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RESPONSE OF DOUGLAS-FIR TO FOLIAR
FERTILIZATION
Richard E. Miller
In 1969, Donald C. Young, research chemist with Union Oil
Company, and I began investigating the use of foliar sprays of
Summarize past research about spray application of 1 0 to 32 per­ concentrated N solutions to fertilize Douglas-fir and associated
cent nitrogen solutions to seedlings and established stands of Doug­ conifers. Our' research questions were: (1) Will Douglas-fir
las-fir. These investigations establish that Douglas-fir and associated absorb N efficiently through its needles? (2) Can conifers toler­
conifers can be foliarly fertilized with concentrated nitrogen solutions
ate practical dosages of 20%-32% N solutions? (3) What is the
at dosages of 50 to 200 pounds per acre; however, fertilization with
growth response?
these solutions requires more critical selection of nitrogen source,
I will share current answers to these questions and describe
dosage, additives, and, perhaps, time of year than does fertilization
some
of our field, greenhouse, and laboratory investigations.
with urea prill. Some burning, up to about 30 percent of the needle
ABSTRACT
surface, is visually disturbing but probably has no measurable effects
on growth.
With low dosages and careful application, gains in cubic volume or
height growth per pound of applied nitrogen were si,hUar for both
spray and prill. Yet costs per pound of applied nitrogen have been
about 25 percent more for 32 percent nitrogen solutions than for
prilled urea. Hence, foliar application of concentrated nitrogen solu­
tions is currently less cost effective than conventional use of urea sol­
ids for fertilizing Douglas-fir and associated conifers.
INTRODUCTION
METHODS
FERTILIZERS AND APPLICATION
Table 1 lists some of the tested fertilizers and their character­
istics. Because of its high N content, solution urea-ammonium
nitrate (SUAN) was used most frequently in our trials. These
fertilizers were either applied by hand t<\ seedlings or by heli­
copter in two field trials in established stands.
Table 1.
i,l,
Foliar application of nutrient solutions in agriculture has
generally been used to supply elements-required in small quan­
tities by plants. To supply nutrients such as N, P, and K, it has
, been generally thought necessary to use dilute solutions to
avoid burning plant foliage. For example, previous investiga­
tors of foliarly 'fertilizing conifers with N used solutions con­
taining less than 3% N (Schultz 1968, Eberhardt and Pritchett
1971, Paavelainen 1972). If, however, such dilute solutions
are useq to fertilize established stands of conifers with N, then
repeated applications of high volume sprays are necessary to
supply enough nutrient before the spray drip-point is reached.
Consequently, the cost of application is less attractive than
conventional soil applications of dry fertilizers.
Yet foliar feeding may provide greater efficiency and econ­
omy of fertilizer use if most of the applied nutrient entered the
plant directly.instead of passing through the soil where losses
and fixations occur.
Fertilizers tested and their characteristics.
Fertilizer
FriZZed
Urea
SoZution
Urea
Ammonium
nitrate
Urea ammonium
nitrate
Ammonium
polyphosphate
Analysis
a
Weight
(lb/gal)
N
(lb/
gal)
Abbre­
viation
46-0-0
PU
20-0-0
9.4
1.9
SU
20-0-0
10.5
2.1
SAN
32-0-0
11.1
. 3-.6
SUAN
10-34-0
a
A ctual analysis of solutions depends on
ambient temperature; these values are for 60°F;
sailing-out occurs at lower temperatures.
62
SAP
LOCATIONS
RESULT
We onducted field trials at·five locations in western Wash­
ington and Oregon ( Figure 1.). These were 6- to 7-yr-old stands
of Douglas-fir or Douglas-fir and associated conifers. Labora­
tory analyses and greenho).lse investigations supported these
.
field trials.
NITROGEN ABSORPTION
We currently have crude measures of the speed of N uptake
and the percentage of applied N that is foliarly absorbed. Pre­
sumably fertilizer that is not foliarly absorbed is eventually
washed to the soil and utilized like conventional dry fertilizers.
Application of 50 lb N/atre as solution urea-ammonium nitrate
to 6-ft tall Douglas-fir before the growing season apparently
increased internal concentration from 1.43% to 1.74% within
48 hours ( Table 2). Greater increases occurred when the same.
or 100 lb of N were applied after the growing season (Table 2);
N concentrations were nearly doubled. Moreover, we esti­
mated that 21% and 45% of the 50- and 100- N dosages,
respectively, were within the foliage of these trees within 48
hours after treatment.
