Environmental Engineering Reference
In-Depth Information
provide anchorage for the plant and a source of water and
minerals for dissolution and uptake. After all, many flower-
ing plants can be grown hydroponically, where the water is
essentially a liquid culture. While certain terrestrial plants
can be grown successfully solely in water containing the
right concentrations of minerals, this situation occurs only
commercially. The only exception is the common duckweed
(
Lemna spp.
), which actually spends its entire life as a free-
floating, hydroponic plant. We also have seen that individual
disciplines have focused on water from the perspective of
plant physiology, hydrogeology, and meteorology, which
has led to various terms to describe the status of water.
This was partly handled by the introduction of
The type and texture of soil where a plant grows is
important and should be investigated as part of phytore-
mediation site-assessment activities, which are discussed in
Chap. 6. For example, clay-rich soil may have higher water
content than sandy soil, but less water availability, whereas
the sandy soil may have less water content but more water
availability. The availability of water to move though soils
to roots is based on the hydraulic conductivity of the soil or
sediment. The magnitude of this hydraulic conductivity is a
function both of the soil type and given water potential. As
soil dries out, the water potential becomes more negative as
air replaces water in the soil pore spaces, which results in
decreased hydraulic conductivity.
The concept of soil-moisture storage is a central concept
to most plant physiologists. Even plants with shallow roots
that rely on precipitation obtain water not from direct infil-
tration, because it is too much to use all at once, but from
what remains in the soil after precipitation ceases. If the
amount of water input is immediately lost, there is no soil-
moisture storage at all. If the amount lost is less than the
amount of water input, then storage can occur, similar to the
relation presented in Chap. 2:
the
integrating concept of water potential.
For plants to survive in soils, a balance is needed between
the ability of the soil to retain water, transmit water, and to
permit aeration. These somewhat mutually exclusive goals
are described by the relative amounts of capillary and
noncapillary pore spaces in a soil sample. Assuming that
half the volume of soil is solid material, the balance is made
up of pore spaces. If the space is less than 60
m, water can
be held against gravity by the capillary forces of adjacent
water molecules as previously discussed in Chap. 2. This
water usually is referred to as field capacity, or the water left
after drainage caused by gravity. Pore spaces larger than
60
m
Soil moisture storage
SMS
ð
Þ¼
inputs
outputs
or
SMS
m
m do not permit water retention, but although this may
seem problematic it actually is essential to provide the intro-
duction of air to respiring roots. Equal proportions of capil-
lary and noncapillary pore spaces are best for water retention
and aeration. As would be expected from just casual
observations, the proportion of capillary to noncapillary
pore spaces differs dramatically for different soil types.
This is because of the difference in soil composition and
texture.
Root hairs can take up water through diffusion and osmo-
sis as long as the water potentials are lower inside the root
hairs relative to the water potential of soil water. However,
the continual intake of water to supply transpiration
demands also is dependent, ultimately, on the supply of
water in the soil as moisture, capillary fringe water, or
groundwater. As would be expected, if the resupply of
water to representative soil is eliminated, the water
potentials of the soil decrease. For a given soil type, the
water potentials can decrease during midday, and plant
cells lose water pressure and wilt. This situation reverses at
night when transpiration decreases as the atmospheric
demand for water decreases, and the plant water potentials
return to equilibrium with soil water potentials. If water
potentials decrease to a maximum negative level, usually
1.5 MPa, however, the wilting can be permanent, even at
night. The exact maximum water potential varies from spe-
cies to species but also is dependent on the soil type.
¼
transpiration
þ
runoff
þ
drainage to groundwater
Þ:
Precipitation
ð
Evaporation
þ
(3.12)
As seen in Eq.
3.12
, drainage to groundwater is not
considered a source of water to soil moisture, but represents
a loss. In contrast, however, plants can use water from the
capillary fringe and water table, not only soil moisture. This
is crucial in the phytoremediation of contaminated ground-
water, as we will see.
3.5.6 Radiation Balance
The energy used to type this sentence as well as to read it
ultimately came from the sun. Fusion reactions transmit
energy from the sun, in the form of light and heat, to the
earth. This light energy is captured as chemical-bond energy
in the structural parts of plants. This energy is released
perhaps most obviously when the parts of a plant, like
wood, are burned. The stored energy is released as light
and heat as the chemical-bond energy is broken down in
the presence of oxygen. At much slower rates, organisms
that consume plant material use chemical-bond energy to
drive their own metabolism and release nutrients back to the
plants to support their growth.
Search WWH ::
Custom Search