Forest Fertilization:
Foliar
pplication of
Nitrogen Solutions
Proves Efficient
By Richard E. Miller
u.s. Fores, Service, Pacific Northwest Forest
& Range Experiment Station
Olympia, Washington
and
Donald C. Young
Union Oil Research Brea, California Pproximately 22 percent of the
2. 3 billion acres of land in the
United States was classified as
commercial forest land! 6 in 1970.
These 500 million acres are each
considered capable of producing at
least 20 cubic feet of timber per year
and are available and suitable for
growing continuous crops of sawlogs,
pulpwood, or other industrial timber
products. Besides producing wood,
these timberlands provide recreation,
watershed protection, wildlife habitat,
and livestock grazing.
A
Foresters who manage commercial
forests' have much in common with
agriculturists. Both foresters and
farmers work with renewable resources
and use good management practices to
ensure a perpetual series of harvests.
Both seek to increase production per
acre and to compensate for decreasing
a cr eage available for production.
Foresters and farmers share another
common goal: to increase benefi ts and
reduce costs of fertilizer programs.
That goal stimulates the search for
more efficient ways of providing
nutrients to the crop.
The purpose of this article is to give
a general introduction to forest
fertilization in the United States and
to summarize our experience with
foliar application of concentrated
nitrogen solutions to improve growth
of Douglas fir and associated conifers.
Forest Fertilization
Nearly a l l o perational forest
fertilization in the United States has
been in the Pacific Northwest and the
Southeast. Between 1965 and 1973,
approximately 824,500 acres were
fertilized in western Washington and
Oregon! ; about 90 percent of this
acreage was in industry-owned forests.
D ur i n g the same period,
approximately 300,000 acres were
fertilized in the Coastal Plain of
the southeastern United States! 2
Operational fertilization in these
two areas is based on -research during
the past two decades. Numerous field
t ri a ls i n the Pacific Northwest
demonstrated that tree growth is
g e ner ally limited by insufficient
available nitrogen, despite total N
supplies in the soil of productive
stands, ranging from 2,000 to 25,000
•
Forest
pounds per acre. Fertilization at time
of tree planting has seldom proved
beneficial, mostly because broadcast
appl ications of nitrogen increase
competition from other vegetation and
reduce seedling survival. However,
broadcast applications of 150 to 200
pounds of N per acre after stand
est a bli shme nts generally increase
growth of Douglas fir and associated
conifers. In the lower Atlantic Coastal
Plain, however, addition of 50 to 100
pounds of phosphorus at time of
p la n t i ng substantial ly increases
seedling survival and growth. In some
of these soils, further addition of
comparable amounts of N also
increases seedling performance! 2
After stand establishment, N is used to
accelerate volume growth.
N Sources: Urea as priUs or granules
i s the most commonly applied
f ertilizer. During recent fertilizer
shortages, some ammonium nitrate
was used in forests of the Pacific
Northwest; however, it is superficially
less attractive because of its lower
concentration of N, somewhat higher
cost per pound of N, and its nitrate
which is more susceptible to leaching.
Since the different sources of N are
subject to various transformations and
f ates within t h e forest, their
comparative effectiveness probably
depends o n s p e c ific ecological
conditions! 9. Thus, research in both
the laboratory and field is attempting
to answer questions that will lead to
more specific fertilizer prescriptions.
For example, under what conditions
do urea applications during dry, warm
weather lead to N volatilization
losses' 7 and substantially reduced
gains in tree response? How do various
N sourcds affect tree growth and
cycling of N and other elements in the
forest?
Placement, Costs, and Bidding: On
level terrain of the Atlantic Coastal
Plain, tractors are generally used to
fertilize and physically prepare sites
for reforestation. In established stands
or steep terrain, aerial application by
fixed wing aircraft or helicopters is
more pract ical. In the Pacific
Northwest, helicopters are used almost
exclusively. Although the operational
program in Washington and Oregon
exceeded 200,000 acres per year by
1974, costs increased markedly when
worldwide fertilizer and petroleum
shortages developed. For example, in
1971, total cost of aflplying urea
averaged 11 cents per pound of N;
•
thus.,applying 200 pounds of nitrogen
cost about $23.00 per acre. In 1974,
costs more than doubled to about 29
cents per pound of N or about $57.00
per acre. Cost of fertilizer represented
60-70 percent of the total treatment
cost including administration.
