Reprinted from the Soil Science Society of America Journal
Volume 52, no. I, January-February 1988
677 South Segoe Rd., Madison, WI 53711 USA
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Urea and Biuret Stimulate Growth of Douglas-fir and Western Hemlock Seedlings
RICHARD
E.
MILLER,* HARRY W. ANDERSON, AND DONALD C. YOUNG
•
Urea and Biuret Stimulate Growth of Douglas-fir and Western Hemlock Seedlings
RICHARD
E. MILLER,*
HARRY
W.
ANDERSON, AND DONALD
YOUNG
found no published information about susceptibility
of conifers to biuret damage. Symptoms of toxic ef­
fects of biuret closely resemble those of high concen­
trations of free NH3 or N02, which can also follow
urea fertilization to the soil (Kilmer and Englestad,
1973). Biuret in urea is generally less damaging to
plants when applied to the soil than when applied as
a foliar spray. For example, biuret concentrations of
2 to 10% in urea fertilizer have been safely applied to
the soil of established agricultural crops, whereas biu­
ret concentration of 0.25 to 3% are tolerable for foliar
applications (Kilmer and Englestad, 1973).
Our objective was to determine if biuret alone or
with urea reduced seedling growth of coast Douglas­
fir or western hemlock. Field trials in established stands
indicated that hemlock does not respond to ground
application of prilled or granular urea as much or as
consistently as Douglas-fir does (Peterson and Gessel,
1983). This difference could reflect susceptibility of
hemlock to toxicity from biuret in urea fertilizer.
ABSTRACT
Two experiments were conducted in a greenhouse to test a theory
that biuret in urea fertilizer reduces growth of Douglas-fir [Pseu­
dotsuga menziesii (Mirb.) Franco] or western hemlock [Tsuga het­
erophy/la (Raf.) Sarg.]. Newly germinated seedlings in 2.5-L pots
were treated with 0, 0.224, 2.24, or 22.4 kg biuret ha I (40% N) in
factorial combinations with 0, 84, or 168 kg N ha 1 as reagent-grade
urea (46% N). The biuret was applied as a solution to either the
sandy loam soil or the foliage. Four of the eight replicates were
harvested 203 d after treatment. For Exp. 2, the remaining seedlings
were retreated as they were initially and harvested 305 d later. In
both experiments, the two species responded strongly to urea; more­
over, seedling dry weight increased with increasing biuret (P 0.003).
A veraged over all other factors, weight of seedlings treated with the
highest biuret dosage was 14% more than the weight of seedlings
with no biuret. Unfertilized hemlock responded as much to 0.2 kg
biuret ha 1 (0.08 kg N ha I ) as to 84 kg N ha 1 as urea; this sug­
gests that biuret functions as a hormone. The results indicated that
biuret is unlikely to reduce growth of seedlings of either species even
when urea heavily contaminated with biuret is applied at high dos­
age.
=
Additional Index Words: N fertilizer, Pseudotsuga menziesii, Tsuga
MATERIALS AND METHODS
heterophylla, phytotoxicity, seedling growth.
We used newly germinated and l -yr-old seedlings of both
species growing in pots in a greenhouse to quantify phyto­
toxicity of various dosages of biuret that were applied to
either foliage or soil and in factorial combinations with three
dosages of N.
Approximately 2.5 kg of Tumwater fine sandy loam
(mixed, mesic Dystric Xeropsamments; pH, 5.6; organic
matter, 24 g kg I; total N, 0.7 g kg I ) were placed in each
of 384 2.5-L pots.' We chose this soil because a large state­
owned tree nursery is located on this soil and it is N deficient
(Radwan and DeBell, 1980; Radwan et aI., 1979). Following
random assignment to either Douglas-fir or western hem­
lock, each pot was sown on 17 May with 10 previously strat­
ified seeds of one species. Approximately 16 h of light per
day was provided; natural light was supplemented by sea­
sonally using fluorescent lamps. Greenhouse temperatures
were maintained between 10 and 32 °C and soil moisture
was maintained near field capacity. Eighty-three days after
sowing, seedlings were thinned to five per pot to achieve
uniform spacing and height, and 24 different treatments were
randomly assigned to each species.
