Predicting Plant Water Uptake
March 6, 2012 
Understanding plant water use begins with substrates, which serve as the water reservoir for plants. There are a few basic concepts that will help you understand the impact that substrate selection has on plant water availability. Container capacity (or water-holding capacity) is the maximum amount of water a substrate can hold.
To calculate the container capacity of your substrate, fill a container with dry substrate and seal the container holes. Once you measure the amount of water you need to add to completely fill the container, place the container over a catch basin and remove the seals. Allow the container to drain for one hour and measure the amount of water that drained from the substrate. The container capacity is: (volume to saturate-drainage) divided by volume of container. Then, multiply that figure by 100 percent. A container capacity of 60 to 70 percent is typical for soilless substrates.
Another important concept is total pore space. Pores are the gaps between substrate particles. Large substrate particles, like bark or perlite, create large pores while small particles, like peat or coir, create small pores.
The total pore space can be calculated by dividing the amount of water needed to fill the container by the total container volume. A total porosity between 80 and 95 percent is typical for substrates.
Finally, air space can be calculated as total porosity minus container capacity, and is typically between 10 and 20 percent. If the air space is less than that, the substrate is likely to become waterlogged, which can result in root diseases.
Generally, substrates with a lot of peat or coir will have a high water-holding capacity, while substrates with a lot of bark or perlite will have a low water holding capacity. This is due to the different sizes of pores that are created in substrates with peat and coir (small pores) versus bark or perlite (large pores). See Figure 1 for details.
Large pores are normally filled with air, not water, and provide oxygen to the roots. Medium-sized pores may be filled with water that can easily be extracted by the roots. Roots may not be able to extract water from very small pores, however, because water may be too tightly bound to the substrate. Good substrates need a mixture of large and medium pores to assure both aeration and adequate water availability.
Water Movement In Plants
Roots absorb water in substrates primarily via bulk flow. When water is taken up into the root, it leaves a vacuum in the pores surrounding the root. Other water molecules fill those pores before eventually moving into the root.
Once water is in the root, it moves inside the plant through the xylem, the system for transporting water throughout the plant. Xylem cells are non-living and resemble narrow straws. Because xylem connects all parts of the plant (from the roots through the stems and into the leaves), the water in the xylem creates a giant chain of molecules that are attached to each other, as well as the xylem.
To transport water from roots to the leaves for use in photosynthesis and other plant functions, it is important to involve another plant structure called stomata. These are tiny pores in the leaves (Figure 2) that plants can open and close as needed. If they are open, water moves out of stomata and evaporates into the air. This occurs because water molecules diffuse from the moist air inside the leaf to the much drier air surrounding the leaf. When that happens, the chain of water will be pulled from the roots all the way up to the leaf.
This is important because water is involved in a number of plant processes. Water is one of the reactants needed for photosynthesis. It transports nutrients throughout the plant and is the driving force for cell elongation and, ultimately, plant growth. This explains why many growers use drought to reduce shoot growth.
Water also helps regulate leaf temperature. Transpiration helps cool the leaves in the same way our bodies regulate temperature through sweat. When water is limited, plants generally favor root growth over shoot growth to help them take up more water from the substrate while losing less water through transpiration. Growers who produce plants for drought tolerant landscapes can take advantage of this; growing their plants relatively dry may help improve plant performance.
Factors Impacting Water Uptake
Water uptake is impacted by quite a few environmental factors. Understanding the effects is helpful in determining when to irrigate and how much water to apply to plants. The primary environmental factors are light, relative humidity, temperature and wind.
We have found that light is one of the environmental factors that has the strongest impact on water uptake. There are many reasons for this, but light impacts water primarily because high light levels increase water loss through stomata.
Vapor pressure deficit (VPD), a measure of the “dryness” of the air that is calculated from relative humidity and temperature, is the driving force for water loss from leaves. VPD and water use increase with increasing temperature and decreasing relative humidity.
When there is less water in the air (high VPD), more water will evaporate from the leaf into the air, pulling that chain of water through the xylem all the way from the root to replenish the water in the leaf. If it is humid (low VPD), however, the air is already so full of water that less evaporates from the leaf. VPD differs regionally (higher in the Southwest compared to the Southeast), and this causes plants in the Southwest to use more water.
Since VPD is so important in determining plant water use, it can be used to automate irrigation. VPD controllers constantly monitor and accumulate VPD and irrigate when a particular threshold has been reached. VPD controllers can help adjust irrigation for different weather conditions but do not take into account the changing size of the plants. To get the full benefits from these controllers, adjustments should be made as the crop grows.
Another factor impacting water use is wind. Higher wind speed strips water away from open stomata, increasing water loss from leaves. While it is easier to imagine the effects of wind outside, wind does affect water uptake in greenhouses. Fans used for pad and fan cooling and horizontal air flow fans can create wind that causes plants close to them to lose water more rapidly. There will also be small wind currents on the edges of benches that, combined with the effects of low relative humidity already described, make drier spots on benches or in beds.
There can be substantial spatial variability in water use throughout a greenhouse because humidity, temperature, light and air movement often are not uniform. And even among plants on a single bench, water use can differ. Plants on the edge of a bench will generally dry out before plants in the center. The center of the bench tends to be more humid and less windy. Additionally, mutual shading of plants in the center of a bench reduces their water use.
Understanding the impact of environmental factors on crop water use is important in scheduling irrigation. Better irrigation practices have many benefits, including less fertilizer runoff, reduced disease pressure and improved plant quality. The bottom line is that understanding substrate and plant water relations can improve your bottom line.
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