In another trial, we applied prilled urea ( PU) and various N
solutions at dosages of 20 to 160 lb of N/acre to Douglas-fir
seedlings prior to bud burst. During the 3-month period after
treatment, were observed faster and greater improvement in
tree color with sprays than with urea prill. Moreover, increased
N in new needles which did not directly receive the N solution
sugg sts that some of the applied N was foliarly absotbed and
then translocated and utilized.
Figure 1. Location and stand age of field trials of foliar fertilization.
\
•
•
.EDDYCR ..
(6 year-old)
:
'--'"'�
.SLABCAMP
(40 & 70 year-old)
•
SHERMAN VALLEY
(4 & 9 year-old)
-.SHIVELEYCR.
(20 year-old)
•
HATCHERY RD.
(B year-old)
Table 2. Nitrogen concentration in Douglas-fir foliage 48 hours after
foliar application of a 32% N solution.
Composite
samplea
o
N dosage
50
- - - - - - - - - - lb N/acre - - - - - -
100
-
-
-
- -
Applied before 1975 growing season (1974 foliage)
A
B
X
1.78
1. 70
1.74 (+22%)
1.44
1.42
1. 43
Applied after 1975 growing season (1974 foliage)
A
B
X
1.98
1.89
1.94 (+48%)
1.18
1.44
1. 31
2.25
2.14
2.20 (+68%)
(1975 foUage)
A
B
X
1. 35
1. 28
1. 32
2.28
2.31
2.30 (+74%)
a
2.48
2.43
2.46 (+86%)
Each composite consisted of equal amounts of foliage from three
randomly located trees.
63
/'
II
i
I
SPRAY DAMAGE
In a lathhouse trial under well-watered conditions, however,
increased growth was associated with increased damage ( Fig­
As indicated by their weight per gallon relations ( Table 1), ure 2). Thus, favorable environmental conditions of this trial
these liquid fertilizers are concentrated salt solutions. There­ apparently compensated for the potentially negative effects of
fore, it is not surprising that foliage and, in some cases, twigs burning. In foliar fertilization of Douglas-fir under field condi­
are burned or damaged by them. We suspect two types of dam­ tions, however, successful use of concentrated solutions
age: (1) Osmotic burning or cell collapse which results from a requires critical selection of materials, dosages, and time of
large difference in osmotic pressure across the cell wall; and year. Maintaining a balance between a large N dosage to
(2) free ammonia damage which increases as solution pH extend duration of response and an acceptable degree of
exceeds 6.5. We believe that osmotic burning is the principal osmotic burning also necessitates c1oser control of fertilizer
type of damage caused by these liquid fertilizers (Miller and distribution than with urea prill.
In the two trials that follow, We compared a 32% urea­
Young 1976). We observed foliar burning within 1 hour of fer­
ammonium
nitrate .solution with prilled urea under operational
tilizing herbaceous vegetation and within severalhours of foli­
conditions. The observed relationships between foliar and twig
arly fertilizing Douglas-fir.
Numerous factors affect foliar or twig burning; these damage and volume growth will have special interest to land
managers.
include:
Factor
Damage
N source N dosage (lb/acre) Season (temperature) Phenology SAP < SU < SAN < SUAN
50 < 100 < 200 < 400
fall < spring < summer
dormant < active
Coverage (N per foliage are) light < heavy
SHIVELY CREEK
The Trial-In a recently thinned, 20-yr-old Douglas-fir
stand near CanyonvH1e in southwestern Oregon, we compared
tree growth in a spray-treated area with growth in an adjacent
nonsprayed area, where plots were either hand-treated with
urea prill or unfertilized (Miller and Wert 1979). Although
prill was applied before the start of diamet r and height
growth, the solution was sprayed in early June after diameter
growth had started and most trees had completed 10% to 25%
of their twig growth. Although our target dosage for both prill
and spray treatments was 150 lb of N/acre, measured dosage in
the three sprayed plots averaged 64, 100, and 123 lb. Drift
onto the six plots in the nonsprayed area, however, was mini­
mal and averaged 1.2 lb N/acre.