The 24 fertilizer treatments for each species were arranged
as a 4 X 2 X 3 factorial that tested four levels of biuret (0,
0.224, 2.24, and 22.4 kg ha I ) , two methods of biuret ap­
plication (solution poured on the soil or sprayed on the fo­
liage), and three levels of N (0, 84, or 168 kg ha 1 as urea).
Fertilizer dosages per hectare were proportional to the area
of the potted soil surface. The 48 treatment X species com­
binations were initially replicated eight times. All solutions
were prepared from reagent-grade urea or biuret and were
first applied when the 60-d-old Douglas-firs were 2- to 6-cm
tall and the hemlocks were 1- to 2-cm tall. Biuret applied as
a 10 g kg 1 solution; dosage was increased by using more
solution either poured or pumped (with a manual sprayer)
from a graduated cylinder. In the initial application, some
solution was inadvertently sprayed directly on the soil be­
cause the spray missed or dripped from the sparse foliage;
however, little, if any, spray was lost from the plant-soil area.
REA FERTILIZER contains small but variable
U amounts of biuret, the result of high tempera­
tures used during manufacture of the fertilizer. Biuret
and NH3 form from urea when urea is maintained in
the molten state: 2(NH2• CO·NH2}....... (NH2 CO· NH .
CO·NH2) + NH3. Although urea fertilizer produced
in modern facilities generally contains 0.3 to 2.0% biu­
ret, concentrations exceeding 5% have been measured
in some batches (Kilmer and Englestad, 1973). Thus,
a typical forest application of 224 kg N ha-I as urea
(46% N) generally would include 1.5 to 9.7 kg biuret
ha-', and occasionally as much as 24.4 kg ha-I if urea
containing 5% biuret was used.
Plants can absorb biuret through their roots and
leaves, but metabolize it slowly, if at all (Kilmer and
Englestad, 1973). Following mineralization by soil mi­
crobes, biuret is a readily available N source for plants.
Biuret does not retard the hydrolysis of associated urea
but does inhibit conversions of NH4 to N02 and of
N02 to N03 (Sahrawat, 1977). Although biuret is re­
portedly phytotoxic, plants vary in their susceptibility
to biuret injury (Commonwealth Agricultural Bur­
eaux, 1977). For example, critical levels of biuret ap­
plied to the soil were 3 to 4 kg ha -I and over 15 kg
ha- I for potato (Solanum tuberosum L.) and maize
(Zea mays L.), respectively (Gadet et aI., 1959). We
•
R.E. Miller, USDA-FS, Forest Science Lab.. Olympia, WA; H.W.
Anderson, Washington State Dep. of Natural Resources, Olympia,
WA; and D.C. Young, Unocal Science and Technology Div., Brea,
CA. Contribution from the USDA-FS, Pacific Northwest Res. Stn.,
Olympia, WA 98502. Received 23 June 1986. *Corresponding au­
thor.
Published in Soil Sci. Soc. Am. J. 52:256-260 (1988).
C.
256
257
MILLER ET AL.: DOUGLAS-FIR AND WESTERN HEMLOCK SEEDLINGS After the first growing season (203 d after treatment), four
of the eight replicates were randomly selected for harvest
and were designated as Exp. 1. The remaining replicates were
designated as Exp. 2; these were immediately rethinned to
three seedlings per pot and refertilized to test the effects of'
large cumulative dosages of biuret. Refertilization of each
pot equalled the original fertilization, thus the experimental
design for both experiments was the same. Seedlings of Exp.
2 were tended the same as those in Exp. 1 and harvested
305 d after retreatment.
Treatments were evaluated on the basis of dry wt. of roots,
tops, their ratio, and plant total. Weight of tops and roots
was determined follo)Ving washing and drying at 105°C to
constant weight. Average seedling weight per pot was com­
puted on the basis of the number of surviving seedlings; i.e.,
five (Exp. 1) or three seedlings (Exp. 2) in almost all pots.
The data were subjected to analysis of variance using or­
thogonal polynomials to compare regressions of average
seedling weight per pot against N or biuret dosages. The
main effects of the four factors (species, N dosage, biuret
dosage, and application method) and their interactions were
analyzed as a completely random design. Means were judged
statistically significant when P :;:: 0.05.
