Water in the Soil – Plant – Atmosphere
In managing water in the landscape, it is useful to keep in mind the ecosphere
and the relationship of the environment to water demand. Simple observation
of plants in their natural environment will confirm that even in environs
with low rainfall rates and no irrigation, some plants will survive and even
thrive. This is possible because the different nutrition and water needs
of those plants are supplied by their environment. As people in an urban
environment manage their desired landscape, understanding the relationship
among the elements of the soil-plant-atmosphere continuum on water use can
help use water more efficiently.
The soil provides a porous reservoir in which water is stored. Water moves
through the soils under the force of gravity and towards evaporative demands
like drier soil and up through the roots of plants. In order for plants
to survive and thrive, the soil profile needs to be deep enough to store
sufficient water for the plants. Soil structure can help hold water in
the root zone or allow it to drain quickly below the root zone. Adding
organic materials to soil can help increase its water holding capacity.
The majority of roots are in the top 4 inches of soil, but roots from many
plants can descend through several feet of soil if it is available. The
deeper the soil profile, the more water can be stored. However, in sandy
soils, water may percolate below the root zone before it is used by the
plants; in rocky soils, including those with caliche, there can be reduction
in water holding capacity. The soil chemistry is also important for plant
health, as soils with too few nutrients or too many salts can limit the
types of plants which thrive. Often, additional watering is used by customers
who see plant stress caused by soil chemistry in order to try to support
plant health. Understanding soil depth and chemistry is the first step
to planning a healthy low water use landscape.
From the water perspective, plants move water from the soil to the air. The
plant benefits from this by also moving nutrients from the roots to the
shoots and leaves, and, on a hot summer day, by cooling the leaf surface.
A healthy plant needs a soil environment which has sufficient water holding
capacity, enough nutrients for plant growth, and a lack of soil pathogens
and chemistry which are antagonistic to the plant. A proper balance of
nutrients and water are necessary for plant health – each species
has different needs for these – and some species thrive better in
similar conditions. Some species need direct sunlight for part or most
of the daylight hours to thrive, while others do best in shaded areas.
The plants transpire water from the leaf to the atmosphere through stomata,
small openings in the leaves. During times of low water availability, the
stomata on many plants partially or completely close,, which lowers transpiration
rates but increases tissue temperature. Other species-specific responses
to water stress or low availability of water can include morphology like
leaf thickening, hairs, different colored leaves, nocturnal photosynthesis
processes, slowed growth rates, and, in some plants, wilting or leaf rolling.
Looking to the environment in areas nearby but outside the urbanized area
can give clues to the native species which grow without soil amendments or
irrigation in a region of the state. However, due to the disturbance of soils
during construction processes, native species may not thrive in the urban
environment. The Urban Landscape Guide database is available to assist customers in making plant choices. Utility personnel
are encouraged to find expert horticulturalists in the local nursery, landscaping,
and agricultural extension service that can also help customers make plant
choices, and answer questions that they have beyond the information available
on this site.
The driving force behind water use in plants is the conversion of carbon
dioxide from the atmosphere, nutrients from the soil, and energy from the
sun through photosynthesis. As seasons change, most plant growth rates
increase with increasing sunlight and temperature. Water use by the plants
rises as well. In addition to the water used by plants to move nutrients
through the plant, as solar radiation heats the plant, many species cool
the plant leaf surfaces by increased transpiration of water vapor. For
shade-tolerant plants, water use rates are typically lower during the hot
summer because less water is used for plant cooling. A landscape plan that
takes into account the amount of heat and direct sunlight that plants will
be exposed to can maximize water use efficiency by applying different amounts
of water to different parts of the landscape.
In planning for a low-water use or efficient landscape, all three of the
factors are important and should be taken into account: the soil type and
depth; plant types; and the availability or lack of shade.