C o ntract or s usually bid , for
supplying and applying fertilizer to
specified areas. Bids are expressed as
dollars per ton of fertilizer applied at a
specified target dosage. Bidders usually
have the option of stockpiling bagged
fertilizer at heliports several weeks
before spreading or delivering on a
daily basis in bulk carriers or smaller
trucks. The forester specifies areas to
be fertilized and schedules access roads
and heliports for completion.
Pattern of Tree Response: Fertilized
Douglas firs usually increase in
diameter and volume growth during
t h e f irst growing season after
treatment. Response in volume growth
generally peaks between the third and
fifth year. Duration of response varies
with location, stand, and N dosage.
Current data suggest that response
gradually approaches zero within 10 to
15 years after treatment; however, few
long-t e rm trials with a single
app lication of either urea or
a m mo n ium nitrate are available.
Application of 150 to 200 pounds of
N per acre generally provides 300 to
400 total additional cubic feet of
wood per acre. These increases in
volume growth are preceded by
i n cr eased w e ight a nd nitrogen
concentration of foliage and increased
crown size; increased photosynthetic
e f f e c t i v e ness h a s a l s o been
demonstrated2•
Nutrient Cycling: Trees conserve
nutrients by internally recycling a
portion of their annual nutrient
uptake. For example, annual uptake of
a 3 5-year - o l d Douglas fir stand
approximates 50 pounds of N and 7
pounds of P per acre per year. About
40 percent of this N, and 10 percent
of the P, are returned each year to the
forest floor through litter fall of
needles and small twigs4. Root
mort ality also returns additional
amounts to the soil. Most the remaining
N uptake is either recyled to growing
tissue or incorporated in the wood.
Dur ing various stages of sta,ld
development, available nutrien ts in the
soil gradually accumulate in the tree
a n d s u b o r d i n at e vegetation;
deficiencies in available nutrients at
some sites may result, especially if
nutrients are removed in harvesting
trees or in preparing the site for a new
crop.
Foliar Fertilization
F o l iar application of nutrient
solutions has been generally used to
supply elements required in small
quantities by plants. To supply
nutrients such as N, P, and K, it has
been generally thought necessary to
use dilute solutions to avoid burning
of foliage. Thus, repeated applications
of high volume sprays would have
been necessary and the cost of
applicati o n less attractive. For
example, previous investigations of
fert ilizing conifers with nitrogen
through their foliage employed the use
of solutions containing less than three
percent N. When dilute solutions of
urea were sprayed on young Scots pine
seedlings, the drip point was reached
before more than 17 pounds N per
acre could be applied; consequently,
the beneficial effects of this spray
were much less than with 112-pound
dosages of various N sources that were
applied to the soil! ! .
In other tests in the southeastern
United States, potted seedlings of slash
pine were dipped in dilute N solutions.
Although soil application of N and P
in creased seedling growth, foliar
fertilization with a diammonium
phosphate solution containing 1.4
percent N did not. Moreover, there
was no significant increase in nutrient
uptake over that of the untreated
control'3. In another series of
laboratory experimentsS, pine
seedlings were dipped in calcium
nitrate solutions containing 0.1 to 2.4
percent N. Needle burning occurred at
0.4 percent N concentrations and
severity increased with increasing N
concentration; no seedlings died
during the two weeks of observation.
When N concentration of the dip
solution was 0.3 percent, the amount
of N absorbed into the foliage was
increase three- to fivefold if a
spreader-sticker was used.
Potential advantages of foliar
fert ilizat i o n ove r co nventional
applications to the soil largely depend
on the extent of nutrient absorption
by the foliage. Potential advantages for
forest fertilization include:
1. Increased utilization of fertilizers
by plants.
