Plant Selection, Installation, and Management to Affect Groundwater - Introduction to Phytoremediation of Contaminated Groundwater - page 179

materials such as lime, a calcium oxide, CaO, formed when
seashells or limestone, CaCO 3 , are heated to 900 C. Lime
addition acts as a soil buffer in that it can liberate hydroxyl
ions as

transport. They found that the maximum observed height
of such trees of 410 ft (125 m) is a function of the maximum
pull of water against gravity and the frictional resistance to
additional flow provided by the xylem surfaces.

Ca 2 þ ð aq Þ þ 2OH ð aq Þ

CaO ð s Þ þ H 2 O ð l Þ ! (7.1)

7.3.4 Soil Moisture Composition

Site-assessment and characterization activities should
include a determination of the chemical composition of the
soil profile before trees are planted. This soil chemistry can
be evaluated by using field test kits or having soil samples
analyzed in a laboratory. Local county or state extension
offices generally provide an excellent starting point for this
type of information. A good thing about site-assessment and
characterization activities is that deficiencies in native soil
can be overcome, although at some cost to the project.
Fertilizer also can be added to the soil during the initial
planting or after the plants have become established. Addi-
tion prior to or during planting can occur either as an amend-
ment to the soil used for backfill, or as the complete removal
of old soil with replacement by an amended top dressing or
fill. Addition to established trees is more difficult, but can be
accomplished with a drop spreader of dry fertilizer or by
injection of liquid fertilizer near roots. The fertilizer can
be added at the beginning of the dormant season to stimulate
root growth or at the end of the dormant season to encourage
shoot growth. For cuttings the fertilizer is added to the
backfill placed near the plant, but for an established tree
the fertilizer is added away from the main trunk to near
the drip edge of the plant's lower branch extent. Holes can
be created with a length of steel rod, filled with fertilizer, and
then water and soil. For small trees, such as 6 in. (15 cm) or
less in diameter, the rate of application using this method is
about 2-3 lb/in. of diameter. For larger trees, the dosage
is increased to 3-5 lbs/in. of diameter. This rate is for a
balanced fertilizer for gardens and trees, such as 10-10-10
(N-P-K), rather than the unbalanced lawn fertilizers that are
high in nitrogen only and commonly applied to lawns and
stimulate shoot growth at the expense of root growth. More-
over, some types of fertilizer can be injected directly into the
tree, but as this wounds the plant, it is best done only as a last
resort.
One of the indications of overall plant health is height
relative to genetic potential. The relation between environ-
mental factors and tree height has been examined for many
years. The controlling factors that have been offered include
resource availability, reduction in stress, and access to light.
One of the more interesting factors is access to water and the
ability to transport water to great heights above land surface
where maximum light levels are also found. Koch et al.
(2004) examined the water-transport characteristics of
coast redwoods ( Sequoia sempervirens ) in relation to maxi-
mum height

The necessary reactants plants need to survive are in limited
supply. This includes CO 2 and nitrogen above ground, and
water and nitrogen below ground. In part, the limitations
below ground are overcome by the movement of water
through plants as part of the hydrologic cycle. The flow of
water couples the plant's requirement for water with nutrient
acquisition from dilute sources in the subsurface. Notice that
the terms flow and movement were used in reference to
subsurface water. This is because the presence of water in
soils or sediments as soil moisture does not always mean that
it is available to plants. Water held by soil particles under
tensions higher than can be removed by plant root-hair cells
results in the absence of water uptake.
This does not mean that soil moisture should not be
measured as part of site-assessment and characterization.
Soils that have too low ( 10%) or too high (100%) soil

moisture can be detrimental to some plants. At sites where
phytoremediation is being considered, low soil-moisture
levels from little annual precipitation could detrimentally
affect plants installed in that water may be limited prior to
the ability of the roots to penetrate the water table. Under
such circumstances, irrigation systems can be installed to
alleviate the water limitations until root growth is
established. Irrigation would be warranted if soil moisture
fell below 80% of field capacity, or when the soil water
reached tensions between

<

0.1 MPa. One way
to determine the magnitude of low soil moisture and its
effect on plant uptake is to use a nest of tensiometers to
provide the necessary soil-water tension data for deciding
whether or not to irrigate—some automated systems can be
used to remotely control irrigation.
The irrigation system designed to overcome low soil-
moisture conditions can be as simple as a water hookup
and sprinkler or as complex as a built-in drip irrigation
system. A drip irrigation system used in this context would
provide a uniformly wet soil horizon rather than a means of
simply limiting evaporation. Short, frequent irrigation could
result in healthy growth but produce a shallow root system.
Less frequent but longer irrigation periods, about 1 in./h,
would be sufficient to deeply wet many surficial soils to
encourage the roots to grow to deeper soil layers. Some
researchers who installed irrigation systems observed that
the roots of planted poplar trees tended to remain shallow
and used the irrigation water rather than groundwater (Van
Epps 2006). Some phytoremediation sites where poplar

0.05 and

relative to physical constraints on water