The statistical analyses estimated and compared the re­
sponse surfaces for N (three levels) and for biuret (four lev­
els). In some analyses, statistically significant interactions
involving N or B (biuret) had a difef rent curve form than
that of the main effects of N or B. We questioned if the
surface identified by the main effect should limit the curve
form of the interaction. For example, when the main effect
of biuret was linear, should a statistically significant inter­
action involving a quadratic B effect be considered as a true
effect? Although the general answer is yes, limitations in
experimental design may preclude a valid estimate of a
quadratic or higher-order form. Thus, we considered that
our three levels of N were inadequate for fitting a quadratic
surface because the quadratic polynomial will fit the three
points exactly and will leave no measure of lack of fit around
the regression (John Hazard, 1978, personal communica­
tion). Because of these considerations, we ignored quadratic
surfaces for N but accepted them for biuret.
RESULTS
To condense the results of factorial analyses of the
four growth parameters (total seedling weight, top
weight, root weight, and top/root ratio), we provide
2.5
:§
..
.c
C)
'iii
3:
iii
...0
...
C
IV
GI
::!
0
l
df
Total
Top/root
Total
Top/root
Species
1
2
(1)
3
(1)
1
1
***
***
***
***
***
***
***
***
NS
NS
NS
N
N, (linear)
Biuret
B, (linear)
Application
SxN
SxB,
SxA
NxA
SxNxB
SxNxA
=
84
0
N dosage (kg ha-1)
=
**
NS
NS
NS
***
NS
NS
NS
NS
NS
NS
NS
*
=
=
=
Total Weight
After one growing season, Douglas-fir seedlings were
1.6 times heavier than hemlock seedlings. Although
average total weight of both species increased with
increasing N dosages (Fig. 1); Douglas fir was more
responsive than hemlock to N fertilization (Table 1).
Seedling weight of both species was increased by the
lowest biuret dosage, 0.224 kg ha - I; little additional
increase in seedling weight accompanied 10- and 100
fold increases in biuret (Fig. 2). Having no statistically
significant interactions of the other factors with biuret
(Table 1), one may generalize over all levels of the
other factors: seedlings receiving biuret were heavier
than those without; at the highest biuret dosage (22.2
kg ha - I), seedlings averaged 14% heavier.
1.0
C
IV
GI
I
I
•
.
Douglas-fir
Hemlock
•
•
::Il 0.5
168
Fig. 1. Mean total oven-dry weight of Douglas-fir and hemlock seed­
lings, as affected by N dosage (Exp. 1). Each species-N dosage
combination pools four biuret dosagesxtwo application methods
'
32 pots. SE 0.045 g.
x four replicates
=
NS
NS
NS
NS
NS
NS
NS
Experiment 1: Fertilization of 60-d-old Seedlings
...o
...
Hemlock
*
tables, figures, and text featuring treatment effects on
total seedling weight and top/root ratio. Treatment
effects on top and root weight were similar to those
on total weight.
GI
-----
*
=
3:
iii
-
***
NS
NS
NS
**
* ,**,*** Significant at the 0.05, 0.01. and 0.001 probability levels,
respectively.
t S species, N nitrogen, B
biuret. and A
application (soil
or foliage). :j: NS
nonsignificant. .f!!
-
*
NS
1
1
1
3
1
(quadratic)
...
.c
-
E xp. 2
Factort
m 1.5
1.5
0.5
Seedling weight
Exp.l
2.0
2.0
1.0
Table 1. Experimental factors and their interactions that were
significant (P :s 0.05) in Exp. 1 and 2.
o
0
20
15
10
5
Biuret dosage (kg ha-1)
25
Fig. 2. Mean total oven-dry weight of Douglas-fir and hemlock seed­
lings, as affected by biuret dosage (Exp. 1). Each species-biuret
dosage combination pools three N dosagesx two biuret applica­
tion methods x four replicates 24 pots. SE 0.05 g.
=
=
258
SOIL SCI. SOc. AM.
2.0
Cl
1.5
Cl
.;
tV
...
0
...
C
nl
Q)
1., VOL. 52, 1988
0
1.0
(5
e
a.
0
I-
0.5
1.5
1.0
0.5
N dosage (kg ha-1)
Fig. 3. Mean total oven-dry seedling weight, as affected by N dosage
and method of applying biuret (Exp. 1). Each application-N dos­
age combination pools four biuret dosages x two species x four
replicates 32 pots. SE 0.045 g.
