hagin and Cohen, 1976).

The decomposition of ureaformaldehyde in soil is a first-order reaction, and the rate increases with an increase of temperature up to 24°C (Hadas and Kafiafi, 1974). The low temperature limit for the decomposition of ureaformaldehyde in soil is 15 "C (Basaraba, 1964). Katz and Fassbender ( 1966) measured the biodegradation of ureaformaldehyde by a mixed cul-

x 100 A1 =

228 W. D. GOULD ET AL.

ture of microorganisms in a Warburg respirometer and found an inverse relationship between polymer length and the degradation rate. Hadas et a/. (1975) found the degradation rate of ureaformaldehyde in the field to be 10-fold higher than the degradation rate measured in the laboratory.

The major advantages of ureaformaldehyde are (1) uniform release of nitrogen during the growing season, and (2) residual soil nitrogen remaining for the following year (Balba and Sheta, 1973; Waddington et al., 1976). Combinations of ureaformaldehyde and occluded herbicide have been for- mulated to produce a slow-release herbicide and nitrogen source (Hewson and Hays, 1972). Beaton et al. (1967) compared a number of slow-release nitrogen sources for orchard grass and found plastic (dicyclopentadiene copolymer)-coated urea to be the most effective and ureaformaldehyde to be the least effective. Ammonium and nitrate sources of fertilizer nitrogen have been shown to be superior to ureaformaldehyde for small-grain production in semiarid regions (Alessi and Power, 1973).

Isobutylidenediurea (IBDU) is an effective slow-release fertilizer for turf- grass (Volk and Horn, 1975) and has reduced nitrogen losses under alternat- ing wet and dry soil conditions (Prasad and Rajale, 1972). Mazur and White (1983) found IBDU to be superior to sulfur-coated urea in field studies. Mechanical breakage of SCU was the major factor adversely affecting its slow-release characteristics (Mazur and White, 1983). Initially, the release of nitrogen from IBDU is very slow, followed by a period of very rapid release of nitrogen (Volk and Horn, 1975). Crotonylidenediurea (CDU) has been successfully used for the fertilization of peach trees (Uchiyama, 1973). How- ever, ryegrass grown in pots took up more nitrogen in a 16-week period from ammonium sulfate than from CDU during a 50-week period (Gasser, 1970). The initial decomposition products of CDU are urea and 2-0x04-methyl-6- hydroxyhexahydropyrimidine (Ishibashi et al., 1969). The decomposition of CDU occurs both by chemical and microbiological means.

3. Mixtures of Urea with Other Chemicals

In order to reduce the volatilization of ammonia, urea has been mixed with other chemicals such as ammonium polyphosphate (Murdock and Frye, 1985) and phosphoric (Bremner and Douglas, 197 1 b), boric (Nom- mik, 1973a), and nitric (Gasser and Penny, 1967) acids. Nommik (1973a) reported that the incorporation of 5% H3B03 in large pellets ofurea reduced ammonia losses using surface-applied urea due to the inhibition of urease by the borate anion (Mulvaney and Bremner, 198 l), as well as by the reduction of soil pH. Gasser and Penny (1967) found urea phosphate to be superior to urea nitrate or ammonium nitrate as nitrogen sources for grass and barley.

UREA TRANSFORMATIONS AND FERTILIZER EFFICIENCY 229

However, Stumpe et al. (1 984) found urea phosphate to be ineffective in reducing ammonia losses on calcareous soils due to the precipitation of the phosphate anions by indigenous soil calcium, thereby increasing the soil pH and resulting in increased loss of NH4 by volatilization.

Urea- thiourea mixtures (Urea : thiourea 2 : 1 w/w) placed in either bands or pellets have been used to retard the transformations of urea in the soil (Malhi and Nyborg, 1979, 1984). Thiourea is both a weak urease and nitrifi- cation inhibitor; alone it has been utilized as a slowly available source of both sulfur and nitrogen.

Another approach has been to coapply calcium or magnesium salts with urea in order to decrease ammonia losses (Fenn and Kissel, 1973). The precipitation of calcium carbonate alters the equilibrium between ammo- nium carbonate, and ammonia and carbon dioxide thus retard ammonia losses (Fig. 6). Fenn et al. ( 198 la,b, 1982b) found a range of 0.25 to 1 .O for the ratio Ca2+ eq to N to be satisfactory for the reduction of ammonia losses from surface-applied urea. On calcareous soils potassium or sodium salts have yielded comparable results (Fenn et al., 1982a; Rappaport and Axley, 1984). The potassium or sodium ions apparently displace calcium ions from the cation exchange complex, thereby producing a net effect similar to the addition of calcium salts.

C. FERTILIZER PLACEMENT

I . Banding

The placement of urea in a band results in a region ofhigh pH and high salt concentration within the soil which inhibits nitrification (Wetselaar et al., 1972). Banded urea is in contact with less soil than broadcast or incorporated urea, and thus N immobilization is reduced (Tomar and Soper, 1981). Banded urea was found to be superior to incorporated urea for barley when the fertilizers were applied during the fall (Mahli and Nyborg, 1985). Fall application of banded urea resulted in lower losses of nitrogen due to deni-

FIG. 6. Effect of calcium salts on ammonium evolution. (1) Hydrolysis of ammonium carbonate to produce volatile ammonia. (2) Reaction of calcium chloride with ammonium carbonate to suppress ammonia volatilization.

