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IV. NH, Losses from Urea

IV. NH, Losses from Urea

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VOLATILIZATION LOSSES OF NITROGEN AS AMMONIA



207



Sufficient water must also be present for the urea to dissolve so that hydrolysis

can occur. Volk (1966) reported data indicating slow dissolution of urea surface

applied to Florida sandy soils. Over 80% of prilled urea field-applied to the

surface of air-dry Leon fine sand did not hydrolyze in 14 days, and 42-72%

persisted on initially moist sand, even though dews formed nightly. On continually moist sand, urea hydrolyzed completely in 7 days and high volatilization loss

resulted from the dissolved urea. These differences in dissolution rates explain

some erratic responses to surface-applied urea.

As previously indicated, loss of N as NH, from surface-applied urea occurs

from both acid and alkaline soils. Losses of dissolved urea depend not only on

the usual factors of drying conditions, soil water content, soil CEC, and temperature, but also on urease activity.



A. UREASE ACTIVITY IN SOILS AND EFFECTS OF

INHIBITORS



Urease activity in 100 Australian surface soils was found by McGarity and

Myers (1967) to be highly correlated with organic carbon content and poorly

correlated with soil pH. Overrein and Moe (1967) found that urea hydrolysis at

28°C was directly proportional to rate of urea applied to Chalmers silt loam and

Plainfield sand (Indiana). Ammonia volatilization increased exponentially with

application rate. Aeration rate had no significant effect on urea hydrolysis, but

NH3 volatilized increased with an increase in gas exchange rate over the soil.

Tabatabai and Bremner (1972) developed a simple soil urease assay technique

involving determination of NH4-N released when soil is incubated with tris(hydroxymethy1)aminomethane (THAM) buffer, urea solution, and toluene at 37°C

for 2 hours.

Dalal (1975) found that urease activity was highly correlated with organic

carbon or CEC in 15 Trinidad soils. Bremner and Zantua (1975) detected urease,

phosphatase, and sulfatase activities in soils at -10°C and -20°C. This was

attributed to interaction in unfrozen water at soil particle surfaces.

Addition of urea to Iowa soils did not induce urease activity (Zantua and

Bremner, 1976, 1977). However, addition of glucose and other organic materials

that stimulated biological activity also increased urease activity. Persistence of

urease activity varied among soils, but activity in all soils eventually returned to

the level of an unamended soil. Apparently, soil constituents protect urease

against microbial degradation and other processes leading to enzyme inactivation, so that each soil has a stable level of urease activity, depending on the soil

constituents.

Zantua et al. (1977) found high correlations of urease activity with organic

carbon, clay content, and CEC in 21 Iowa soils. Organic matter accounted for

most of the variation in urease activity. Tabatabai (1977) found that urease



208



G. L. TERMAN



activity decreased with soil depth, in proportion to a decrease in organic carbon

content. Thus, a given soil apparently has a stable urease level and temporary

levels, depending primarily on soil organic matter content.

Numerous compounds have been found effective in inactivating urease in

solution, but relatively few are effective in soil. Waid and Pugh (1967) showed

that acetohydroxamic acid inhibited hydrolysis of urea in an acid loamy sand and

delayed loss of NH,. Pugh and Waid (1969a,b) found that acetohydroxamic acid

was more effective in retarding NH, loss than was benzo-, capro-, or

salicylohydroxamic acid.

Bremner and Douglas (1971b) evaluated more than 100 compounds as urease

inhibitors in soils. Dihydric phenols and quinones were the most effective organic compounds, and silver and mercury salts the most effective inorganics. In

later studies (1973) they found that of eight urease inhibitors 2,Sdimethyl-pbenzoquinone was most effective in reducing urea hydrolysis and NH, loss from

soil. This compound reduced NH, loss from urea applied to a sandy soil from 63

to 0.3%.

Methyl-, chloro-, bromo-, and fluoro-substituted p-benzoquinones markedly

inhibited urease activity in soils, whereas phenyl-, t-butyl-, and hydroxysubstituted p-benzoquinones had little, if any, effect (Bundy and Bremner,

1973). Various nitrification inhibitors retarded nitrification of NH4-N but had

little effect on urease activity (Bundy and Bremner, 1974).

Bremner and Bundy (1976) found that potassium azide (KN,) retarded urea

hydrolysis in soils, but did not retard nitrification of NH4-N. Tabatabai (1977)

reported that numerous metals inhibited urease activity in eight Iowa soils in the

order Ag 2 Hg > Cu > Cd > Zn > Sn > Mn. Additional trace elements also

inhibited urease.