=
=
The N X A interaction (N dosage X application
method) was statistically significant. Thus, depending
on the N dosage, the two methods of applying biuret
significantly differed in their effects on seedling weight.
Seedlings that received 0 and 84 kg N ha I and that
also received biuret as a soil application were slightly
heavier than those treated with foliar spray. When
combined with 168 kg N ha I, however, soil appli­
cation of biuret slightly depressed the response of both
species to this high dosage of N (Fig. 3).
Top/Root Ratio
Relative changes in top and root weights during the
first growing season are expressed by the ratio of top/
root weight. Significant interactions indicated that the
top/root ratio of the two species differed in response
to N dosage and to method of biuret application (Ta­
ble 1). Although top weight of both species increased
more than root weight with increasing N dosage (Fig.
4), the top/root ratio of hemlock increased more than
that of Douglas-fir. Nitrogen fertilization strongly
stimulated top growth of hemlock, but had little effect
on root weight.
N applied, kg ha I
Species
168
0
84
root weight, g
Douglas-fir
0.88
0.45
0 . 76
Hemlock
0.28
0.22
0.27
Both species also differed in their response to the
method of applying biuret. As tabulated below, top
growth (relative to root) of Douglas-fir was slightly
stimulated only by foliar application of biuret, whereas
hemlock responded more to soil application.
Species
Douglas-fir
Hemlock
Biuret application
Foliage
None
Soil
-----top/root ratio----1.48
1.40
1.32
1.92
1.85
2.25
0
0
N
168
64
dosage (kg ha-1)
Fig. 4. Mean top/root weight ratio as affected by N dosage (Exp.
1). Each species-N dosage combination pools four biuret dosages
x two application methods x four replicates
32 pots. SE 0.084.
=
=
Experiment 2: Refertilization of l-yr-old Seedlings
Total Weight
Species and N were significant in both their main
effects and their interaction, but a significant three­
way interaction with A (application method) pre­
cluded generalizations (Table I). Thus, total weight of
both species increased linearly with cumulative N dos­
age; foliar application of biuret depressed response of
Douglas-fir to N, whereas soil application depressed
response of hemlock to N (Fig. 5).
From the combined efef cts of the first and second
treatments, average weight of both species increased
linearly with increasing biuret dosage (Fig. 6). As in
Exp. 1, seedlings treated with the highest biuret dosage
(44.8 kg ha I in Exp. 2) averaged 14% heavier than
seedlings that received no biuret . This generalization
about the stimulating effect of biuret held for both
species and to both methods of applying biuret be­
cause the SX Band BX A interactions (where S
species) were nonsignificant.
=
Top/Root Ratio
The decline in top/root ratios after the second grow­
ing season indicated that the three seedlings in each
pot collectively increased root weight more than top
weight during the second growing season.
Species
Douglas-fir
Hemlock
Top/root ratio
Year I
Year 2
1.39
2.02
1.06
1.21
The top/root ratio was greater for hemlock than for
Douglas-fir in both years. Moreover, the average (for
both species) trend of top/root ratio and N dosage
(1.06:1, 1.10:1, and 1.24:1 respectively for 0, 168, and
336 kg N ha I, respectively) was linear and statisti­
cally significant (Table 1). This significant main effect
of N, and nonsignificant interactions, indicated that
top growth of both species increased more than root
growth with increasing amounts of N.
I
259
MILLER ET AL.: DOUGLAS-FIR AND WESTERN HEMLOCK SEEDLINGS
6
Total
5
:§
4
.
3
....
.t::.
;
OL- L-----------------J
- ---------------O
3
1 68
N
dosage (kg ha-1)
2
--_T=_= _ ______________________.. .
. . ._ Roots
.
_____________
_
_
-___-.
. _
_ -__
. -._.. .-_
.
.
•• '--- .•
--
-- ----.... ----------
.
.
-
.
.
.
..
.
.
6
5
Fig. 5. Mean total oven-dry weight of Douglas-fir (DF) and hemlock
(H) seedlings as affected by cumulative N dosage and method of
applying biuret (Exp. 2). Each species-N dosage-application
method combination pools four biuret dosages x four replicates
16 pots. SE 0.29 g.
15
25
35
Cumulative biuret dosage (kg ha
45
-1)
Fig. 6.
