Feature Article: February, 2010
The frequency, amount, timing and duration of nutrient application to a hydroponic substrate are some of the most commonly discussed concerns by new growers. Even those with some experience can be caught out when switching from one substrate to another without realizing that the physical properties can vary considerably between growing media. More hydroponic plants are lost through over saturation in the root zone and the risks that occur with root suffocation and opportunist pathogen attack, then end up suffering through a drought. And while commercial growers can make use of computerized moisture sensors and complex formulas to determine the rate of irrigation, smaller growers need to be in tune with the growing environment, crop water uptake and media moisture levels.
Why media moisture matters
Hydroponics is often seen as a foolproof way of growing plants under automatic control, so every aspect of plant growth is optimized. However, the majority of hydroponic systems still use some form of substrate or growing media to support the plant and retain a reserve of moisture and nutrients around the root zone between nutrient applications. Even many solution culture systems rely on a small amount of some substrate or material to hold the plant in position, while seeds and cuttings also need support during the propagation stages.
Some crops are very intolerant of high levels of moisture and prefer a coarse and very free draining medium.
Plants require water, nutrients and oxygen in the root zone to both survive and obtain maximum growth and yields. In a hydroponic substrate these requirements are determined by the physical and chemical properties of the media, such as the water holding capacity, cation exchange capacity, pore size distribution and porosity. The substrate has to retain water and nutrients and at the same time must have enough oxygen and remove sufficient carbon dioxide for the plant to thrive. This water retention versus aeration within a growing media is a delicate balance as over watering reduces the amount of oxygen in the media for root respiration, leading to anaerobic conditions. Drainage can be a problem in some hydroponic systems – the water held in the growing container is effectively a balance between the gravitational pull on the water, the tendency for water to adhere to particles by surface tension and also the tendency for water to be retained in the fine pores of the substrate by capillary action. ‘Container capacity’ refers to the amount of water held in a container of a given depth after drainage is complete and most cropping plants will perform best in a substrate with a high percentage of air filled pores when at container capacity. To achieve this high rate of both air filled porosity and water holding capacity a particular type of structure is required and this is seen in many good hydroponic substrates.
Although it may appear that a hydroponic substrate has better aeration and drainage than field soil, this may not always be the case; the bottom of the growing container, or bed, creates a barrier to downward drainage. Once the medium’s pores at the container base become saturated with water, the moisture moves out of the container’s drainage holes. The rate at which nutrient moves through a hydroponic substrate and the amount which is left behind after irrigation are vital properties of a substrate, which are dependant on a number of physical properties of the media.
Over watering is a more common problem in hydroponic substrates that have a high moisture holding capacity. These include the finer grades of coconut fiber, peat, some grades of rockwool slabs, decomposed sawdust and vermiculite. Cool growing conditions, high humidity and low rates of growth also predispose heavier media to over irrigation as moisture is not taken up as rapidly by the plants. Under slow growth and cooler conditions, a lighter growing media with a higher degree of aeration and more rapid drainage is often a better choice, with more moisture retentive media of greater use under warm, faster growing conditions.
Over saturation – signs and symptoms
Early signs of over saturation of a growing media may be as simple as the presence of shore flies and fungus gnats, which are attracted to the overly damp surface of a substrate. Fungus gnats are a particular problem as they can persist in hydroponic systems, which are continually over wet and where there is also organic matter such as compost. The adult gnat fly lays eggs in the wet substrate, which hatch into small larvae and can chew into the root system causing damage and creating wounds for root rot pathogens to enter. The best form of prevention of gnat problems is to keep the surface of all growing media slightly dry to prevent the gnats from laying eggs. Large amounts of algae may also grow on the surface of the media if over watering has been occurring. In seedling trays, high levels of moisture often lead to problems with damping off caused by opportunist pathogens such as pythium and rhizoctonia who prey on young plants stressed by over saturation and lack of oxygen. Cuttings and clones may suffer from stem rot and die back as over saturation cuts out much of the oxygen required for callus and root formation.
