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Article 5-3 Nutrient Solution Problems

Nutrient Solution Problems
By Dr. Lynette Morgan

Introduction
Bad odors, furry growths, strings of jelly, slimes, algae of all colors and fungus gnat larvae are all problems that can affect the nutrient solution and media in hydroponic systems. Nutrients and media provide an ideal breeding ground for many fungi and bacteria since they contain mineral elements, usually some organic matter, moisture and are often warmed to just the right temperature for optimum microbial growth. Bacteria, fungi and algae are also very common - carried in water supplies, media, on seedlings and old root systems, on equipment, transported by humans, animals, wind and in dust, making complete avoidance very difficult. Despite this, most well run hydroponic systems don't end becoming a slimy, smelly mess and most nutrient solution problems are easily avoided.

The Microbial equilibrium
What happens in a normal hydroponic system (one where there is no constant disinfection of the nutrient with ozone, UV or chemicals) is that not long after the nutrient is made up and the seedlings planted, an equilibrium begins to establish between the microbes which are naturally present. These microbes may have arrived in the water supply. If pure distilled or RO water has been used, the microbe populations have been shown to take a little longer to establish, but they will still develop. Seedlings introduced to a new hydroponic system will bring with them their own populations of microbes and microbes present in the surrounding environment will also find their way into the system.

At this stage, the newly planted hydroponic system is very much like a soil garden - the mixtures of different microbes will compete with each other, just as they do in soil, and populations of different species will begin to dominate. In most healthy soils, it is the beneficial microbes which predominate. Many of these have a suppressive effect on certain plant pathogenic fungi and bacteria, others act to break down organic matter releasing plant nutrients, while others might have other roles such as the bacteria which colonise legume plants and fix nitrogen from the atmosphere. Research into the species and population of microbes which exist in hydroponic nutrient solutions and media have found a similar process - many of the species are in fact beneficial microbes which, in a healthy system, will predominate and have a suppressant effect on many of the pathogenic microbes which might be present. In commercial hydroponic operations it has been found that the species and populations of microbes which establish in the nutrient solution are in fact the same ones present in the soil under the system and around the greenhouse. Many of the naturally occurring beneficial microbes which colonise nutrient solutions are species of Pseudomonas, Bacillus, Trichoderma, and Gliocladium virens, although a huge range of microbes may be in existence in difference systems.

Because these types of species are known to have a natural suppressant action to many of the root diseases we encounter in hydroponics such as Pythium, Furasium wilts and others, the use of mixtures of beneficial microbes for diseases control became an area of intense research and study. As a result many products are now available on the market containing certain microbe species or mixtures of microbes to assist with the control of not only root diseases in horticulture, but also many pathogens which affect the leaves and fruit of crops.

Slow sand filtration of nutrient solutions to suppress disease causing pathogens is one rapidly developing and promising technology for hydroponics. This method uses mixtures of beneficial microbes which are 'seeded' into the layers of sand material in the filter where they multiply and live, working on the organic matter and pathogenic fungi in the nutrient as it slowly filters through the system.

Nutrient problems
Cloudy nutrients, floating 'fur' and 'cotton like' growths are most likely to be caused by fungi in the nutrient solution. Bacteria can also make the nutrient cloudy, but tend to produce more of a'slime' or jelly like mass in the system and a bad smell. Bad odors can be both strong and persistent and are a result of bacteria metabolizing proteins which releases amines and sulfur containing organic molecules into solution. Microbial growth in the nutrient itself is a result of having organic materials in the system somewhere (fungi etc need organic matter to feed on). When these fungi or bacteria are present in the system, feeding on organic matter, they use up just about all the oxygen in the nutrient and this ends up smothering the plants roots. They can also release toxic compounds into the nutrient as they grow, breed, die and decompose, and many of these are harmful to plants. The microbial species which produce the bad smells, slime and other undesirable problems are not the ones we want to encourage, since their growth results in stagnant, oxygen starved conditions and root death. Once root death has started to occur, opportunist pathogens such as Pythium will then invade the plant's root tissues, making disease control difficult. They also make running a hydroponic system unpleasant, will block up emitters, drippers and other equipment and require the whole system to be cleaned out and disinfected.

