You need to know how much water your irrigation system delivers in a given amount of time.
Automating irrigation based on plant needs is an option that can save a tremendous amount of water.
Water issues are a major concern around the world. Population growth and increased urbanization put increasing pressure on water supplies, making it increasingly important to use water more efficiently.
It’s not just the quantity of water that matters, water quality is important as well. Excessive irrigation inevitably results in leaching of water and fertilizer. This results in runoff that can end up in the ground or in surface water if it is not captured on site. Fertilizer runoff, especially nitrogen and phosphorus, can result in algae growth in ponds and lakes. Many growers face strict fertilizer runoff regulations. The best method to minimize runoff is to water more efficiently.
Less water needed
Although the question about how much water your plants really need is simple, the answer is not. Surprisingly, there is little information on plant water needs. The last few years, researchers at the University of Georgia and University of Maine have been studying the water needs of plants. The good news is that many plants need much less water than most people expect.
The plants are not given just enough water to survive, but all the water they need to grow. However, they are only watered when they need to be and no excess water is provided. Soil moisture sensors should be used to irrigated when volumetric substrate water content drop below 40 percent.
Determining water needs
Since each greenhouse environment is unique, and weather changes daily and differs from region to region, it is important to have some idea of your plants’ water needs. There is a simple way to determine this.
Thoroughly water the plants in the morning and let them drain for at least 30 minutes. Then weigh the pots, come back 24 hours later and weigh the pots again. The decrease in weight is the amount of water that has been used by the plant. Water use will differ from day to day.
The main environmental factors that affect water use are light, temperature and relative humidity. Plant size also plays an important role with larger plants needing more water than smaller ones. Determining daily water use several times during the production cycle, both on warm, sunny and on cool, overcast days will provide you with valuable information for water management.
In order to use this information to water more efficiently, you also need to know how much water your irrigation system delivers in a given amount of time. With drip irrigation, this is simple. Put a few drippers in a beaker, run the drip system for a specific period of time and measure how much water was applied per minute.
Now that you have determined how much water the plants use and how much water is applied per minute, you should be able to make better decisions about how much water to apply. However, dealing with day-to-day fluctuations in plant water use due to changing weather conditions may make frequent adjustments necessary. If you prefer to not have to make these adjustments manually, it is possible to automate your irrigation system in a way that assures that the plants receive only the water they need.
Sensors improve efficiency
Suppliers have developed irrigation systems that can water plants when needed with the appropriate amount of water. These systems use sensors to measure substrate moisture. Soil moisture sensors have become available that are relatively inexpensive, reliable and low maintenance.
In many cases, the sensors can be connected to computerized greenhouse control systems to directly control irrigation. Sensor operation is simple. When a plant is transpiring, it takes up water from the substrate, which dries out. Since the sensor measures the substrate water content frequently, the greenhouse environmental computer notices when the substrate water content drops below a particular set point and can turn on the irrigation system. The duration of the irrigation period can be either a set amount of time or it can be sensor-controlled. For example, the irrigation system can be turned on when the substrate water content drops below 40 percent and turned off when it has reached 45 percent.
Plants control watering needs
By irrigating based on substrate water content, the plants essentially are in control of their watering. On warm, sunny days the plants use water quickly, which results in a quick drop in substrate water content and more frequent irrigation. Another approach to using soil moisture sensors for irrigation control is to use them like a cut-off switch. A timer can be set to water at a specific time. At that time the irrigation valve opens only if the sensors detect that the substrate water content is below a designated set point. If the substrate is still moist, the sensor prevents the irrigation valve from opening. If the sensor does allow irrigation to occur, it can then turn off the irrigation automatically when the set point for substrate water content is reached.
A tablespoon per day
We have studied how different irrigation set points affect a variety of plants. Plants were grown at substrate water levels ranging from 5-40 percent for three weeks. In a peat-lite substrate, a 5-10 percent substrate water level is the lowest most plants could survive and 50 percent is near container capacity. Plant growth increased with increasing substrate water content, although there was little difference between 25-, 30-, 35- and 40-percent treatments. Even in the substrate maintained at 40 percent water level, there was no leaching.
A higher substrate moisture set point result in more frequent waterings, so the amount of water that the plants receive increase with increasing substrate moisture levels. Over a three week period, plants can receive anywhere from 3½ to 22 ounces.
Overall, there is a good correlation between plant growth and the amount of water applied. The refered irrigation can be an effective method of controlling growth.