2. More flexibility in applicatiofl
timing
3. Reduced risk of watershed
contamination
4. Improved logistics of delivery and
application
forest
70 ...
5. More options in formulations
60
( 1) Greater utilization of fertilizers
would occur if most of the applied
fertilizer entered the plant directly,
rather than passing through the soil
where inevitable losses and fixations
occur. In foliar applications, one seeks
a r a p i d a nd complete nutrient
penetration into the foliage. This
insures high recovery of applied
nutrients and rapid availability to the
plants. To provide an extended period
of response in conifers, one also
attempts to get large amounts of N
into the plant without excessive
b u r n i ng. Non-absorbed nutrients
probably have the same pathways as
solid fertilizers after eventually being
washed to the soill s. Volatilization
losses of urea from foliage are also
p ossible, h owever, t h e rapid
penetration of urea into foliage
reduces time of exposure and
susceptibility.
Rate and total amount of
penetrat ion dep end largely on
n u me r ou s c haracteristics of the
fertilizer solution, plant, and climate
at and after treatment] 8 6 . For
example, urea penetrates more rapidly
than inorganic ions such as ammonium
and nitrate. In isolated cuticular
membranes from tomatoes and onions,
r a te 0 f u re a p enetrations was
ten- to twentyfold faster than rates of
cation a n d a n i o n p e netration.
Increasing the concentration of urea
solution also increased the rate of urea
penetration, but not proportionally 2 0
When slash pine seedlings were dipped
in 0. 3 percent nitrogen solutions, the
rate of Nt 5 absorption was also faster
from un!a solution than from nitrate
or anunoni m solutionss. Eberhardt
and Pritchett calculated that 71
percent of the nitrogen retained on the
needles after dipping was absorbed
from the urea solution within24hours.
In contrast, 45 and 39 percent of the
retained nitrogen was absorbed from
the nitrate and ammonium solutions,
respectively. Thus, faster and greater
penetration of nitrogen into foliage is
likely when urea or urea-containing
soluti ons are f ol i a r l y applied.
Regardless of N form, absorption rates
after foliar fertilization are many times
faster than after soil applications; it is
also likely that total recovelY will be
greater.
(2) In the Pacific Northwest, forest
fertilization is currently done during
•
_
Figure 1: Osmotic burning on Douglas fir and relative osmotic
pressure of various N solutions at 160 Ibs. N/acre. SAP = ammonium
polyphosphate, SU = urea, SAN = ammonium nitrate, and SUAN =
urea-ammonium nitrate.
•
X SAP
•
A
•
•
1
I
SAN
SUAN
A
X
.5
SU (1.0)*
7( =C RT"
I
1.0
I
I
1.5
I
L
2.0
L J
2.5
RELATIVE* OSMOTI C PRESSURE, IT'
the late fall through early spring
period, because of limited, sporadic
rainfall in June through September.
There is general recognition that urea
prills and granules require sufficient
precipitation shortly after treatment
to carry the fertilizer to the plant
roots and away from the soil or forest
f loor surface where volatilization
losses are m o s t l ikely. Foliar
fe rt i lization could provide more
flexibility in application timing, by
extending the season of fertilization
into summer months when weather
conditions are also more favorable for
application by helicopters.
Application of foliar sprays during
rainy weather is also feasible providing
the spray material doesn't drip or wash
off within a short period after
treatment. In other crops, such as
sugar cane, cotton, pineapple, corn,
and potatoes, that Union Oil
researchers tested, foliar absorption was
essentially complete witllin 24 hours
after application-usually four to six
1
hours proved adequate. (See also 8.)
For example, 24 hours after pineapple
was sprayed with 150 pounds of
nitrogen per acre as ammonium nitrate
or urea solutions, there was no
nitrogen in repeated washings of the
foliage and the nitrogen content
within the plant was at maximum
levels. Finally, storing or applying N
solutions during sub-freezing weather
is also feasible, providing the
concentrated solutions are diluted
with water to reduce the salting-out
temperature.
