FERTILIZATION OF FRUIT AND NUT TREES
by Jerry Parsons, Ph.D.
Horticulture Specialist, Texas Agricultural Extension Service
in San Antonio
March is an ideal time to fertilize fruit and
An annual application of fertilizer is critical
to the maintenance of healthy, productive fruit trees. The
fruit plants must achieve sufficient growth each season to
replenish the food reserves exhausted the previous year.
Carbohydrates and other food materials required
to develop fruit on the plant are produced in the leaves by
photosynthesis. When the plant is under fertilized, it produces
fewer leaves and fruit production is often limited. Therefore,
it is important that fruit plant leaves are dark green and
there is adequate annual shoot growth. Yellowish or scorched
leaves and a deficiency of shoot growth normally indicate
What kind of fertilizer to use---organic vs.
chemical---is often confusing.
The advantages of one type of fertilizer over
another have been debated for many years. The debate probably
will continue for some time as long as people won't admit
that there is a place for more than one type.
It is well known that chemical forms of nitrogen
are very readily available to plants. As soon as ammonium
or nitrate fertilizers dissolve in the soil solution, they
can be absorbed by many plants. Thus, they are extremely useful
in rapidly correcting a deficiency of nitrogen in the soil.
However, nitrates are readily washed out of the soil since
they cannot be absorbed (fixed) on the surface of the clay
particles. Therefore, watering or rain leaches them away easily.
Ammonium is absorbed to considerable extent, but the free
form (not fixed) of ammonium is rapidly changed to nitrates
through chemical oxidation by certain soil organisms and can
be lost through leaching (washing of nutrients from the soil).
Organic nitrogen sources are slower to act--a
fact which appears to be a rather serious disadvantage at
first glance. The nitrogen in most organic sources is in the
amino acid or protein form and must be changed by enzymes
in the bodies of micro-organisms to the ammonium form of nitrogen.
This process is called ammonification and is brought about
by a number of micro-organisms including bacteria and fungi.
The ammonium thus formed can be changed to
nitrates as described above by two groups of bacteria. One
group converts the ammonia to nitrites and the other group
changes nitrites to nitrates. Oxygen is needed for both of
these nitrification processes, hence, the desirability for
good soil aeration. The ammonium can change to ammonia and
escape as a gas, resulting in a loss of nitrogen from the
soil and, of course, is no benefit to plants. Sometimes under
conditions of poor soil aeration, nitrates or ammonium are
changed to free nitrogen which escapes into the atmosphere.
Plant growth is generally poor under such conditions because
of limited aeration and nitrogen deficiency (water plants
apparently tolerate the conditions of limited oxygen).
These processes related to the change in the
form of nitrogen take time and proceed very slowly when the
soil is cool. When soil temperatures are cold (below 50 degrees
F.) an application of organic nitrogen might take a long period
of time to have any visible effect in improving a condition
of nitrogen deficiency of a plant, whereas a chemical nitrogenous
fertilizer would correct the trouble at once.
The organic forms of fertilizer are considerably
lower in analysis than the chemicals. As such, more of the
organic material must be applied to obtain the same amount
of fertilizer per unit area. In some instances, it is impractical
to do this because the bulk of material would be too great
- - 20 pounds of manure for 1 pound of ammonium sulfate.
Because of their high analysis, quick solubility,
and rapid availability, chemical fertilizers must be applied
carefully. Overdoses cause the concentration of fertilizer
in the soil solution to be greater than that in the plant,
resulting in withdrawal of water from the roots. In severe
cases, this induces wilting, or death. In milder cases the
growth is stunted because of root injury.
Organic materials such as peat, rotted manure,
etc., have long been known to improve soil structures. This
can take place either through actual dilution of soil by particles
of organic matter that themselves possess an environment favorable
for root growth (lumps of peat hold air and water) or by furnishing
food for microorganisms which secrete glue-like substances
that cement the soil particles into granules of aggregates.
The addition of chemical fertilizers does nothing
to promote soil improvement in this respect. Organic fertilizers
promote these beneficial effects, but the amounts added are
rather small so that their influence in this regard is not
too great. No one should hope for continued improvement or
maintenance of favorable soil structure by use of organic
fertilizers alone - - incorporation of large amounts of organic
materials (not fertilizers) are needed for this effect.
Because soil conditions vary greatly from place
to place a generalized, blanket recommendation can be only
used as a general guide. However, because of the abundance
of potassium in this area's native soil and because of the
build-up of phosphorus which occurs after several seasons
of fertilizing with a complete fertilizer such as 10-10-5
or 15-10-10, a high nitrogen fertilizer can be recommended
as the best for most growers. To continuously use a balanced
fertilizer in gardens and around trees invites the occurrence
of nutrient deficiencies of minor elements such as iron and
zinc. If you don't believe it, look how many yellowing plants
from iron chlorosis are ever-present in this area - - we do
not need to accentuate the problem with improper fertilization
Peach and plum trees should receive one-half
pound or 1 cup of ammonium sulfate (21-0-0) per year of age
or per inch of trunk diameter with a maximum limit of 5 pounds
per tree. In addition, mature trees should receive an additional
cup of ammonium sulfate in late August. First and second year-old
trees should receive one-half cup of ammonium sulfate during
the months of April, May and June.