Inhibition of soil urease by several heterocyclic mercaptans was investigated

by Gould et al. (1978). Several quinones, catechol, and 1,3,4-thiodiazole-2,5dithiol were most effective.



B . ADDITIVES TO UREA IN RELATION TO NH3 LOSSES



Chemical amendments are applied not only to inhibit urease activity, but also

to have other direct effects on potential NH, losses from urea. These include

( a ) reduction in pH, (b) coating of urea granule surfaces, and (c) reduction in

NH3 loss through precipitation of Ca and Mg carbonates.

Urea forms addition compounds, or adducts with numerous acids and salts.

For example, urea and Ca(N03), form an adduct, as follows:

4CO(NH2),



+ Ca(NO&



4CO(NH2)z.Ca(N03)z



This product, known as Calurea (34% N), was manufactured in Germany and

exported in the 1930s. It was a usable product, but often was in poor physical



VOLATILIZATION LOSSES OF NITROGEN AS AMMONIA



209



condition. Other adducts having industrial and other uses (Terman and Fleming,

1968) include urea nitrate [CO(NH,), -HNO,] and urea phosphate [UP,

CO(NH&. H,P04]. Both form strongly acid solutions.

Gasser and Penny (1967) found that urea nitrate applied to barley and

grassland produced lower yields than did AN, likely as a result of toxicity of the

resulting extremely acid solution. Urea phospate and UP plus urea were superior

to AN as sources of N; however, P was not equalized in the experiments, and

urea alone was not included.

Bremner and Douglas (1971a) found that H3P04formed from decomposition

of UP in soils retarded enzymatic hydrolysis of the urea by urease. This resulted

in reduction of NH, loss from urea. Loss of N as NH, from urea applied to Iowa

soils was 5-61%, but only 0.1-1% from UP. Paulson and Kurtz (1969) found

that urea N in a pelleted urea-aliphatic acid complex plus p-benzoquinone inhibitor released NH,-N slightly more slowly than did urea alone.

Watkins et al. (1972) measured lower NH, losses from mixtures of urea and

NH4Cl than from crystalline or pelleted urea alone (50%less, with 25% of the N

as NH,Cl). Terman and Hunt (1964) were apparently the first to report higher

crop recoveries from surface-applied SCU than from uncoated urea. Recoveries

in pot experiments with corn were also higher (less NH3 loss) from granules

containing urea phosphate (UAP) than from urea alone. The lower NH3 losses

with SCU probably occur as a result of slower dissolution and hydrolysis of urea

in SCU and from some oxidation of S , which lowers the pH at the granule sites.

In the case of UAP, the presence of MAP or NH,-polyphosphates tends to reduce

the fertilizer solution pH and thus restricts NH, loss. The presence of DAP in the

granules would not be effective in reducing the solution pH, since DAP solution

is alkaline (pH 8.0). However, Phipps (1964) found that NH, losses from

fertilizers applied to Nebraska soils were in the order urea > UAP > DAP, and

they decreased with a decrease in soil pH from 8.2 to 5.5. Losses from AN and

APP were negligible.

Matocha (1976) reported that < 1% of AN, AS, and SCU (20%and 30% initial

dissolution in water) surface-applied to unlimed Darco fine sand (pH 6.0) was

lost in 14 days as NH,, whereas 18.5% of uncoated urea N was lost. Topdressing

lime with N increased N loss more initially from AS than from urea, but cumulative losses in 14 days were 52%, 23 %, 9%, and 2% from urea, AS, SCU-30, and

SCU-20, respectively.

Urea-AN (UAN) solutions and suspensions are used widely to supply N for a

wide range of crops. The question frequently arises as to whether AN with urea

reduces NH, losses from topdressed UAN and whether such solutions are more

effective than solid urea, especially under no-till .