(3) The increasing use of N
fertilizers in the Pacific Northwest to
i n crease forest growth triggered
concerns about the effects of this
practice on water quality. Most
research results indicate that direct
application to surface water is the
principal route of fertilizer N to forest
streams; small but increased amounts
of nitrate in streams several months
after fertilization indicate nitrification
of fertilizer N or native organic N is
another source9. Providing 1 00­ to
200-foot-wide buffer strips beside
major s t reams within the area
scheduled for fertilization is the
current means of reducing risk for
these potential sources of reduced
water quality. Foliar fertilization is
likely to reduce this risk further if
most of the fertilizer is not required to
pass through the soil to reach the tree
and m o s t of t he material is
accumulated in the plant within a few
hours.
(4) After a critical minium scale of
fertilizer application is exceeded, the
delivering, handling, and applying of
liquids are generally more economical
than using solids. This reflects the ease
of mechanization; one man with a
pump can readily move thousands of
pounds of liquid fertilizet per hour.
Moreover, some fertilizer solutions
very nearly equal solids in their
content of active ingredients per unit
weight.
( 5) F i n a l l y , p r e s c r iption
formulations can be prepared for
special requirements by the simple
blending of liquids. Since fertilizer
solutions are generally compatible
with most herbicides and insecticides,
two jobs can be done in a single
application.
Forest
Our Research Experience
In 1969, we began a series of field
experiments to compare the effects of
various nitrogen solutions and prilled
urea on the growth of Douglas fir
seedlings and 6- to 70-year-old stands
of Douglas fir and western hemlock in
northwestern W a shi ngton. Our
questions were:
Douglas fir absorb N
efficiently through its
needles?
2. Can Douglas fir and associated
c o n i f e r s t o l e rate
practical dosages of
concentrated nitrogen
s o l u t i o n w i t hout
excessive burning?
3. What is the growth response to
t h i s
type o f
fertilization?
Some of the materials we've used
are:
Material
Analysis
46-0-0
prilled urea
solution urea
20-0-0
solution ammonium nitrate
20-0-0
solution urea-ammonium nitrate 32-0-0
solution ammonium
polyphosphate
10-34-0
Nitrogen Absorption: We currently
have crude measures of the speed of
nitrogen uptake, but not of the
percentage of applied N that is foliarly
absorbed. Application of 50 pounds of
n it roge n per acre as solution
urea-ammonium nitrate (SUAN) to
Douglas fir seedlings increased internal
nitrogen concentration from 1.43 to
1.74 percent within 48 hours. In
another trial, we applied prllied urea
(PU) and various nitrogen solutions at
dosages of 20 to 160 pounds of N per
acre to D?uglas fir seedlings prior to
bud burst in the spring. During the
3-month period after treatment, we
o b s e r v e d faster and g reater
improvement in tree color with sprays
than with urea prill. Moreover, absence
of inorganic N in both new and old
foliage and increased protein in new
needles which did not directly receive
the N solution suggest that at least
some of the applied N was foliarly
a b so r bed t hen translocated and
utilized.
Osmotic Burning: Osmotic burning
or cell rupture is a type of foliar
burning resulting from a large
difference in osmotic pressure across
the cell wall. It is our observation that
osmotic burning is the principal type
of burning caused by foliar application
I
.
Will
o f n u t r i e n t s o lut ions. The
concentrated fertilizer solution outside
t h e cell established an osmotic
pressure gradient across the cell wall.
Eq ualization of this pressure is
attained by fluid moving out of the
cell and the feliilizer solute moving in.
Presumably, the cell wall is like a
membrane penetrable at a finite rate.
When cells are covered by a large
quantity of solute, a large pressure
differential develops; if transport
cannot occur fast enough to equalize
the pressure, the cell ruptures. We
observed foliar burning within one
hour of fert ilizing herbaceous
vegetation and within several hours of
foliarly fertilizing Douglas fir.
