Fertilizing Trees & Shrubs
With years of research and reports, we still are unable
to unanimously agree on the importance of tree and shrub
fertilization. When it comes to applying fertilizers,
there is a lack of consensus among landscape and tree
professionals whether to fertilize: what are the
benefits and needs of the trees, shrubs and soil.
Fertilizers are applied as preventive medicine,
supplementing mineral elements in an effort to ward off
marauding insects and disease pests. Also they are
applied as a rescue treatment for plants besieged by any
number of biotic and abiotic stresses. Still others
adopt a policy of non-intervention, avoiding the
application of fertilizers altogether.
Forest trees are able to thrive, usually without
additional fertilizer. In their natural setting, trees
benefit from the accumulation of layers of leaf litter
and natural organic matter on the forest floor. Nitrogen
and nutrient levels do fluctuate just after leaf fall,
both in the autumn and early spring. The soil microbes
during increased organic decomposition temporarily tie
up soil nitrogen. With the rains in fall and early
spring, this also acts as a boost to the nitrogen
depletion by volatitilization and leaching. Although
nitrogen levels are low in most natural forest
Eco-Systems, trees slow their growth rate to sustain
acceptable green and healthy looking foliage over all.
(See Nitrogen article) light
application of a high nitrogen fertilizer
(21-0-0 ammonium sulfate or 36-6-6 miracle grow)
will help supplement for Mother Nature at these low
nitrogen intervals.
A
balanced availability of nutrients is one of the most
important factors affecting tree vitality.
An
unbalanced nutrient status can predispose a tree to
disease or insect damage. The resistance of trees
against insects has been found to be dependent on
protective compounds, the concentrations of which are
dependent upon tree nutrition.
Fertilizer cannot remedy poor growth resulting from
causes other that nutrient deficiencies such as soil
compaction, poor drainage, restricted rooting areas,
asphalt or landscape asphyxiation, mechanical injury, or
turf competition. The only valid reason to fertilize
landscape plants is to correct nutrient deficiencies.
Urban
soils are strongly affected by construction development,
human activities and by various landscape practices.
These impacts adversely affect tree growth, vigor,
longevity and general overall appearance. Top soils of
high organic content and microbial activity is routinely
removed during construction and the sub-soils become
severely compacted. The result is a hard, nearly
impenetrable, poorly aerated, nutrient-poor root
environment with reduced water holding capacity and with
restricted water and air penetration. The soils that are
replaced for the landscaping process are usually void of
organic material and microbial life. In simple terms
landscapers are just adding even greater woes that are
near to impossible to remedy. This will take a barrage
of fertilizer and organic applications and soil
management to restore the soils with microbial life and
conditions conducive to providing healthy tree and shrub
development.
We
must preserve balance without impairing growth.
We must encourage growth without destroying balance.
W. R. Nixon
Trees and shrubbery can be mulched to help restore
microbial life and moisture availability. A moderate
fertilization program with light and repetitive
applications will in time restore or at least somewhat
simulates a forest type eco system. For our Hill Country
type soils, approximately four applications, Dec., Feb.,
May, and July, using ammonium sulfate
21-0-0 or a Miracle Gro 36-6-6 is sufficient.
Additional Iron supplement such as
Iron Plus may also have to be applied at
some point during the year.
(See article Iron Chlorosis).
Apply according to manufacturer recommendations.
However, a light application and more frequent
applications provide optimal benefits.
Over
fertilizing trees, encouraging rapid succulent growth
can promote insect and disease attacks. Studies have
shown that resistance to certain insects and diseases
decreases in rapidly growing trees and such trees are
more nutritionally suitable to some pests. In simple
words, the faster the growth, the less defense ability,
there is. When conditions restrict growth, more energy
can be diverted for defense. Trees under moderate stress
produce higher levels of defensive
(pest inhibiting) chemicals.
Trees
naturally grow in response to their environmental
conditions, adjusting root growth and mass to provide
adequate nutrients. In deficient soils, roots increase
mass and area and the canopy is in ratio to those
particular conditions. In nutrient rich soils, the root
mass will occupy a smaller volume area and the canopy
would respond with greater mass volume relative to the
available nutrients.
Trees
do not obtain energy directly from mineral nutrients in
the soil. Sunlight energy along with carbon dioxide from
the air, water and oxygen from the soil converts
chemical energy (sugar)
during photosynthesis. During Transpiration, glucose is
broken down to release stored biochemical processes.
Mineral elements are the basic building blocks for new
growth and cellular function. Mineral elements, usually
positively charged particles or ions from organic matter
decomposition and fertilization are dissolved in the
water and absorbed by the roots as ions The negative
charged particles are anions, and it is this positive
and negative property of each ion that affects its
behavior in the soil.
(See article Cation-Exchange
Capacity),
and
(See article Trees are
Vital)
As
trees age, we see distinct changes in their ability to
respond to stress. Young trees have a high ratio of
photosynthetic (leaves) to
non-photosynthetic (woody)
tissue. Consequently for their size, they produce a
relatively high amount of energy. Healthy young trees
produce enough energy for growth and abundant storage.
They tolerate environmental changes and maintain
treatments. In contrast, mature trees have greater
demands on their energy supply. They utilize some energy
for growth, but a larger percentage is used just to
maintain the massive amounts of existing woody tissues
in the trunk, branches and roots. Additional energy is
needed to seal wounds that occur from wind breakage,
insect and disease attacks and for the development of
reproductive structures. Because of their tighter energy
budget, mature, healthy trees are in a delicate balance
with the environment. The key to preserving this
balance, and their health, is to maintain environmental
stability around mature trees.
Comprehensive tree care programs should always be
considered long term, beginning before planting and
continue throughout the life of the tree. Practices
should be geared to mesh with natural changes and
development and to be modified to meet the trees ability
to withstand and cope with change.
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