Spray Damage-Despite twig extension on most trees, only
4% of the sprayed trees on the plot receivin 123 lb N/acre had
as much as 30% of the foliage damaged ( Figure 3). Greater
NET EFFECTS ON GROWTH
From a practical standpoint, needle or twig burning is unac­
ceptable ,if crop trees are top-killed or show permanently
deformed boles or if long-term growth is less than that of
untreated trees or stands. With foliar sprays, our measured
growth reflects the net effects of fertilization; i.e., improve­
ment of nutritional status minus negative effects of burning.
Our field trials show that damage begins to offset benefits of
fertilizing when more than about 30% of the needle surface
area of Douglas-fir is injured.
_
g
Figure 2. Average first-year height response and initial foliar burning
of potted Douglas-fir after fertilization.
GROWTH RATIO (POST/PRETRERTMENT)
Figure 3. Foliage burning and N dosage in a 20-yr-old
I S'£l
Douglas-fir stand at Shively Creek.
•
Tree. (% of total)
SUAN
I. £l£l
J
-----
----
+
SU
100
lb••
•
.•
Nla
75
N/SEEDLING (G)
+
D
.
£l.£l£l+-
____
--
____
, I £l
l!£l
____
25
----
____--
3il
50
£l.B
I E
NONE
liil
o L..-_.J.-.IllIl lIIIIL-..I-.I""
10
20
Foliage Bum Cia•• (%)
NEEDLES DRMAGED (PERCENT)
64
30
ume growth of the urea-ammonium nitrate-treated plots aver­
aged 80 ft3/acre more than that of the unfertilized plots. Yet the
difference between the control and the prill- and spray-treated
plots was not statistically significant. To validly compare the
effectiveness of these soil and foliar applications, we would
have to know the actual dosage released from the helicopter·
when it was over the plots. If we assumed (1) the helicopter·
actually released an average of 96 1b N/acre, and (2) a .linear
response surface between 0 and 150 lb N/acre, then the gain
per 1b of applied N was 0.83 ft3 instead of 0. 53 (Table 3).
Under these assumptions, foliar fertilization was 36% more
,effective per 1b of applied N than was the prill application. The
most conservative approach, however, was to assume that 150
1b N/acre were released by the helicopter but an average of 53
1b/acre were lost before the spray reached the plots. With this
assumption of loss, and again with no correction for a shorter
growth period after spray fertilization, foliar application was
about 13% less effective than prill during the first 4 yr after
treatment.
crown damage however occurred in other portions of the
spray-treated area which probably received heavier dosages of
spr y (e.g., where the helicopter pivoted to change direction).
Thus, our target dosage of 150 lb N/acre was probably close to
the maximum sage dosage for the weather and tree conditions
at time of treatment.
Visual estimates after the first growing season showed that
as average N dosage per plot increased, more of the trees had
injured needles and that a larger percentage of the needles on
these trees were injured.
Ten percent of the trees on the sprayed plots had curled or
dead leaders at the end of the first growing season, compared
to 8% broken or dead on the prill-treated and, surprisingly,
18% on the control plots. Yet these differences were not
statistically different, thus indicating that fertilizer treatment
neither decreased nor increased incidence of malformed lead­
ers (Miller and Wert 1979).
Volume Growth-Adjusted volume growth on the prill­
treated plots exceeded that on the control plots by 92 ft3/acre
during the 4-yr period (Table 3). Moreover, the adjusted vo1­
SLAB CAMP
Table 3. Average initial volume and gross volume growth of all trees. 1.6 in. dbh and larger. at Shively Creek. per acre basis.a Fertilizer
None
Prill
Spray
N/acre
(lb)
Initial Annual
volume
growth
- .;. _ft3_ _ _
204
190
166
o
150
150
b
(96)
62
85
82
The Trial-We applied prilled-urea and urea-ammonium
nitrate solution by helicopter to four 40- to 70-yr-old site IV-V
stands near Sequim, Washington. Nominal dosages of 50,
100, 200, and 400 lb N/acre applied both before and during the
1969 growing season.