Based on this hypothesis, we can
predict t h e relative d&maging
t e ndencies of various fertilizer
solutions from their respective osmotic
pressures. Figure I shows visually
estimated percentage of needle burn
three weeks after spraying various
concentrated nitrogen solutions at 160
pounds of N per acre vs. the calculated
osmotic pressure for each material
relative to urea solution. Osmotic
pressure, II, was calculated in the
standard thermodynamic equation:
'/(= C ART
C is molal concentration of the
solution, R is the gas constant, T is the
absolute temperature, and A is the
activity coefficient.
Thus, we can readily predict the
relative damaging tendencies of various
chemicals for given plant and weather
conditions by comparing calculated or
measured osmotic pressures of their
saturated solutions. For example,
ammonium nitrate solution (32-0-0)
causes more osmotic burning than
does urea solution (20-0-0) at the same
N dosage (Figure 2); this was true in
applications made before, during, and
after the growing season.
From a practical standpoint, needle
or twig burning is unsatisfactory if
burning occurs on crop trees, resulting
in crop tree mortality, top killing with
permanently deformed boles, or
long-term growth that is less than that
of untreated trees or stands. If damage
to crop trees is minimal and response
occurs, then a desired thinning of
surplus, less vigorous trees is likely.
For example, the largest trees in
densely-stocked stands have the largest
amount and proportion of their
crowns exposed to light and also to an
aerial application of liquid fertilizer.
Conversely, the smallest and generally
least valuable trees have much less
exposed crown surface and are the
least likely to be feliilized. Hence,
fertilization with foliar sprays is more
likely to improve the competitive
status of dominant trees to the
disadvantage of smaller trees.
We've applied concentrated
nitrogen solutions at dosages ranging
from 20 to 800 pounds of N per acre.
Corresponding percentages of needle
surface area or needles with osmotic
burning ranged from 0 to 100 percent.
Moderately to severely burned needles
are lost from the tree within a few
months of treatment; however, even
complete defoliation does not always
result in tree kill.
Growth Response: Comparative
increases in height, diameter, or
volume growth per unit of cost are the
ultimate standard for judging foliar­
and soil-applied fertilizers. With foliar
sprays, our mea ured growth reflects
the net effect· of fertilizing (i.e.,
improvement of nutritional status
minus negative effects of osmotic
burning). Our current data suggest that
damage begins to offset benefits of
fertilizing when more than about 30
percent of the needle surface area of
Douglas fir is injured. The N dosage
that can be applied without causing
excessive needle damage depends on
numerous factors including: N source,
additives, season of year and/or stage
of tree growth, spray volume relative
to amount of foliage, and uniformity
of spray application.
In one experimental area, we
applied p r il led urea (PU) and
urea-ammoni um nitrate solution
( SUAN) by helicopter to four,
70-foot-tall stands. Treatment was
applied both before and during the
growing season. Both materials were
applied at nominal dosages of 50,100,
200, and 400 pounds of N per acre.
Our results in Table I show growth of
treated plots as a percent of growth of
control plots. Four-year growth of the
four untreated plots averaged 640
cubic feet per acre.
C ombining results from both
seasons of application, growth of plots
treated with prilled urea exceeded that
of SUAN-treated plots at all dosages.
Maximum gain from urea was with
400 pounds of N in both seasons. The
50· and 100-pound N dosages of urea
did not stimulate growth in spring but
did in summer. The strong response to
low summer dosages of PU was
u n e x p e c t e d b e c a use weather
conditions during and following
t r ea t ment s ho uld have led to
considerably more volatilization losses
A d d it i onal response to both
nitrogen sources is likely. Increment
cores taken from the 40 largest
Douglas fir trees per acre showed that
positive response began in the third
and fourth years after treatment. This
response is slower than usual and
undoubtedly reflects the extremely
low site quality and/or overstocked
conditions of the experimental stands.
Operational Experience
Figure 2: Tree on left (sprayed with concentrated urea solution) had less osmotic burning
and needle l oss than tree on right (sprayed with ammonium nitrate solution at same dosage
of 50 Ibs. of N/acre).
