For many growers, both new and experienced, irrigation control and scheduling in substrate-based systems is often one of the most confusing aspects of hydroponics. Frequency, volume, duration, programming and timing of nutrient solution application can seem daunting at first, and the fact that irrigation requirements change throughout the growing process seems to make this even more complex. Fortunately, many of the substrates available these days are somewhat forgiving of minor irrigation errors. However, maximizing growth and minimizing root problems depends on a certain moisture balance being established and maintained.
Why is Irrigation Control so Important?
Nutrient application via the irrigation system serves a number of purposes in substrate-based systems:
- It replenishes the moisture taken up the plant that is lost via the process of transpiration from the leaves.
- It flushes through fresh nutrient solution, providing mineral ions for uptake by the roots.
- It helps correct and adjust the pH and EC around the root zone.
- It plays a role in oxygenation as the process of solution influx and drainage flushes fresh air through the pores in the substrate.
While these are all beneficial to plant growth, the downside of getting the process wrong can leave root zones either desiccated between inadequate irrigations, or more commonly, oversaturated, deoxygenated and prone to root dieback and pythium attack. Unlike soil in a field situation, hydroponic systems have a highly restricted root zone volume, which makes moisture and nutrient control and supply far more critical for maximum growth.
Figure 1. Careful selection of emitters in drip systems helps ensure the correct level of irrigation is maintained.
Irrigation for the Right Substrate
Most hydroponic substrates are designed to hold a certain amount of air and moisture between irrigations. However, there are wide variations between different growing mediums with regard to these properties. Luckily, substrates are usually selected based on their suitability for different crops, climates and irrigation systems. Porous and highly free-draining substrates such as LECA (light expanded clay aggregates) chunky perlite, various grow rocks and similar materials drain freely, are highly aerated, but hold less moisture between irrigations than many other mediums.
Fine-grade coconut fiber, peat, some grades of stone or rockwool, vermiculite and organic mixes tend to have a much higher water-holding capacity, giving a greater degree of buffering capacity when it comes to root zone moisture content, but are more prone to overwatering, root suffocation and die back.
For these reasons, irrigation schedules for different substrates need to be matched to the properties of the medium being used. For more frequent, smaller irrigations go for those that are highly free-draining, and for less frequent watering, go for those that are highly moisture retentive.
Irrigation for Different Growth Stages
A plant’s stage of development also influences the number and volume of irrigations per day. As plants increase in size, more moisture is lost in the process of transpiration from the larger leaf surface area and more nutrients are required in the root zone. Recent transplants may only require one or less irrigations per day, while a mature tomato plant may need as many as six to eight depending on environmental factors. Since most of the water supplied to substrate-based crops is lost through leaf transpiration, the growing environment is largely what drives irrigation requirements.
Plants grown under low humidity with a high rate of transpiration will need significantly more frequent and larger irrigations than those under high humidity and low temperatures with minimal water lost from the foliage. Temperature, root size and health and light also play a major role in plant water and irrigation requirements. This is why automatic irrigation programs are often linked to the measurements of environmental factors.
Symptoms of Incorrect Irrigation Scheduling
Most growers are familiar with the symptoms of under-watering or irrigation failures—plants typically wilt rapidly in the restricted root volume of a hydroponic system if the irrigation fails. Longer term, slight under-watering may have very minimal effects, apart from smaller or shorter than normal plant size, as plants have the ability to adjust to the moisture status in the root zone to a certain degree. Chronic underwatering, however, may look like nutrient deficiencies, particularly of calcium, which moves within the transpiration stream of the plant. It may also lead to reduced growth and yields as photosynthesis is restricted when plants shut down stomata to conserve moisture. Water-stressed plants may also be more prone to pests and diseases and other physiological disorders.
Overwatering is a far more common occurrence in hydroponic systems than under watering and can have severe implications for plant health. The number one cause of pythium and other root rot pathogen infection is roots that have become damaged by oversaturation and the subsequent root suffocation this causes. Root zone saturation can be difficult to diagnose in the early stages, as symptoms like wilting, or epinastic leaf drooping during the warmest time of day, foliage yellowing and leaf drop, may appear very similar to water-stressed plants. However, unlike water-stressed plants, oversaturated root zones do not respond to increasing the volume or frequency of irrigation, and often by this stage, root dieback and browning can be seen.
Figure 2. Overwatering is a common mistake that can lead to pythium and other pathogens invading plant tissue.
Manual Methods of Irrigation Control
The volume of nutrient to be applied or irrigation schedule to be followed is typically based on each grower’s particular crop, substrate and environment, and needs to be adjusted over time as plants develop. For this reason, using recommendations for set irrigation times and duration of nutrient application should only be considered a starting point.