Apples, pears, and persimmons, normally require
little fertilization. This is to prevent the stimulation of
excess growth which is very susceptible to the disease fire
blight. Severity of fire blight infection can also be decreased
by applying a cover spray of streptomycin or Kocide to apple,
loquat and pear trees immediately before, during and after
the bloom period. The trees should make 6-12 inches of annual
growth, otherwise nitrogen fertilizer will be required. Follow
the formula above for peach trees if growth is poor.
Fertilizer requirements for fig trees should
be gauged by the amount of growth made in the previous year.
The shoots should grow about 12-18 inches each year. In general,
one-half pound of ammonium sulfate per year of tree age should
be applied with a maximum limit of 10 pounds.
Pecans require one pound of ammonium sulfate
per inch diameter of tree trunk of established trees. Fertilizers
with zinc added are of little benefit in alkaline soils- -
zinc will have to be added to the tree foliage by spraying
zinc sulfate or NZN solutions directly on the leaves.
Blackberries should be fertilized twice each
season; in early March and when harvest is completed, usually
in June. About one-half pound of ammonium sulfate fertilizer
scattered around plant is recommended for each application.
Grapes should receive one-half pound of ammonium
sulfate per vine depending on the vigor of the vine. Overly
vigorous grapes are undesirable because they are not as productive.
When applying fertilizer for trees around which
lawn grass is growing, it is advisable to apply a little extra
to take care of the fertilizer utilized by the grass. You
may also want to punch four-inch deep, randomly spaced holes
within the canopy drip line of the tree in which to distribute
the recommended quantity of fertilizer if phosphorus is needed.
However, this is not necessary for nitrogen only applications
since tree roots will uptake surface nitrogen applications.
The important consideration to remember is
that plants, as most living things, need food to grow and
produce. The "food" of plant life is fertilizer
and it must be available to the plant when needed if optimum
growth and production are expected. In addition the plants
do not care nor does it matter whether the source is organic
Research on pecan trees has shown that nitrogen and zinc are
by far the most needed elements. Nitrogen is always needed
in combination with deep, well drained soil, irrigation with
clean water, and frequent (every two weeks) early season zinc
foliage sprays for excellent tree growth and regular crops
of high quality pecans. Zinc and nitrogen are not the same.
Nitrogen is applied to the soil as a commercial fertilizer
while zinc is sprayed onto the leaves in early spring. Soil
applications of zinc or foliar applications of nitrogen are
not as effective.
Too little or no nitrogen results in poor vigor
or no tree growth. Too much nitrogen can result in excessive
growth on large trees and late season growth on young trees.
Excessive growth on bearing trees uses plant food and can
reduce the nut producing capacity of the tree. Keeping this
in mind, bearing trees should not make over 12 inches of shoot
growth on non-bearing terminals. Also, young trees should
stop growing in September to prevent freeze injury in October
The perfect situation is to have young trees
make fast growth in April, May and June, moderate growth in
July and August and stop growth in September.
On mature trees, shoot growth and leaf expansion
should occur in April, May and early June.
On young trees, fertilize only once the first
year in late May or early June. Salt burn on the leaves can
occur if fertilizer is placed in the planting hole or applied
around the trees before roots are established and pecan roots
begin growth late in the spring.
From the second to seventh year, apply 4 light
applications, one each month in March, April, May and June.
Do not apply the first applications until the trees start
to grow. Nitrogen burn or small black leaves can result if
nitrogen is applied too early. Never apply nitrogen after
the month of June to young trees because it can encourage
late season growth and result in severe freeze injury in November
The first year apply one cup or ½ pound
of ammonium sulfate (21-0-0) to the tree in late May 18 to
24 inches from the tree. Never apply granular nitrogen closer
than 12 inches to the trunk. If the tree is not making good
growth by mid-June, do not fertilize the first year.
The second year make March, April, May and
June application of 1/2 pound of ammonium sulfate fertilizer.
The third, fourth and fifth year apply 1 pound
in March, April, May and June.
The sixth and seventh year apply 2 pounds in
March, April, May and June.
As the trees become larger, use 1 pound of
ammonium sulfate per inch of trunk diameter. Split this amount
into applications-March and May. Broadcast this fertilizer
toward and slightly beyond the drip line.
Very few cultural practices will give better
pecan tree growth responses than good fertilization. Nitrogen
fertilizer is essential for good tree growth. Consistent annual
nitrogen fertilization is very important in insuring regular
nut crops year after year.
Most of the fertilizer recommendations above
call for ammonium sulfate (21-0-0). It is readily available
and fairly inexpensive. In the home landscape with few trees,
the slow-release fertilizer formulations such as 19-5-9 and
15 -5-10 can be substituted for ammonium sulfate for the first
application. The use of a slow-release formulation will reduce
the risk of plant damage by accidental over-application of
fertilizer and will also not burn plants growing near and
around the target tree. Regardless of how it is applied or
what type is used, remember that fertilizer is the tree's
food. Don't let your tree go hungry this year.
For more information about fertilizing fruit
and nut trees, see: