There are additives lurking in most gardening sprays, but not to worry, that's a good thing! However, as with any potentially beneficial product or compound being added to hydroponic nutrient solutions, there are issues with surfactants that growers should know about.
There is nothing more satisfying than finding an exciting new application for a horticultural or hydroponic product. Many of us love to trial new ideas for old problems, and search out solutions for those troublesome aspects of plant production even to discover what might or might not boost crop growth that little bit more. One such idea, which is increasingly being backed up by some good science, is the use of non-ionic surfactants in the hydroponic nutrient solution.
Most of us are familiar in one way or another with surfactants, such as in our everyday life (as household detergent) and as indoor gardeners with these being a common component of plant foliar, insecticide and fungicide sprays. Surfactants, however, have much more potential than just a wetting agent in plant spray products; they have been proven to assist with root disease control in the nutrient solution and more recently have been proven to have plant growth boosting effects under certain conditions.
What are surfactants
Surfactants are one of the most widely used additives in agriculture. They essentially lower surface tension and allow the spreading and sticking of liquids as well as enhance the absorption of compounds and sprays. In this way they allow a foliar applied spray to form a uniform, thin, easily absorbed layer over a leaf surface rather than just running off the waxy cuticle in large droplets. Surfactants are also used as wetting agents in soil and in soilless growing mixes, allowing the substrate to initially saturate up easily as media such as peat can be water repellent when fully dry. However, what is vital to remember with any use of surfactants in hydroponics is that not all are created equal; it is only the non-ionic surfactants that have proven effects on pathogen control and plant growth when added to the nutrient solution.
Other types of surfactants have been proven to have toxic effects, so choosing a good quality, non-ionic type is essential. Commonly used synthetic non-ionic surfactants include those used in many plant growth studies such as those conducted on Tween and Agral, although many others exist. These days the organic market has also been catered to with a number of bio-surfactants becoming available. Many of these non-synthetic, saponin surfactants are derived from plants such as yucca and quillaja and while they are non-ionic, they are often required to be used at higher rates than other more concentrated synthetic products. Luckily there is now non-ionic surfactant products designed specifically for the hydroponic market so basic mistakes in choosing the right formulation and dose rate can be avoided.
Surfactant uses and potential
Apart from their common use as spray additives and wetting agents, non-ionic surfactants have two major potential uses in hydroponic nutrient solutions. The first has been known since the 1960s—the fact that non-ionic surfactants have the ability to kill pathogen zoospores and control the spread of certain root rot diseases in the nutrient solution. Another potential use is the more recently reported effect of surfactants boosting plant growth in some species under hydroponic conditions.
Surfactants and pathogen control
Many of the common root rot diseases problematic in hydroponics are spread via zoospores. Zoosporic fungi include the dreaded pythium, but also phytophthora root rot. In fact, there are approximately 143 species of zoosporic plant pathogens. What makes these particularly damaging in hydroponics is that diseases producing zoospores release these into the nutrient solution or irrigation water. Zoospores survive easily in water and are able to swim, locate and infest new root systems. Therefore, zoospores in hydroponic systems can rapidly spread an isolated disease outbreak fairly rapidly through this highly efficient system of zoospore infection. Zoosporic fungal diseases such as pythium actually thrive under warm, wet hydroponic conditions that favor the spread via motile spores, particularly if plants have been weakened or stressed in any way.
The effect of non-ionic surfactants have on controlling disease zoospores in hydroponic nutrient solutions was actually discovered by accident when researchers applied a fungicide containing a surfactant to a hydroponic lettuce crop. It was discovered that the synthetic surfactant in the fungicide was effective in destroying the zoospore plasma membrane, thus reducing the ability of the spores to move and infect plants and eventually kill them. While non-ionic surfactants had little or no effect on any other stage of the disease life cycle (that have cell walls rather than plasma membranes), in killing or retarding the zoospores in the nutrient solution, the potential for the control of these root infecting pathogens was realized.
Further studies were carried out on a number of different hydroponic crops prone to zoosporic fungal disease outbreaks. Most found highly promising results in that non-ionic surfactant used at the correct rate could give a high degree of disease control of pythium and phytophthora in the nutrient solution. However, while the surfactant could destroy large numbers of zoospores being carried by the nutrient solution, hence preventing or slowing the spread of these diseases, it had no effect on plants already infected when the disease was present inside plant tissue. Therefore, use of non-ionic surfactants in the nutrient solution is more of a preventative action rather than curative. Non-ionic surfactants destroy zoospores released by active disease outbreaks and prevent these from infecting new plants. For this to be effective, the correct rate of non-ionic surfactant products needs to continually maintained in the nutrient solution and when this has been achieved, growers have reported good results.
Non-ionic surfactants also have an important role to play in general system and indoor garden hygiene. Since surfactants can not only assist in removal of biofilms and general cleaning, but also in the destruction of disease zoospores, they are a great tool for prevention of pathogen problems.
Surfactants and growth enhancement
While there have been many research trials since the 1960s examining the effects of non-ionic surfactants on pathogen control in the nutrient solution, the re-evaluation that these same surfactants may have other growth benefits has not been as extensive. The non-ionic surfactant called Tween 20 has been found to stimulate tobacco root respiration and succinoxidase activity in tissue culture media at rates of 0.001%. Whether this same effect occurs in intact mature plants growing hydroponically is not clear.
A more recent study reported that the addition of non-ionic surfactant to hydroponic nutrient solutions increased fresh and dry weights of peace lily and impatiens by 17% and 33% respectively. It was also found that water use efficiency of peace lily and impatiens with the Tween 20 solution was increased by 166% and 221% compared to the untreated control. The most likely mode of action of surfactants when added to the nutrient solution is that they may make root membranes more permeable, thus allowing more nutrients (or other compounds) to be absorbed by the roots. The plant growth promoting effects of non-ionic surfactants probably go even further than this and it has been suggested that these compounds can also act as synergists with plant hormones such as IAA (and other auxins often used on cuttings for root formation) giving further growth boosting effects. It is likely that further research will clarify exactly how non-ionic surfactants applied at the correct dose for each species can positively benefit growth and development. It appears that the advantageous effects are a combination of both physical and biochemical interactions.
As with any potentially beneficial product or compound being added to the hydroponic nutrient solutions, there are issues growers should know about. First, growers who have found the addition of non-ionic surfactant useful for reduction in root rot pathogens such as pythium also reported foaming in the nutrient reservoir was common, particularly in the few days after the surfactant was added. Foaming is to be expected when what is essentially a strong detergent is added to moving water; however, the degree of bubble formation differs between surfactant products.
Dosage rates can be another issue. Commercial lettuce growers have had success with using 0.85 oz. per 264 gal. of nutrient solution of Agral and other similar non-ionic surfactants for assistance with root disease control; however, different surfactant products may need different dosage rates depending on the concentration of active ingredients. Many of the newer bio surfactant products designed for hydroponic use give dose rates on the product label and these should be carefully followed. As with most products and compounds available to add to the nutrient solution, over-dosing does not give better results and in many cases can be quite toxic to the plants. Another issue with dosage rates is that surfactants are gradually broken down by microbial action over time so determining how often to re-dose may be difficult and require some trial and error.
While the use of ionic surfactants in the hydroponic nutrient solution is not a new idea and has been proven to battle disease spread via mobile swimming zoospores, the potential for growth enhancement is a relatively new area of study. There exists a great deal of potential to define exactly how these compounds may assist plant growth and the exciting possibility of using new and even more effective non-ionic surfactant products in the future.