Irrigation determination in hydroponic systems may be manual or automatic. Many smaller indoor and greenhouse growers base decisions on when and how much to water on basic observation of the moisture status of the substrate. These may include visual clues such as the color of the substrate surface, which for many may be darker when moist and lighter when dry, or a finger test to determine how moist the substrate feels below the surface. These methods are largely based on experience with different types of growing substrates and can be difficult for people new to soilless growing.
Another method, which is more accurate and commonly used by commercial greenhouse growers, is to collect and measure the volume of leachate after each irrigation. Leachate refers to the nutrient solution draining from the base of the growing slabs or containers. Most commercial growers aim for a five to 20 per cent leachate/drainage volume at each irrigation, that is five to 20 per cent of the volume of irrigation applied to each plant drains from the base of the container, bucket, bag or bed. If the drainage volume is higher than this, the irrigation time is cut back, if it is lower, it is increased in volume and/or frequency. Using the drainage volume measurement method is particularly effective in drip irrigation indoor gardens as the growing environment is less prone to daily changes in sunlight, humidity and temperature, which determine irrigation requirements.
Along with monitoring of leachate volume to help adjust irrigation programs, basic system checks are vital with indoor gardens. Small drip irrigation systems in particular are prone to using under-pressurized pumps, which can lead to poor flow rates, increased occurrence of blocked drippers/emitters and uneven amounts of nutrient delivery around the system. Using a jug to collect and measure the volume of nutrient solution coming from each dripper/emitter in the system is an important process to ensure all plants receive the same amount of irrigation. Growth differences between plants in the same system are often found to be attributed to uneven irrigation volumes in drip fed systems.
Figure 3. Free-draining, shallow substrates need shorter but more frequent irrigations then those that retain more moisture.
Advanced Methods of Irrigation Control
There are more advanced automatic irrigation control methods to take note of. These methods involve measuring the moisture status in the substrate, which then triggers nutrient application accordingly. These include the use of substrate moisture sensors or irrigation devices that weigh the substrate to determine moisture loss. Other methods commonly used in commercial greenhouse production are solar integrators linked to irrigation controllers that base plant water requirements on incoming light levels and computer models that estimate crop transpiration.
Newer technology, much of which is still under development for soilless systems but is likely to be a beneficial innovation for indoor gardeners, is the use of plant-based sensing of moisture status. Plant-based methods of irrigation determination rely on direct or indirect measurement of plant water status based on plant physiological responses to drought using tissue water content sensors or measurement of growth, sap flow and stomata conductance. Measurement of the water status of the plant itself is may be highly beneficial when growers are using methods of controlled deficit irrigation to direct growth in a certain way, for example into more generative and less vegetative growth, or to apply some controlled degree of stress to improve plant or fruit quality.
Substrate moisture sensors are one way of irrigating to the plant’s actual water needs rather than just relying on pre-programmed, timed applications that may overwater or underwater plants. Soil moisture sensors have been in use in field crops for decades, but more recent developments have seen accurate sensors available for soilless substrates, including stonewool, coco fiber and soilless container mixes. These root zone sensors measure the substrate moisture status and allow the irrigation program to replenish the water in the growing medium to a preset level. Some irrigation sensors not only control the nutrient application program but may also measure EC and pH in the root zone as well as temperature. While most substrate sensors operate by providing data to a computer-controlled irrigation program, for smaller growers there are also hand-held moisture sensors that can be used to check root zones, which are connected to timer-based irrigation schedules, to see if they are providing the correct level of nutrients.
As with most indoor gardens, the general recommendations are that nutrient application only occurs during the lights-on period when plants are transpiring through their open stomata. The first nutrient application of the day is scheduled within an hour of lights on and is usually the largest irrigation of the day, with the highest amount of drainage percentage to restore root zone moisture and nutrient levels after the overnight dry down. Irrigations are then scheduled throughout the day—the frequency and volume of each can be based on either targeted leachate volume percentages, as well as plant appearance and substrate visual clues, or based on using substrate moisture sensors linked to an irrigation controller. Avoiding irrigation at night when moisture requirements are lowest helps prevent root saturation, increase aeration and lower the risk of pythium infection.
Irrigation scheduling is a vital aspect of maintaining a high-yielding substrate-based hydroponic system, but it takes some understanding of plant water usage and requirements to get a handle on this process and avoid oversaturating the root zone. The use of increasingly sophisticated technology for plant and substrate moisture status monitoring will become increasingly important for many indoor gardeners when it comes to the process of irrigation programming.