Mean oven-dry seedling weight. as affected
5y two cumulative biuret dosages (Exp. 2).
Each
biuret dosage pools two species x three N dosages x
biuret
application
o
methods
of
x
four
re
licates
tw
4 pots.
SE
g ( otal weight); SE
.17 g
(tops); and SE
.10 g (roots).
=
=
DISCUSSION
=
0'89
=
s
=
Effect of Urea
In both experiments, addition of reagent-grade urea
solution to this fine sandy loam soil clearly stimulated
growth of both species. The N response surfaces were
linear for total seedling weight, top weight, and root
weight; however, N stimulated top growth more than
root growth. The initial applications of 84 and 168 kg
N ha I increased growth of 60-d-old Douglas-fir more
than hemlock. Tpe initially slower growth and devel­
opment of hemlock compared to Douglas-fir probably
explains its slower response during the first growing
season.
tors of both experiments, the highest biuret dosage
(22.4 and 44.8 kg ha I in Exp. 1 and 2, respectively)
increased growth about 14%. Unexpectedly, soil ap­
plication of biuret to unfertilized hemlock seedlings
in both experiments was especially stimulating.
Soil application of biuret depressed response of both
species to a concurrent fertilization with 168 kg N
ha I in Exp. 1 (Fig. 3) and response of hemlock to N
in Exp. 2 (Fig. 6). This reduced response to urea could
be explained by biuret either increasing salt concen­
tration in the soil or inhibiting conversion of NH4 to
NO, or NO, to N01• These inhibitions can lead to
NH; or NO toxicity (Court et aI., 1964; Sahrawat,
1977). For example, Sahrawat (1977) reported N02
accumulation in a sandy loam soil when the biuret
Effect of Biuret and Method of Application
Biuret generally stimulated growth of both species,
but especially of hemlock. Averaged over all other fac-
Table 2. Mean seedling weight and weight gains with specified N dosage from biuret (B) or urea (U) sources.
Douglas -fir
N dosage
Source
Applied to
kg ha-'
Mean seedling
weight
g
Hemlock
Weight gain/N
gt
Mean seedling
weight
Weight gain/N
Rel.:j:
g
gt
Rel.:j:
-44
78
17
-13
0.1
1.4
1.0
0.7
0.36
0.86
0.47
0.54
0.44
0.94
0.50
0.75
0.95
5.56
1.22
0.20
0.089
0.064
0.Q16
0.005
0.004
1110
244
40
18
13
3.2
1.0
0.8
-190
31
-26
-32
2.4
1.4
1.0
1.1
0.65
1.89
0.74
1.62
0.74
2.17
1.60
4.34
8.69
6.89
0.50
0.54
0.050
0.084
0.053
0.022
0.024
310
23
25
2.3
3.8
2.4
1.0
1.1
Exp_ 1
0
0.09
B
B
0.90
B
9.00
U
U
84
168
0
0.18 B
B
1.80 B
18.00
168
336
t Weight gain/N
=
U
U
Soil
Foliage
Soil
Foliage
Soil
Foliage
Soil
Soil
0_90
0.86
0.97
1.05
0.78
0.89
1.03
1.70
2.15
Soil
Foliage
Soil
Foliage
Soil
Foliage
Soil
Soil
2.97
2.43
3.06
2.24
2.04
3.68
3.36
5.60
8.62
-0.44
0.78
0.17
-0.13
0.001
0.014
0.010
0.007
Exp. 2
weight gain over control divided by N dosage.
-3.0
0.50
-0.42
-0.52
0.039
0.022
0.Q16
0.017
:j: ReI.
=
gainiN with 84 and 168 kg N ha-' (Exp. 1 and 2. respectively)
=
1.0.
260
SOIL SCI. SOc.
AM. J.,
concentration was only 1% of the pplied urea. B s d
on this biuret/urea ratio, the potentIal for N02.tOX CIty
clearly existed in five of six . biur t-urea co.mbInatIOns
in both experiments. DespIte thIS pot ntIal for N 2
toxicity, only a few seedlings temporanly showed VIS7
ual symptoms of toxicity.
.