If these problems have developed in a hydroponic system it is usually an indication that large amounts of organic matter have been introduced which have given the fungi and bacteria a food source and resulted in rapid population growth. Organic matter may have come from large amounts of old rotting root systems or vegetation from a previous crop, use of organic growing media or it could even be unintentional organic contamination - (such as the large, dead rat found rotting at the bottom of one growers nutrient tank!). There are organic additives specifically designed for hydroponic systems - meaning they are suitable for a'soilless' system and following the manufacturers instructions for the use of these will get the most benefit from these types of products. As with all hydroponic nutrients - more is not better when using organic additives in a hydroponic system and the dose stated on the product will be the optimum one. Use of organic fertilizers which are used and designed for soil based systems have in the past caused major problems in many hydroponic systems - many of these rely on boosting the population growth of microbes in the soil which in turn breaks down organic matter, releasing nutrients. Also many of the organic compounds are not fully mineralized and putting these soil based organic fertilizers (such as fish emulsions) into an NFT or even media system in large quantities can have rapid and unpleasant results.

Solutions and prevention of nutrient problems
There has been a major move away from the'kill everything and sterilize approach' for nutrient solutions to a more integrated and environmentally friendly method of allowing microbes to exist naturally in a well run, fully aerated system. Sterilization of the nutrient has proven many times to be difficult and expensive to carry out, often resulting in plant damage, nutrient problems and residues of sterilization chemicals. Using sterilization techniques such as H2O2 and Chlorine in nutrients solutions requires a high dose to kill some of the persistent plant pathogens and this has been shown to damage young and sensitive roots in many crops. 100ppm H2O2 is required to kill spores of some of the common hydroponic pathogens, but even a level of 8ppm was found to damage lettuce seedling roots. The same problem exists with the use of chlorine. Studies have also found that after nutrient sterilization when all microbes and algae have been killed, re-growth of these occurs very rapidly in the nutrient, and this can in fact result in some of the pathogenic microbes re-establishing fastest, causing many new problems in the systems. A better approach, which is used by many commercial growers, is to start with a clean, sterilized water source, add in the nutrients, then inoculate the nutrient solution with a mixture of beneficial microbes (the same ones which may also be used in a sand filter). This gives the beneficial microbes a head start and results in a healthy system where any pathogens should be suppressed before plant damage occurs.

How to keep your hydroponic system healthy
Keeping the nutrient well oxygenated swings the equilibrium in factor of the beneficial microbes in the nutrient, as does starting with a clean system and equipment. Planting only healthy seedlings into a hydroponic system and removing any sick or stressed plants as soon as they are noticed also assists with preventing problems occurring. Some researchers have found that having a layer of inert clay or diatomaceous earth on the bottom of the nutrient tank is beneficial - this provides a 'home' for the beneficial microbes to colonise and breed, particularly where mixtures of 'pro-biotics or 'beneficials' are added in when the system is initially planted out. Watching out for problems such as odours, or bacterial/fungal growths in the nutrient or media is important - often complete replacement of the nutrient and rinsing equipment will help for one off problems. Long term prevention of nutrient problems relies on having the right combination of microbes, nutrients, oxygen and small amounts of organic matter which do not encourage rapid growth of pathogenic bacteria and fungi.

Part two of this series will cover more nutrient and system problems - algae, fungus gnat larvae and shore flies.

References and Sources of information:


Paulitz, T. C., 'Biological Control of Root Pathogens in Soilless and Hydroponic Systems'. HortSceince, Vol 32, No 2 (1997) pages 193-196.

Stanghellini, M. E., Rasmussen, S. L., 'Hydroponics: A Solution for Zoosporic Pathogens' Plant Disease, Vol 78 No 12 (1994) pages 1129-1137.

Utkhede, R. S., Levesque, C. A., Dinh D. 'Pythium aphanidermatum Root Rot in Hydroponically Grown lettuce and the Effect of Chemical and Biological Agents on its Control' Canadian Journal of Plant Pathology, Vol 22 (2000) pages 138 - 144.

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