Believe it or not, more hydroponic plants are lost due to an oversaturated root zone than because of drought. This is due to the risks associated with oversaturation, including root suffocation and opportunistic pathogen attacks. Given that the majority of hydroponic systems still use some form of growing media to support plants and retain a reserve of water and nutrients around the root zone between irrigations, maintaining the correct moisture level is essential to plant health and yields.
All substrates handle moisture a bit differently, and it’s this variation that gives growers an important tool when it comes to directing plant growth, improving fruit quality and shelf life, and preventing issues associated with overwatering, such as pythium root rot.
Why Moisture Matters in Hydroponics
Within a plant’s root zone, there must be a good balance between water, nutrients and oxygen. The substrate has to retain water and nutrients while also providing enough oxygen and removing a sufficient amount of carbon dioxide for plants to thrive. The water retention versus aeration within a growing media is a delicate balance, as overwatering reduces the amount of oxygen in the media for root respiration, which leads to conditions that eventually become anaerobic.
Starting off with a growing substrate that has a suitable moisture-holding capacity and air-filled porosity is only one part of the equation for successful plant growth. Irrigation methods and frequency also determine moisture and oxygen levels in the root zone, so nutrient application rates and timings are often quite different for the same crop growing in substrates with different physical properties. This means growers must determine their irrigation programs based on their particular crop, system, substrate and climate, along with their regular observations of moisture levels. Following pre-determined irrigation settings isn’t always the best practice.
Restricting Moisture to Control Plant Growth
Plant growth is often divided into two parts: vegetative (leaves and stems) and generative (buds, flowers and fruits). Many plants, such as indeterminate tomatoes, capsicums and cucumbers, have both vegetative and generative growth occurring at the same time, and the balance between these two stages is referred to as growth balance.
A highly vegetative crop will have large, thin, soft leaves, minimal flowering and poor fruit set. They may drop small fruitlets, or their set fruit may end up undersized. Generative plants may sound like a good thing, but much of their energy goes into flower/fruit production, with little left over for vegetative growth to support the developing fruit with a sufficient photo-assimilate supply to continue crop development.
One way of manipulating plant balance is using moisture control via substrate selection and the use of careful irrigation programming that controls the supply of nutrient solution. High levels of moisture in the root zone tend to have a vegetative effect on many crops, so controlled deficit irrigation is used on a wide range of crops—both in hydroponics and in the soil—to help direct growth in a more productive or generative direction with controlled levels of stress.
Deficit irrigation may refer to reducing the volume applied at each irrigation, allowing more time between irrigations and allowing the media to dry slightly overnight by restricting early morning and evening irrigation. Along with restricting moisture levels via a reduction in nutrient application, growers can select substrates that are coarser and more free-draining, with a higher air-filled porosity and lower water-holding capacity to help control overly vegetative growth.
This type of media is a useful tool for certain crops or cultivars that are naturally vegetative and benefit from some growth control, or for those growing under environmental conditions and that may favor vegetative over generative growth. For plants that tend to be overly generative and set large numbers of fruit at the expense of foliage growth, using a substrate that has a naturally higher water-holding capacity is beneficial. A fine-grade coconut fiber is a good option for favoring vegetative growth.
Along with influencing growth balance, mildly deficit irrigation can be used to help improve fruit quality, shelf life and even volatile concentrations in some plants. For example, tomato fruit flavor can be improved by restricting moisture in the root zone, which increases the dry matter percentage of the fruits and reduces their water content, giving them more concentrated flavor. Plus, they tend to last longer! Mildly deficit irrigation can also be used to harden off plants before they are transplanted into a different environment.
Substrates and Water-holding Capacity
Many hydroponic substrates, such as perlite, vermiculite and coconut fiber, are available in a number of different grades, including fine, medium, coarse and mixed. This means growers can select different grades for different crops, different growth stages and different hydroponic systems.
Let’s look at an example using horticultural coconut fiber in a hydroponic production setting. While orchids prefer coarse coco chips, using coco for the propagation and germination of small seeds requires a much finer grade. However, too fine a grade and you invite new problems into the garden. The higher water-holding capacity of coir dust is great in some situations, but it can oversaturate the root zone.
Grades of coco commonly used in grow slabs tend to consist of a mixture of longer, coarser fibers or chips that keep the substrate open and aerated, and finer particles that hold more moisture. These mixed grades of coco are ideal for longer-term hydroponic crops, such as tomatoes, cucumbers, melons, peppers and cut flowers, as the fibers help prevent the substrate from packing down over time.