Spray Damage-Damage to needles and leaders was much
greater in the July application of urea-ammonium nitrate than
in the May application (Table 4). For example, 3.5% of trees
in the control plots had brown-or unhealthy-appearing
crowns. The incidence of such unhealthy crown was increased
by the 400-N dosage of urea-ammonium nitrate in May, but by ,
as little as 100 1b N/acre in July. Conversely, lower dosages of
50 and 100 lb N in May and 50 lb N in July had a lower propor­
Total gain
Per lb
of N
rela­
tive
0.61
0.53
0.83
1.00
0.87
1.36
92
80
a
Growth adjgsted for initial differences in
stand volumes.
Average measured dosage on three
plots.
Table 4. Average incidence of crown damage after foliar fertilization
by treatment in four stands near Slab Camp. per acre basis.
Fertilized
May
July
Control
N dosage
(lb)
50
100
200
400
50
100
200
400
0
Trees with
unhealthy crowns
(%)
(No.)
15
40
40
120
10 '
98
348
990
40
Damaged
a
crown surface
(%)
1.1
3.0
3.7
12.3
1.0
7.4
22.7
90.2
3.5
a
For 57 to 60 height trees per treatment.
10 to 80%.
65
16
b
43
2
Trees with
top kill
(No.) (%)
25
5
12
8
15
12
55
318
10
1.8
0.4
1.2
0.8
1.5
1.0
3.6
29.0
0.9
b
Range for 57 trees was
tion of trees with unhealthy crowns than did the control plots.
At the 4.o.o-N dosage, average surface area affected was 16%
in the May application and 43% in the July application.
Moreover, 29% of the trees sprayed in July with 4.0.0 lb of N as
urea-ammonium nitrate had top or leader kill, compared to
.0.9% in the four control plots. In contrast, May application of
the same dosage did not increase numbt?r of trees with top or
leader kill. Thus, one could anticipate that July application at
the high dosage should have reduced response or even growth
below that of the control plots.
Volume Growth-The growth period after the July applica­
tion was about 3.5 versus 4..0 growing seasons. Therefore, we
increased the measured growth of plots treated in July by 14%.
Based on the combined results from both seasons of applica­
tiop. and all dosages, annual growth of plots treated with urea
prill (2.06 ft3/acre) exceeded that of urea-ammonium nitrate­
treated plots (164 ft3/acre) (P < .0. .0.01). Moreover, the superi­
ority of the prill treatment was significantly greater in the July
than in the May application (P < .0..038).
The strong response to summer application of low dosage of
prilled urea was unexpected, because weather conditions dur­
ing and following treatment should have lead to considerably
more ammonia volatilization losses than after our May applica­
tion. A possible explanation is that control of fertilization dis­
tribution on the treatment area was better in the July applica­
tion (the second one) and, thus, the plots received lllore N.
Unfortunately, we cannot prove and/or support this specula­
tion. The maximum gained from urea fertilization was with
4.0.0 lb of in both seasons (Table 5).
Response to urea-ammonium nitrate varied between the two
seasons of application. Spring application of 10.0 through 4.0.0
lb N/acre was less stimulating than the corresponding dosages
applied in spring. For no apparent reason, spring application of
the 5.o-N dosage apparently depressed growth, whereas sum-
mer application of the same nominal dosage apparently
enhanced growth. In summary, 4-yr response to urea­
ammonium nitrate during the first four growing seasons after
treatment was probably reduced by foliar burning; this burning
was much greater in the summer than the spring application at
each N dosage.
Future additional response to both N sources is likely
because increment cores taken from the 4.0 largest Douglas-fir
trees per acre showed that positive response began in the third
and fourth years after treatment. This response was slower than
usual and undoubtedly reflected the low vigor or overstocked
conditions of the experimental stands.
COST EFFECTIVENESS
The initial cost of fertilizing includes the contract cost
(applying the fertilizer, transporting it in the forest, and apply­
ing it to specified' areas by helicopter), the administrative costs,
and the costs providing access roads and heliports. Since the
cost of fertilizer is usually more than half the initial treatment
cost, the price of fertilizer strongly influences initial costs.
Fertilizer Cost-Fertilizer prices have increased since 1971,
particularly in 1974 when demand greatly exceeded supply
(Table 6). On a cost-per-lb of N basis, urea-ammonium nitrate
averaged $.o.12Y2 or 3% more than N from .prilled urea.