Table 1 Relative Volume Growth on Research Plots' in Douglllsfir/Western Hemlock Stands 1 Treatment
Month
Dosage (lb. N/acrel
N source
_ -
May
Mean
- _
-
- Percent
400
200
21
-
_
-
_
-
-
100
97
122
137
84
104
118
106
Prill (46-0-0)
116
128
129
133
Solution (32-0-0)
101
84
94
81
Prill (46-0-0)
108
113
125
135
93
94
106
93
Prill (46-0-0)
Solution (32-0-0)
July
100
50
Solution (32-0-0)
11 Gross growth over a 4-year period; based on four O.l-aere plots for each treatment
combination, 68 p lots total. 21 Percent of a verage control or untreated growth. Forest
of urea than in our May application.
Moreover, the measured growth period
was shorter-three and one-half vs.
four growing seasons after treatment.
Response to SUAN varied greatly
b et w e e n the two seasons of
application. Spring application of 100
through 400 pounds of N resulted in
greater growth than that of the
control; however, response to 400
pounds of N was less than that with
200 pounds of N. This probably
reflects the osmotic burning noted in
the field following the heavier dosage.
July application burned foliage-even
with 100 pounds of N. The incidence
a n d severity of needle burning
increased with increasing dosage.
The ma mum grun from SUAN
was 18 percent from 200 pounds of
nitrogen applied in spring (Table 1);
gain from urea prill at the same dosage
and season was 22 percent. Response
to SUAN during the first 4 years after
treatment was inversely related to
foliar burning; this burning was much
greater in summer than in spring
applications. Application of 100 or
more pounds per acre of nitrogen in
the summer reduced growth below
that of untreated plots.
The logistics and effectiveness of
concentrated SUAN were investigated
in the spring of 1973 in cooperation
with the Bureau of Land Management
(BLM) near Roseburg, Oregon. Two
hundred acres were spray-fertilized by
helicopter (Figure 3). In a recently
thinned stand, we monitored spray
distribution and dosage and also
established three 0.2-acre plots. We
then compared tree growth in the
spray-treated area with growth in the
non-spray area, where plots were
either hand-treated with urea prill or
unfertilized. Although prill was
applied before the start of diameter
and height growth, solution was
sprayed in early June after diameter
growth had started and buds had
burst; most trees had completed 10 to
25 percent of their twig growth.
While our target dosage was 150
pounds of N per acre, dosage in the
three spray plots averaged 5 1, 102,
and 137 pounds of nitrogen per acre.
Drift to the seven plots in the
non-spray area was minimal and
averaged 1.2 pounds of N per acre.
Although some of these targets were
within 100 feet of the spray-treated
plots, maximum dosage on any target
in the non-spray area was 3.4 pounds
of N per acre.
Despite twig extension on most
trees, only four percent of the sprayed
trees on the plots receiving 137
pounds of N per acre had as much as
30 percent of their foliage injured_
However, greater crown damage
occurred in other portions of the
spray-treated area which probably
received heavier dosages of spray (e.g.,
where the helicopter pivoted to change.
direction). Thus, our target dosage oli
150 pounds of N per acre was
probably close to the maximum safe
dosage for the weather and tree
conditions at time of treatment. We.
concluded that a reduction in target
dosage is advisable if uniformity of
spray dosage cannot be assured.
Growth Response: Relative to
growth on control plots, volume
growth was increased 26 percent by
Forest
helicopter application of SUAN and
33 percent by the hand application of
urea prill (Table 2). Because the
spray-treated plots averaged only 97
pounds of N per acre compared with
150 pounds on the plots treated with
prill, we have expressed the results as
cubic-foot gain per pound of applied
nitrogen. Our assumption of linear
growth between N dosages of 97 and
150 pounds of N per acre is readily
supported by past research 7 1. Thus,
one pound of N as prilled urea
produced 0.25 cubic foot during the
first two years after treatment and one
pound of N as solution produced 0.29
cubic foot in a shorter period of
growth.
Treatment Costs: Including its first
operational spray fertilization in 1973,
the Bureau of Land Management in
Roseburg, Oregon, has spray-fertilized
nearly 2,000 acres through 1975.