Other manufactured substrates, such as stonewool blocks and slabs, also have a range of differences, not only in overall moisture and air-filled porosity levels, but also in carefully calculated moisture gradients between the top and the base. Many of these products are aimed at different crops, climates and systems to ensure optimal levels of moisture in the root zone for different applications.
Matching Substrates to Plant Species
Some plant species are highly prone to problems with overwatering, while others are quite tolerant. For example, strawberries have zero tolerance for a saturated substrate, and many strawberry crop losses have resulted from overwatering the crown. Many cacti and succulents will also rot when overwatered.
They prefer a coarse, free-draining substrate such as perlite or coarse sand. Other plants—most notably those that are grown under warm conditions—have large leaves and a rapid growth rate, making them better suited to highly moisture-retentive media that will hold sufficient water between irrigations. Cucumbers, tomatoes, squash and similar crops perform well in a medium that has a high water-holding capacity and also a good rate of air-filled porosity.
Transpiration, temperature and humidity levels also play a role in substrate selection. Crops growing under warm, high-light, dry conditions require frequent irrigation and benefit from a moisture-retentive medium, which helps prevent the root zone from drying out and provides more of a safety cushion during failures with pumps or the power supply occur.
For smaller plants in cooler conditions, substrates that are highly free-draining and retain lower levels of moisture help prevent the common occurrence of oversaturation. Free-draining substrates are also more forgiving when it comes to the application of higher levels of nutrient solutions that may be required by some plants.
The Problem with Too Much Moisture in a Grow Medium
When a nutrient solution is irrigated onto a substrate, it displaces the air-filled pockets. When it has completely drained, more air is drawn down into the root system. If watering occurs too frequently, the air-filled pores remain saturated and the plant gets less oxygen. This is a dangerous situation because plants exhibit a survival strategy known as oxytropism, where roots avoid growing into oxygen-deprived areas such as water-logged soils, overwatered hydroponic substrates and stagnant nutrient solutions.
To determine if this is a problem in your garden, be on the lookout for areas in containers or slabs of substrates that are void of any root growth. The roots that are present are often small, thin and browning, having died back due to suffocation and oversaturation.
If plants have been performing poorly, a quick check of root outgrowth into all areas of the substrate in the growing container or slab, particularly the base, will quickly reveal if oversaturation has become a problem. Other symptoms of waterlogging include chlorosis (yellowing) of the new foliage.
Older leaves may also yellow and abscise, and flower and fruitlet drop is common. One of the more extreme symptoms of too much irrigation is epinasty, a condition where ethylene gas builds up within the plant and causes the upper side of the leaf petiole cells to elongate.
This results in a severe downwards bending of the leaves, which may be mistaken for wilting caused by a lack of moisture. Growers need to carefully check whether wilted plants are actually suffering from a lack of irrigation, or epinasty due to waterlogging in the root zone.
Large amounts of algae may also grow on the surface of the medium if plants are frequently overwatered. In seedling trays, high levels of moisture often lead to problems with damping-off caused by pathogens such as pythium and rhizoctonia that prey on young plants that are stressed out by oversaturation and lack of oxygen.
Cuttings and clones may suffer, too, as they are prone to stem rot and die back as oversaturation cuts out much of the oxygen required for callus and root formation.
Measuring Substrate Moisture Levels
There are a number of ways to assess moisture levels in different hydroponic substrates. Keep in mind that just looking at the surface of the media can be misleading in some systems.
Coco can look slightly dry on the surface and still be fully moist in the root zone, so checking the moisture level a few inches below the surface is recommended. Moisture should appear when the coco is squeezed between the fingers, but the surface should never appear wet. Overly damp coco can attract fungus gnats in addition to reducing the oxygen in the root zone.
Other substrates have different physical properties. With some, the moisture level can be gauged easily with a finger test. With others, it can be more difficult to determine what is occurring down in the root zone due to differences in moisture gradients between the surface and the base of the container.
Hand-held and electronic moisture meters are becoming more common for use in hydroponics and can be particularly useful for newer, inexperienced growers who are still learning to judge irrigation frequency and healthy moisture levels. Many moisture sensors have been developed for use in field soils, and there are some good models that have been designed for many soilless substrates. There are even some that feed the information to a computer-controlled irrigation program.
With so many tools at your disposal, maintaining a healthy hydroponic system has become easier than ever, but it still takes a bit of due diligence to get those high yields we’re all after. Stick with it and you’ll be a moisture level master in no time.