Kresge and Satchel1 (1960) showed that higher AN/urea ratios of fertilizer

solutions (158 ppm of total N) surface-applied to two Pennsylvania soils (pH 6.3)

decreased NH, volatilization. However, it is not clear (Table IV) whether the

reduced NH, losses were due to less urea N applied or to the effects of increasing



+



210



G. L. TERMAN



TABLE IV

Volatilizationof NH, in 11 Days, as Affected by ANRTrea Ratio of Solutions Surface-Appliedto

Two Soils"

NH,-N volatilized

Momson sandy loam

AN/urea

ratio



N applied

as AN (ppm)



01100

36/65

45/55

56/44



0

45



70130



101



Hagerstown silt loam



N applied

as urea (ppm)



% of

urea N



% of

total N



% of

urea N



% of

total N



158



17

42

37



77

30

23



19



32

28



19

23



36



18



34



12



I13

98



60

77



81

51



25

23



17

13

8



Calculated from Kresge and Satchel1 (1960).



the proportions of AN, or to both. It was shown in other studies that NH, losses

from AN or AS alone were negligible.

Results obtained in Mississippi (Table V) also were not definitive in regard to

possible effects of AN on NH, losses from urea. Negligible NH, losses occurred

from AN surface-applied to an acid (pH 5.9) sandy loam. Losses were low

(5-13%) from urea or AN

urea. Percentage losses of NH, decreased with an

increase in the amount of AN if expressed on the basis of total N applied, but not

if expressed on the basis of urea N applied. Losses of NH3-N were 6-9% from

AN surface-applied to an alkaline (pH 7.8) sandy loam, and much higher from

urea or urea AN. Percentage losses again decreased with increasing amounts

of AN if expressed on the basis of total N applied, but increased greatly with

amounts of AN if expressed on the basis of urea N applied. Thus, susceptibility

of AN to NH, losses on the two soils and method of expressing losses greatly

affected interpretation of the results. -This experiment was also not adequate to

determine effects of AN on NH, losses from urea. Adequate answers to comparative effectiveness of urea + AN solutions versus urea alone await more definitive

results.

Terman et af. (1968) found the usual increases in NH3-N losses with an

increase in urea content of fluid fertilizers. Negligible losses occurred from AN

and from solutions of 75% AN, 25% urea. Higher losses occurred with drier than

with moist aeration.



+



+



V. NH, Losses from Anhydrous NH, and NH40H



Intensive studies on equipment for applying anhydrous NH, and on crop

response were carried out in Mississippi in 1945-1947 (Andrews et al., 1948).



21 1



VOLATILIZATION LOSSES OF NITROGEN AS AMMONIA



Use has grown rapidly since then. Jackson and Chang (1947) found little loss of

NH, from anhydrous NH, injected 2.5-5.0 cm into soil varying widely in physical properties.

Laboratory studies of aqua ammonia surface-applied to Hawaiian soils by

Humbert and Ayres (1957) showed losses of up to 15% from surface application

to acid soils and 50% or more from alkaline soils, even with injection 10 cm into

the soils. With injection into irrigation water, about 20% volatilization loss was

found from water flowing about 60 m in irrigation furrows.

McDowell and Smith (1958) found negligible loss of NH, from anhydrous

NH, injected only 7 cm deep in a calcareous clay soil. Losses were much higher

from injections 15 cm deep in an acid sandy soil.

Robertson and Hansen (1959) reported that NH, losses from low-pressure N

solutions dribbled on the surface of Kalamazoo sandy loam in Michigan increased with rate of application, were greatest immediately after application, and

decreased with time. No appreciable loss occurred with injection to a depth of 5

cm. Baker er al. (1959) also reported negligible losses from anhydrous NH, with

injection 10 cm or deeper at practical rates of application. In contrast, Henderson

et al. (1955) reported NH, losses as high as 60% when anhydrous NH, was

applied through jet sprinklers at NH, concentrations greater than 100 ppm.

Chao and Kroontje (1964) observed a linear relationship between rate of

application of NH,OH to soils and rate of NH, loss during drying. Rates of NH,



TABLE V

Volatilization of NH, in 18 Days from AN and Urea Surface-Applied to Acid and Alkaline Soils"

N as AN

applied

with urea

(mg)



Urea applied and NH,-N loss



Urea applied and NH,-N loss

100mg



200mg



300mg



None



100mg



100

6

200

6

300

7

Sandy loam, pH 7.8

0

18

50

28

100

33

200

42

300

47



8

8

9

9

9



12

13

13

12

II



18



16

16

19

22

25



20

21

24

31



300mg



8 of total N



96 of urea N



Sandy loam, pH 5.9

0

5

50

5



200mg



5

0.3

0.6

0.5



9



7

8

6



4

3

2

2



8

6

6

4

4



12

11



18

16

16

14

12



18

16

14

12

12



16

14

14



10

7

6



13

13



Calculated from unpublished Mississippi Agricultural Experiment Station data reported to the

TVA in 1966.



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