Contract costs for SUAN and spraying
at a dosage of 1 50 pounds of N per
acre ranged between $33 and $58 per
acre. This is equivalent to $0.22 to
$0.38 per pound of N compared with
$0.20 to $0.27 for prilled ur a (Table
3).
Despite t he smaller acreages
invo l v e d i n past contracts for
spray-fertilization, costs per pound of
N applied as solution averaged only 2 1
percent m o r e t han for u r ea
fertilization. These higher costs of the
foliar application apparently reflect
higher application costs, since nitrogen
purchased as solution 32-0-0 averaged
only five percent more than prilled
,
-
Table 2
Average Two-year Volu me Growth of Douglas fir
Per Acre Basis on Operationa I Plots
Fertilized
-Lb. N/acre0
Initial
stand
volume
Annual!
growth
Two-year gain
/
/
Absolute
Relative
Per lb. N
-Cu.ft.-
-Percent-
·Cu.ft.·
-Cu.ft.-
-Cu.ft.-
!
206
54
-
-
-
97 as spray
164
68
28
26
0.29
150 as prill
190
73
38
33
.25
1/ Growth adjusted for initial differences in stand volume.
urea during this three-year period
(Table 4). During the 5-year period,
197 1 through 1.975, cost per pound of
N as SUAN averaged 1 1.5 cents; this is
two percent more than N from prilled
urea. Thus, cost of the solution
averaged 50 percent of total contract
price, compared with 58 percent for
prilled urea (Table 5).
These past costs indicate that the
o v e r al l b enefit s f r o m forest
fertilization with SUAN would need to
be about 2 1 percent greater per pound
of applied N to be as cost-effective as
fertilization with urea prill.
Our two- and four-year data from
foliarly ferti lized stands show
responses in volume growth that range
between 16 percent more to 52
.
Figure 3: Helicopter with 500-lb. payload (136 gallons of SUAN) was used to fertilize a
15-year-old stand of Douglas fir with 150 Ibs. N/acre.
percent less than those from fertilizing
the same stands with urea prill. Where
heavier dosages of solution severely
damaged f o liage, initial volume
response was probably depressed.
Although spray and prill both
increased mortality of smaller trees in
overstocked stands during the four
years after treatment, fewer than five
percent of the initial trees died. This
intensity of thinning is unlikely to
measurably affect residual trees. Thus,
we must await longer term growth
data to determine full benefits.
Alternately, we can improve the
benefit: cost ratio of spray treatments
by reducin g a p p l i cation costs.
Increasing the scale of spray projects
and using more efficient spraying
eq uipment should reduce costs.
Moreover , combining n it r o gen
solutions with herbicides which are
frequently used to reduce competing
vegetation in young conifer stands8
should reduce costs and increase
benefits. In western Washington and
Oregon, approximately 100,000 acres
per year are sprayed with herbicides in
a water carrier to release young
conifers from brush and hardwood
competition2• An additional 50 to 100
thousand acres of older stands of red
alder (a nitrogen-fixing tree species)
are also sprayed, but it's unlikely that
conifers in these stands would benefit
f r o m f e r t i lizer n itrogen.
Herbicide-fertilizer solutions also may
have use for intensively managed
Christmas tree production. Herbicides
are currently applied on 20,000 to
30,000 acres per year primarily for
grass control2; fertilizers are also used
on these Christmas tree plantations.
