is the general rule in forest settings and usually
light applications at seasonal changes can give Mother
Nature a helping hand. In urban landscapes with
already depleted nutrient levels due to original
topsoil removal, the continual removal of natural leaf
litter and organic materials further depletes soil
nitrogen, nutrients and soil microbial activity.
In the landscape
settings, it is here that a balanced fertilization
program needs to be established to recreate and
simulate a forest type Eco-system.
article Fertilizing Trees and Shrubs).
In the Texas Hill
Country, phosphorous, potassium and nutrients are
usually in the soils but bound up due to the high (pH)
alkaline type soils.
article Soil pH).
levels are usually sufficient in most soils for tree
growth. However, potassium is usually deficient in
soils of low organic matter. Applying high nitrogen
fertilizer with sulfur will lower the pH and increase
the solubility of the other nutrients. Iron supplement
may also have to be included in the fertilization
program. Although nitrogen levels are usually low in
forest settings our native trees have adapted to the
soil and environmental conditions
article on Iron Chlorosis).
grow in their own preferred soil and eco-systems,
giving us a variety of trees through our Texas
Studies have shown
that trees in the natural forest Eco-systems and with
natural low-level nitrogen adjust and grow reasonably
well. In fact, trees respond to their environmental
conditions and slow their growth rate to maintain a
healthy canopy and adjust their root: shoot ratio to
provide adequate nutrients.
There is no conclusive research indicating that
fertilizing trees will prevent, combat or cure
bacterial or fungal disease problems or ward off
The opposite has more truth, research has shown that
the slow to moderate growth is normal and desirable
for most trees. In depth studies show that resistance
to certain insect and diseases decreases in rapidly
growing trees and that such trees are more
nutritionally suitable to these predator pests. More
of the available energy is shifted to growth in
periods of new and increased rapid growth and less
energy is diverted to defense. Trees under low to
moderate stress and (slow growth, produces higher
levels of defensive pest inhibiting chemicals.
Cankerworms or Leaf Roller attack Live Oaks
immediately after bud break while the new leaves are
still palatable, tender and before the insect defense
toxins have fully developed. Forcing rapid green
growth with large applications of nitrogen is like
ringing the dinner bell for the wonderful array of
predator insects in love with your trees.
Fertilization decreases insect resistance of trees in
two key ways:
By increasing the nutritional value of the plant.
By decreasing concentrations of defense chemicals
known as phytochemicals or secondary metalolites.
alkaloids, terpenes, phenols and compounds that
release cyanide. Insects adapt to these defense
chemicals and usually feed on one or a few closely
related species. Studies have shown that
phytochemicals play key roles in protecting plants
from abiotic stress, including drought, heat, air
pollution and damaging radiation. Carbon plays an
important roll in the production and support of
defense chemicals. Fertilization increases tree growth
primarily and is dependant on carbon sources which has
to trans-located from existing leaves to support
active growth of leaves, shoots and stems. Exported
carbon from existing leaves, is unavailable to produce
or sustain defensive chemicals in those leaves.
Fertilization increased tree growth, with no effect
on photosynthesis, however, foliar nitrogen levels
increased and phytochemicals levels became
substantially lower in every case thus decreasing the
resistance of the tree to insects.
Oak wilt infects and
moves faster in a healthy tree than a tree that is
stressed or already in a weakened condition. It is
better to help a tree maintain a somewhat natural
health, than to just apply fertilizers at random and
with the intention that you’re going to make your tree
healthy with vigorous new growth.
Nitrogen deficiency is
most prevalent in sandy or silt type soils, poorly
drained soils and soils with low organic content.
There is a small input of nitrogen from the
atmosphere, and certain microorganisms can fix
(convert) elemental nitrogen from the air to forms
that can be absorbed by trees. Some nitrogen fixing
organisms are free-living in the soil while others
occur in specialized root nodules. Appreciable amounts
of nitrates (from acid forming nitrogen-based air
pollutants) and lighting also fixes smaller quantities
of nitrates that are deposited on the soils by the
rains. New leaves with sever nitrogen deficiency are
typically smaller than normal and appear relatively
green. Mature leaves however, are yellow. These
symptoms are typical in Post Oaks in early spring,
just after leaf out. The mobility of nitrogen in the
plant allows it to move from the older leaves to the
new, developing foliage. Nitrogen deficiency can be
corrected by adding nitrogen-based fertilizer.
Young trees need
increasing supplies of minerals to grow well. Nutrient
demand is usually met when root growth and soil volume
are unrestricted and soil is relatively fertile or if
nitrogen is applied. Older trees, on the other hand,
can adapt to reduced soil fertility by slowing growth.
However moderate to sever nutrient deficiency can
cause abnormalities and poor growth. Moderately slow
growth in mature trees is normal and desirable. As
trees grow older and larger, demands for minerals,
particularly nitrogen, to maintain growth and life
functions increases, while availability decreases as
minerals are increasingly bound in living and dead
tissue. Thus, the nutrient availability may not be
able to satisfy the demand of large, old trees. Such
trees may benefit from moderate fertilization, but the
over stimulation of mature trees with fertilizer can
result in excessive growth, reduced drought
resistance, and greater susceptibility to predator
pests and diseases, and additional maintenance cost.
Trees stressed by such problems as drought, poor soil
aeration, and inadequate light or root disease usually
to not respond to fertilization until these factors
article Trees are Vital).
There is a role for
prescription fertilization in a tree health care
program, especially in high maintenance landscapes.
However, fertilization programs should be implemented
with an understanding of potential consequences for
pest resistance and stress tolerance.
(See article Oak
“Diseases are created chiefly
when we destroy the harmony reigning among mineral
substances present in infinitesimal amounts in air,
water, food and, most crucially soil”
Dr. Alex Carrel, “Nobel Prize”