230 W. D. GOULD ET AL.

trification; this can probably be attributed to a reduced rate of nitrification because of the inhibitory effect of the urea. In contrast, no difference in N loss between incorporated and banded urea was observed when the fertilizers were applied in the spring (Mahli and Nyborg, 1985). Banded urea was equal to or better than urea applied by other placement techniques for rice (Cao et al., 1984b; Rao et al., 1985). Urea banded twice (2 and 16 days postemer- gence) was superior to incorporated, single-banded, and surface-applied urea for dryland sorghum (Moraghan et al., 1984).

One of the major disadvantages of banding is the inhibition of root growth in the region surrounding the band (Creamer and Fox, 1980). Both side banding and seed application of urea - ammonium phosphate fertilizer caused delayed germination and a reduced stand of corn seedlings (Liegel and Walsh, 1975a). The same effect was not observed with fertilizers not containing urea (Liegel and Walsh, 1975a). Liegel and Walsh ( 1975b) mea- sured high ammonium ion concentrations and high pH values in the soil surrounding the banded urea - ammonium phosphate and suggested that the presence of free ammonia was responsible for the observed plant toxicity.

2. Large Pellets

Incorporation of urea in large pellets or granules is an alternative means of retarding nitrogen transformations, by concentrating urea in the soil (Nom- mik, 1973b). Pellets have an additional advantage in that they can easily be formulated as mixtures of urea with other chemicals (Nommik, 1973a). Large urea pellets (2.0 to 2.2 g) had lower initial volatile nitrogen losses than smaller pellets or prilled urea when these fertilizers were applied to the surface of a forest soil, but cumulative ammonia losses after 20 to 30 days were equivalent for all particle sizes (Nommik, 1976). Nyborg and Malhi ( 1979) found fall-applied pelleted urea to be equivalent to spring-applied urea, and to be superior to fall-applied banded urea for barley. However, Watkins et al. (1972) found that pelleted urea was equivalent to prilled urea.

Deep placement of urea supergranules has shown promise for the fertiliza- tion of flooded rice (Craswell et al., 1981, 1985; Prasad et al., 1984). Deep placement of large urea pellets resulted in higher recoveries (50 - 6 1 %) of the applied nitrogen by rice than when urea was applied in split treatments (25-34% recovery) (Savant et al., 1982). The advantages ofthe deep place- ment of urea pellets with rice are (1) reduced diffusion of urea into the floodwater, thereby reducing the potential for ammonia loss and denitrifica- tion (Cao et al., 1984a; Eriksen et al., 1985); and (2) proliferation of roots through the zone of high ammonium ion concentrations at a later stage of growth when nitrogen requirements are higher (Savant and De Datta, 1980).

UREA TRANSFORMATIONS AND FERTILIZER EFFICIENCY 23 1

However, in permeable soils with high leaching rates, the deep placement of large urea granules resulted in higher leaching losses of nitrogen than was observed with other methods of application (Vlek et al., 1980a).

V. SUMMARY

A. FUTURE TRENDS IN THE USE OF UREA FERTILIZERS

Although the use of nitrogen fertilizers has steadily increased during the past 20 years, the greatest percentage increase has been in the use of urea. Urea utilization is expected to increase, particularly in the developing coun- tries. Urea currently constitutes 80% of the nitrogenous fertilizer that is used on rice (Vlek and Craswell, 198 1). High nitrogen losses have restricted the use of urea on some crops but this may change as new technologies develop. Slow-release urea preparations may find widespread acceptance for the fer- tilization of forages, grasslands, and forests.

B. FUTURE DEVELOPMENTS IN IMPROVING THE EFFICIENCY OF UREA FERTILIZER

Under most conditions urea is equivalent to other nitrogenous fertilizers, but under certain conditions conventional practices must be modified for urea to be used successfully. Coating urea with sulfur or adding urease inhibitors to retard the hydrolysis rate of urea appear to be the most effective means of preventing ammonia losses. On soils where extensive leaching occurs, sulfur-coated urea might be more effective than urea containing a urease inhibitor, due to the high mobility of urea. Fall application of nitro- genous fertilizers can result in immobilization, leaching, and denitrification. Placement (i.e., banding or large granules of urea) may be appropriate for fall fertilization and also for flooded rice crops. The recently discovered class of urease inhibitors, the phosphoroamides, are the first group of inhibitors that are sufficiently active at low concentrations to be agronomically feasible for field application. One can anticipate that more effective compounds in this class of inhibitors will be developed in the future.

Further research with respect to urea fertilizers is required in the following aspects: (1) the measurement of urea transformations in the field under agronomically realistic conditions, (2) improved understanding of urea transformations in no-till or minimum till soils, (3) the effect of placement and time of application on mobility and uptake of nitrogen from urea, and (4) the development of improved urease inhibitors.

232 W. D. GOULD ET AL.

ACKNOWLEDGMENTS

The authors express gratitude to Mr. G. H. Bishop of the Canadian Fertilizer Institute and Mr. G. M. Gould of Shemtt Gordon Mines for data concerning world consumption of nitro- genous fertilizers. A critical review of the manuscript by Dr. L. L. Hendrickson is also appre- ciated.

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