Fall 2015: A simple approach to knowing when to irrigate ornamental potted crops
Regional Report for Santa Cruz and Monterey Counties by Steve Tjosvold
A grower knows that a wilting plant is a sure sign of a need for water. But a grower also knows that wilting is bad for the plant. So at what point before wilting occurs should a grower water plants? To figure out how that process might be improved, think about what a grower normally does to evaluate a plant’s need for water. If it is a potted crop, he or she intuitively picks up the pot and feels its weight! The grower is sensing how much water is in the potting soil. If a pot feels heavy with water it may not need water, or if that the pot feels relatively light it probably needs water.
This process could be improved by actually measuring the weight of the plant container. There is a nifty relationship; 1 milliliter of water weighs 1 gram. So the weight change in grams, with no intervening irrigation, represents the milliliters of water lost from the container during that time (Fig 1). It is important to select representative plants when measuring water use by any method. Large plants tend to use more water than small ones, and those on the borders of fields, benches, or nursery blocks tend to use more than those in the interior. It is probably better to use select plants that might use relatively more water to meet at least the water requirements of these plants. Since accurate scales can be purchased for less than $100, this method provides a fast, accurate, inexpensive way to monitor plant water use. Try a scale that is accurate to a gram for 1 gallon or smaller pots. One that is accurate to 5 grams might be used for larger pots.
Figure 1 Plant water use in container nurseries can be measured directly using a scale. Photo S. Tjosvold
Not all the water in the soil is available for use by the plant. After the pot has been fully watered, the water is readily available to the plant. But as the plant uses water and the soil dries, the water held by the soil is less available. Eventually the plant wilts. To determine the total amount of water that is available to the plant, first measure the weight of a representative pot just after it is fully watered and drains. Then measure the weight daily until the plant just starts to wilt. The difference between the beginning and end weights in grams (and therefore milliliters) is the available water.
So when should water be applied and how much? Generally, irrigation is initiated when one-half of the available water is used. This minimizes the energy that is needed by the plant to extract water and the hazard to roots by concentrating salts in the soil solution. So if there are 300 mililiters of available water in a pot, then irrigation should be made when 150 mililiters of water has been used. (That occurs when 150 grams are lost as you are measuring with your gram scale).
How much water to apply to each pot? In this case, the pot needs at least 150 mililiters to replace water the plant used. The grower is probably thinking about the total water that needs to be applied to a particular irrigated block (perhaps the total time that the sprinklers or drip irrigation will be on). There are couple more factors that should be considered.
Salts in the soil solution can concentrate because not all them contained in irrigation water are extracted by the plant. This can cause harm to the plant, so a little extra water is usually needed to “leach” salts out the bottom of the pot. Leaching is best achieved on crops by applying the proper leaching fraction. This is the ratio of the volume of water leached out of the bottom of a pot to the volume of water applied to a pot. This factor depends on the salt concentration of the irrigation water (with soluble fertilizer) and the tolerance of the plant to salt. Often 10 to 15% more water has to be applied just to take into account the need to leach. Editors note: Please see Richard Evans UCNFA article for more information on the management of salinity.http://ucanr.edu/sites/UCNFAnews/Feature_Stories/Leaching_to_Manage_Salinity_in_Ornamental_Crops/
Another important factor that will affect the final amount of water to apply is the efficiency of the irrigation system to apply water uniformly to a nursery crop. The irrigation system’s distribution uniformity is commonly calculated using measurements made in the field. But in general, sprinkler irrigation is relatively inefficient and drip irrigation is relatively efficient in distributing irrigation water to plants in a field. For sprinkler systems another 10 to 60 % more water might be needed to account for these inefficiencies, while a drip system might only need up to 10% more water. Editors note: Please see the chapter “Irrigation Management Practices” in Container Nursery Production and Business Management Manual
So the sum of the water used by the plant (P) plus the water needed for salinity (S) control plus the water needed to account for sprinkler irrigation (I) inefficiency is equal to the total water that is applied. That is, P+S +I = total water that must be applied.
Growers that have used this method often find it to be enlightening. At the very least, they find that it supports an already successful irrigation program. It often helps fine tune irrigation practices. It often points to the importance of irrigation systems as a crucial factor in using water efficiently. Because of the diversity of crops and crop water needs, it might be best to start with similar plants and pot sizes. Use the method through a crop cycle (from placing the pots in the field or greenhouse to establishment) and see if generalizations can be made and followed, and then maybe move on to another group of similar crops and pot sizes.