T7
forest
Table 3
Acreage and Costs of Operational Fertilization with
Urea-Ammonium Nitrate Solution and Urea Prill
Outlook
Our investigations establish that
Douglas fir and associated conifers can
be foliarly fertilized with concentrated
n i t r ogen solutions at reasonable
dosages. Application of concentrated
fe rtilizer solutions requires more
c r i t i c al selection of N sources,
additives, and dosages than does
fertilization with urea prill. In foliar
fertilization of Douglas fir, maximum
nitrogen dosage is set by anticipated
degree of osmotic burning. Until
practical effects of osmotic burning on
tree growth are better defined, burning
up to about 30 percent of the needle
surface appears acceptable. In our
experience, Douglas fir and western
hemlock show more susceptibility to
osmotic burning when new foliage and
twigs are in e a r l y stages of
development (spring) and in hot, dry
w ea ther (summer). M aintaining
balance between a large dosage to
extend duration of response and an
acceptable degree of osmotic burning
necessitates closel control of fertilizer
distribution than with urea prill. When
we bypass fixations of the soil system,
we also lose its integrating or buffering
capacity. With careful formulation and
applica tion to control the degree of
osmotic burning, practical dosages of
n i t r ogen can be applied as
concentrated sprays.
Our short-term results suggest that
when less than about 30 percent of the
needle surface is osmotically burned
by concentrated nitrogen solutions,
growth response in Dougl<\s fir and
western hemlock is similar to that
from urea prill at the same N dosage.
Development of a spray additive that
would reduce osmotic burning, yet not
d ecr e ase penetration of applied
n itrogen, is most desirable. For
example, a silicone-based surfactant
was particularly effective in preventing
leaf burn on bean seedlings and in
facili tating penetration of applied
nutrients lO.
Co mbining N solutions with
herbicides which are frequently used
to reduce competing vegetation in
young conifer stands should reduce
costs and increase benefits. We are
currently investigating interactions of
nitrogen solutions with herbicides in
terms o f (1) nitrogen solution
modifying herbicidal effectiveness on
both target and non-target vegetation
and (2) herbicides modifying the
uptake of nitrogen from the solution.
Prill (46-0-0)21
Solution (32-0-0)11
Year
Acres
Costs
Acres
Costs
-No.- -Per acre- -Per lb. N- -No.- -Per acre- -Per lb. N1973
1974
1975
Mean
203
200
1,544
649
$33.09
42.38
$0.22
.28
6,100
306
$40.75
52.00
$0.20
.26
57.68
.38
1,691
53.77
.27
44.38
.30
2,699
48.84
.24
l
/Total cost of solution (32-0-0), delivery, and application at 150 pounds of N per acre..
Source: Steve Wert, Soil Scientist, Bureau of Land Management, Roseburg, Oregon, October
22, 1975.
2
/Total cost of urea prill (46-0-0), delivery, and application at 200 pounds of 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.
Table 4
Costs of Urea-Ammonium Nitrate Solution (32-0-0) and
Prilled Urea (46-0-0), F.O.B. Portland
N cost per pound
Fertilizer cost per ton
Year
32-0-01
46-0-02
32-0-0
46-0-0
Ratio
1971
1972
1973
1974
1975
$ 43
49
61
96
120
$ 67
65
75
147
165
$0.067
.077
.095
.150
.188
$0.073
.071
.082
.160
.179
0.92
1.08
1.16
.94
1.05
Mean
74
104
.115
.113
1.02
1
Source: Allied Chemical Company, Portland, Oregon, December 5, 1975
2
Source: Average posted, delivered prices of a major producer. To convert to F.O.B.
Portland, shipping costs were estimated at $7 per ton and subtracted from the delivered
price.
Table 5
Average Cost of Fertilizer as a Percentage of Total Contract Price
Solution
Prill 1 1
II
Year
32-0-0
Contract
Percent
46-0-0
Contract Percent
- -Per lb. N- -
--Per lb. N- 1973
1974
1975
$0.095
.150
.188
$0.22
.28
.38
43
54
49
$0.082
.160
.179
$0.20
.26
.27
41
62
66
Mean
.144
.29
50
.140
.24
58
II Sources: See Tables 3 and 4. Actual cost of fertilizer not known for some contracts.
Forest
Further expansion of this work is
desirable.
Costs of nitrogen solutions and
their application have been and are
likely to continue to be competitive
with those for prilled urea. Hence, we
believe that foliar application of
conc entrated nitrogen solutions can be
an efficient alternative means for
fertilizing Douglas fir and associated
@
oo fu
Atkinson,
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