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Texas AgriLife Extension Service Texas Water Development Board Texas Nursery and Landscape Association Texas AgriLife Extension Service Texas AgriLife Extension Service
A supplement to the Best Management Practices Manual produced by the Texas Water Conservation Task Force.

Water in the Soil – Plant – Atmosphere Continuum

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
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.

"Landscape planning and design is essential to water use efficiency..."

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