Although in 1979 N in urea-ammonium nitrate cost 7% more
than that in prilled urea, the future price differential between
the two materials is unclear. Although theoretically more
energy is required to produce prills from solution because of
the additional energy requirements of the drying process,
nonenergy factors may be stronger determinants of current
prices.
Application and Total, Cost-During he 3-yr period,
1973-1975, the Bureau of Land Management in Ro_seburg,
Oregon, foliarly fertilized nearly 2.0.0.0 acres. Contact costs for
urea-ammonium nitrate and spraying at dosages of 15.0 lb
Table 5. Relative volume growth in four Doug1as-fir/
western hemlock stands near Slab Camp.a
Month
Treatment
N source
, _
May
July
Mean
Prill (46--0-0)
Solution (32-0-0)
Prill (46--0-0)
Solution (32-0-0)
Prill (46--0-0)
Solution (32-0-0)
Table 6. Costs of urea-ammonium nitrate solution
(32-0-0) and pri11ed urea (46-0-0), F.O.B.
Portland.a
Dosage (lb of N/acre)
50 '100
200
100
_
100
84
132
116
116
100
_
_
(%)
97
104
146
96
122
100
b
_
_
122
118
147
107
134
112
_
_
137
106
152
92
145
99
,Year
a
Gross growth over a 4-yr period; based on four
O.l-acre plots for each treatment combination, 68
plots total. Treatment means were adjusted by co­
variance techniques for initial differences in stand
golume, site index, and percent of Douglas-fir.
Percent of average control or untreated growth
(159 cu ft/acre/yr).
Fertilizer cost/ton
32-0-0
46--0-0
1971
1972
1973
1974
1975
1979
$ 43
49
61
96
120
111
Mean
80
32-0-0
N cost/1b
46--0-0
ratio
$ 67
65
75
147
165
148
$0.067
0.077
0.095
0.150
0.188
0.173
$0.073
0.071
0.082
0.190
0.179
0.161
0.92
1.08·
1.16
0.94
1.05
1.07
111
0.125
1.121
1.03
a
Average posted, delivered prices of a major
producer.
66
N/acre ranged between $33 and $58 per acre (Table 7). The
cost per lb of applied N averaged $0.30 for urea-ammonium
nitrate and $0.24 for urea prill. These past costs indicate that
the overall benefits from forest fertilization with urea­
ammonium nitrate would need to be about 25% greater per lb
of applied N to be as cost effective as fertilization with urea
prill.
The short-term response in the two trials where cubic vol­
ume growth was measured indicated that this break-even point
was exceeded at Shively Creek only if we assured that 97 lb
rather than the target dosage of 150 lb N/acre was actually
applied. Comparable cost effectiveness of foliar fertilization
will probably be achieved at lower N dosages than the conven­
tional 200 lb N/acre, because heavier dosages of solution can
severely damage foliage and presumably reduce growth. At
lower N dosages, foliar application should theoretically be
more effective than application of urea to the soil because more
of the applied fertilizer will enter the plant directly, rather than
passing through the soil where inevitable losses and fixations
occur. To test this"theory and to compare the full benefits of the
two treatments, however, we need long-term data; our existing
field trials will provide such data.
IMPLICATIONS FOR LAND MANAGERS
Foliar fertilization is a potential alternative to conventional
fertilization with urea prills or granules. When we use foliar
sprays to bypass fixation of the soil system, however, we also
bypass its buffering capacity and thus risk tree damage and
reduced growth. Although the potential exists for application
of N in solutions to be more cost effective than with urea prill,
Table 7. Acreage and costs of operational fertiliza­
tion with urea-ammonium nitrate solution and urea
prill.
b
Prill (46-0-0)
Costs
Acres
(per
(per
acre) lb N)
(No.)
Year
a
Solution (32-Q-O)
Costs
Acres
(per
(per
acre) 1b N)
(No.)
1973
1974
1975
203
200
1544
$33.09
42.38
57.68
$0.22
0.28
0.38
6100
306
1691
$40.75
52.00
53.77
$0.20
0.26
0.27
Mean
649
44.38
0.30
2699
48.84
0.24
there is also greater risk or uncertainty in foliar fertilization.
'
Moreover, reduced application costs and/or greater growth
stimulation are needed to improve cost effectiveness of sprays.