While most plants can handle a short stint of over watering, if the saturated conditions continue, further damage is inflicted and root respiration rates begin to decline through a lack of oxygen. Reduced root respiration often displays itself in a few different ways, some of which are species dependant. Some plants may, despite the high amount of moisture in the root zone, wilt during the warmest part of the day. This is a response to suffocation and hypoxic conditions in the root zone, as without a high degree of respiration the plant can’t take up sufficient water and nutrients. Longer term nutrient deficiencies may show on the waterlogged plant, including chlorosis (yellowing) or paleness in the new foliage. In more mature plants, flower and fruitlet drop is common in waterlogged crops and older leaves may also yellow and abscise. One of the more extreme symptoms of waterlogged roots is ‘epinasty’ where ethylene gas builds up within the plant causing the upper side of the leaf petiole cells to elongate whereas those on the lower side do not. The result is a severe bending downwards of the leaves in response to long term water logged conditions that cut out sufficient oxygenation in the root zone. Epinasty in response to saturated root zone conditions are often mistaken for wilting caused by a lack of moisture, so growers need to carefully check whether wilted plants are actually suffering from a lack of irrigation or not. Some plants such as tomatoes may also respond to over saturation of the root zone with the production of adventitious roots up on the stems. These root initials can be numerous and can even erupt as large bumps and slits which may be mistaken for a pest or disease attack.
“Cool growing conditions, high humidity and low rates of growth predispose heavier media to over irrigation.”
Some plant species are highly prone to problems with over watering, while others are quite tolerant. Strawberries, for instance, have no tolerance for a saturated substrate and many strawberry crop losses have resulted from over watering the crown leading to root. Many cacti and succulents will also rot when over irrigated and prefer a coarse and very free draining substrate such as perlite or coarse sand. Other plants – more notably those that are grown under warm conditions - have large leaves and a rapid rate of growth are better suited to highly moisture retentive media that will hold sufficient water between irrigations. Cucumbers, tomatoes, squash and similar crops perform well in a media that has a high water holding capacity and also a good rate of air filled porosity.
Nutrient application programs
Nutrient application rate, frequency and timing needs to be carefully matched to the crop, stage of growth, type of hydroponic system and the water holding capacity and drainage ability of the substrate. A crop growing in coarse perlite or expanded clay would be expected to have a different irrigation program to the same crop growing in fine coconut fiber or rockwool slabs. The coarser media would be expected to have a higher frequency of smaller volumes of irrigation, while the more moisture retentive media would have less frequent irrigation of a greater volume.
Determining how often to irrigate a particular substrate to avoid over saturation in the root zone can be difficult for newer growers. With experience growers soon develop an instinct of knowing how often to water certain media and how to adjust this for growing conditions and plant size. To start out inexperienced growers may want to chose one of the more free draining substrates such as coarse grade perlite and monitor how much drainage solution occurs after each nutrient application as a guide to irrigation amounts. However, more moisture retentive substrates such as coconut fiber are more forgiving of a lack of watering and probably a better choice in systems that can’t be monitored through the day. The moisture status of coconut fiber and similar substrates can be determined by poking a finger into the top of the substrate – this should be almost dry on top, but moist underneath the surface, with some nutrient run off at each irrigation.
Systems such as ebb and flow, NFT and anywhere that a small volume of media is being used to support a plant in a larger system are also prone to over watering problems. While the nutrient solution may carry some dissolved oxygen, this is only a very small amount (six to 13 ppm on average) compared to that contained in moist air surrounding roots, so aeration is still a priority. NFT relies on a thin film (two to three millimeters deep) of nutrient solution flowing over the base of a growing channel; if the flow becomes slower and deeper, especially under warm growing conditions, problems can arise with insufficient oxygenation and root death. The requirement for both oxygen and moisture in the root zone is a trade off that needs to be considered in any type of hydroponic system as both are essential for optimal plant functioning.
Moisture meters
Moisture meters are not a substitute for experience when it comes to plant irrigation and management; however, they are a useful tool for some hydroponic substrates. Most moisture meters have been developed for use in field soils; however, there are some models which can be used for soilless substrates. These should always be selected as they give a more accurate result with hydroponic mediums. Moisture meters range from the relatively inexpensive type sold for home gardeners to professional sensors for larger scale greenhouse growers. The inexpensive type only gives a rough approximation of moisture levels but is useful for houseplants and small container systems.
Other articles by Dr. Lynette Morgan