,
In Table 2 we compare the relatIve effiCIency of
urea and soil:applied or foliar-applied biuret for in­
creasing weight of unfertilized Douglas-fir and hem­
lock seedlings. Total weight of hemlock was clearly
.
increased by supplying N via biuret (40% N);. soIl ap­
plication of biuret was consistently more .effectIve than
foliar application. In contrast, total weIgh of unfer­
tilized Douglas-fir was increased by some bIUret treat­
ments and decreased by others.
The weight gain of hemlock s edlings per ilogram
of applied N was greater with bIUr t than WIt urea.
Yet, our comparisoIl: of urea and .bIUret effectIveness
.
required an assumptIOn because bIUret was applIed at
1/10 or less of the lower urea dosage. Consequently,
we had to assume that N response was linear between
o and 84 kg N ha-", our lo er urea ra e in xP. 1 (168
kg in Exp. 2). Based on thIS assumptIOn, b uret as an
N source for hemlock was from 2 to 1110 tImes more
effective per unit of N than was urea. (Table 2). The
high efficiency of biuret, when applIed to hemlock
seedlings at the lowest biuret dosage (0.09 and 0.18 kg
.
N ha - I in Exp. 1 and 2, respectn:ely), suggests a non­
nutritional or hormonal effect from biuret. The co ­
sistently greater stimulating effect to hemlock of soIl­
applied vs. foliar-appli d biu et i.n the absence of u ea
fertilizer needs further InveStIgatIOn. Res lts cou d In­
dicate that biuret may be more readIly avaIlable
through hemlock roots than through foliage.
Extremely large dosages of iuret (22.1 and 44.8 kg
ha -I in Exp. 1 and 2, respectIvely) appbed t the f?­
liage of newly germinated and l -yr-old seedlIngs dId
not result in the biuret toxicity symptoms of yello.w
leaf tips and tip die-back for many plant speCIes (Kil­
mer and Engelstad, 1973). This strongly suggests t at
these species are tolerant of biuret. U exp.ected y, bIU­
ret as a single treatment and in COmbInatIOn WIth low
or moderate N dosages clearly stimulated hemlock
seedling growth.
.
Relating the results of these greenhouse exper ments
to forest fertilization is tenuous (Mead a d Pnt h tt,
1971), but necessary to serye silvic,;!ltuflSts untIl In­
vestigations are conducted In establIshed .stands. We
found that biuret applied to the potted soIl and com­
bined with a heavy urea dosage slightly reduced re­
sponse of Douglas-fir and hemlock seedlin s to N;
however, levels of biuret commonly found In com­
mercial grade urea would generally be well below the
highest dosages used in these experiments. We also
found that the l-yr-old seedlings of Exp. 2 showed
greater tolerance of larger dosages of biuret and N t.han
did new germinants of Exp. 1. We therefore belIeve
that growth of established stands of Doug as-fi or
.
western hemlock or their response to N fertIhzatIOn,
is not likely to b reduced by biuret in urea fertilizers.
VOL.
52, 1988
CONCLUSIONS
1. Seedling biomass of both species was at least dou­
bled when urea was added to a loamy sand soil. Doug­
las-fir responded more to urea than did hemlock.
2. In unfertilized seedlings, biuret stimulated growth
of hemlock, even when excessive dosages were applied
to foliage or to the soil. Unfertilized Douglas-fir showed
slight but erratic response to biuret; growth slightly
increased or decreased depending on biuret dosage and
method of application .
3. Unfertilized hemlock responded positively to as
little as 0.2 kg biuret ha-' «0.1 kg N ha-'). This
small biuret dosage applied to the soil under newly
germinated hemlock seedlings stimulated their dry
weight more than did 84 kg N ha 1 as urea. Biuret
may function as a plant hormone.
4. Excessive biuret applied to the soil and combined with heavy N fertilization reduced response of 60-d­
old seedlings to N, but not response of l-yr-old seed­
lings. The reported inhibiting effect of biuret on NH4 and N02 conversions may explain this reduced re­
sponse to urea. 5. Our experimental levels of biuret exceeded those found in commercial-grade urea; therefore, biuret con­
tamination of urea is unlikely to reduce growth of young seedlings of either species, even where heavily contaminated urea is applied at high dosage. -
ACKNOWLEDGMENT
The authors thank John Hazard for technical assistance
and advice about statistical analyses. Our thanks also to M.A.
Radwan, Paul Heilman, Robert F. Powers, and three anon­
ymous people for technical review.
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