With careful formulation and application, the gain in cubic
volume or height growth per lb of applied N was similar for
spray and prill applications in our study; longer term compari­
sons are still pending. Clearly, more information is necessary
before foliar fertilization can be recommended as .a standard
operations procedure.
RESEARCH NEEDS
To reduce uncertainty and improve the cost effectiveness of
foliar fertilization, additional research on the following topics
is desirable:
1. Spray losses-Our experience at Shively Creek indicated
that about two-thirds of the prescribed fertilizer reached the
stand. Although excessive dosage in nonmeasured areas possi­
bly explains this difference, a certain level of loss may be
inherent to spray applications. Determining the magnitude and
fate of this potential loss and some means for reducing it is
desirable.
2. Application costs-Increasing the scale of spray projects
and using more efficient spray equipment should reduce costs
of foliar fertilization. After a critical minimal scale 6f fertilizer
application is exceeded, delivering, handling, and applying
liquids is generally more economical than using solids. This
reflects the ease of mechanization; one person with a pump can
readily move thousands of pounds of liquid fertilizer per hour.
Moreover, some fertilizer solutions very nearly equal solids in
their content of active ingredients per unit weight.
3. Spray additives-Development of a spray additive that
would reduce osmotic burning, yet not decrease penetration of
applied N is appealing. For example, our recent screening test
has indicated that foliar damage from urea-ammonium nitrate
was reduced by three of four surfactants tested.
CONCLUSIONS
Our research results indicate that:
1. Douglas-fir can absorb N efficiently through its needles;
internal N concentrations and color are much more rapidly and
markedly improved than when urea prill was used.
2. A risk of excessive foliar burning exists when these salt
solutions are applied to the foliage. Douglas-fir and associated
species can tolerate practical dosages of 20% to 32% N solu­
tions; however, burning that exceeds about 30% of the needle
surface can reduce response or growth.
3. Short term growth data and past costs indicate that foliar
fertilization is currently less cost effective than conventional
fertilization with urea prills or granules.
otal cost of solution (32-Q-O), delivery, and
application at 150-N per acre. Source: Steve Wert,
Soil Scientist, Bureau of Land Management, Roseburg,
Oregon, October 22, 1975. bTota1 cost of urea prill
(46-0-0), delivery, and application at 200-N per
acre; however, in 1974, ammonium nitrate (34-0-0) was
used. Source: Robert Bergland, Fertilization
Forester, Washington State Department of Natural
Resources, Olympia, Washington, November 13, 1975.
67
4.
Pending data analyses will clarify uncertainty about the
comparable long-term effects of spray and prill application of
N on stand growth and mortality.
5. The reliability and financial gains from foliar fertilization
can be improved by additional research of spray formulations
and application techniques.
Miller, R. E., and D. C. Young.
1976. Forest fertilization: Foliar application of nitrogen solutions
proves efficient. Fert. Solutions 20(2):36, 40, 42, 44, 46, 48,
59-60.
Miller, R. E., and S. Wert.
1979. Effects of soil and foliar applications of nitrogen fertilizers
on a 20-year-old Douglas-fir stand. USDA For. Servo Res. Note
PNW-329, 12 p. Pac. Northwest For. and Range Exp. Stn., Port­
land, OR.
Paavelainen, E.'
1972. Reaction of Scots pine on various nitrogen fertilizers on
LITERATURE CITED
drained peatlands. 44 p. Comm. Inst. For. Fenn. 77.
Schultz, R. P.
Eberhardt, P. J., and W. L. Pritchett.
1971. Foliar application of nitrogen to slash pine seedlings. Plant
and Soil 34:731-740.
1968. Soil or foliar fertilization of well-drained and flooded slash
pine seedlings. USDA For. Servo Res. Pap. SE-32, 8 p. Southeast
For. and Range Exp. Stn., Asheville, NC.
\
68
Proceedings
Forest Fertilization
\
Conference
September 25,26, and 27,1979
Alderbrook Inn
Union, Washington
Sponsored by
Institute of Forest Resources
College of Forest Resources
U niversity of Washington
•
U .S.F.S. Pacific Northwest Forest
and Range Experiment Station
•
British Columbia Ministry of Forests
Price per copy $17.50