Maximizing the Hydroponic Nutrient Environment – Part 2
Grow Media Trouble and Water Worries
We all know that the water we use to make up our nutrient solution is a key factor in pH and conductivity variation – it is not the only source of trouble however. Another common cause of unstable pH and salinity is poor quality growing media. Industrial grade rockwool and gravel are notorious for having very high pH levels that can cause your nutrient pH to rise, often to dangerous levels. Coco fiber or ‘coir’ can contain high levels of sodium and chloride, typically from seawater since coconuts are often harvested near the ocean; high levels of sodium can be very harmful to plants. This is where the conductivity, or total salt content, of your nutrient solution becomes a source of stress to your plants – these problems can come from your media or from your water supply.
Consider this example: you want to make up a 1,000 ppm (.5 miliSiemen) nutrient blend for your plants. You test your tap water and find that it is 300 ppm TDS (total dissolved solids). This means that you can only add 700 ppm of nutrient to make a 1000 ppm blend, 300 of which is unknown or problematic. With hard water most of this 300 ppm is probably going to be Calcium and Magnesium. Immediately you have a nutrient imbalance. At first it is not so bad, but with time the salts will accumulate in the media or soil and leave a salt residue on everything. Your plants start out stressed and over time it just gets worse. This is a case where an R/O filter (reverse osmosis) could make a very big difference since it will remove the salts from your incoming water. An alternative solution is to collect rainwater to mix with your nutrient.
A simple way to test new growing medium is to put some of the medium – rockwool, gravel, soil – into a clean cup, then immerse (soak) the sample in distilled, reverse osmosis purified or “deionized” (purified) water. Let this sit for a while (say, an hour) and then test the pH and EC of the water, note the pH and the EC and continue to let the sample sit. Test the pH and EC occasionally until it has stabilized, it may take days in extreme examples. Has the pH risen to 8.0, perhaps 9.0? Construction grade gravel can go as high as 10.0. On this scale ‘10’ is absolutely not the ideal – it spells torture for roots and death to your plants! If the EC rises very much then the media has soluble minerals of unknown character, probably harmful since neutral media is what we’re after here.
Never underestimate growing media as a source of pH problems. This is one of the primary reasons that “waterculture” hydroponic methods are gaining popularity over “media-based” hydroponics. Water-culture systems require less water and nutrient than media-based methods, due to higher efficiencies and reduced evaporation. However, they are much less forgiving of high nutrient temperatures than media-based systems and lack the ‘buffering’ power of excellent quality media that can help stabilize pH. If you have to use hard water to mix your nutrient then use a specialty fertilizer made for hard water. Hard water formulations usually will have reduced calcium. Alternatively blend soft water with your hard water to at least get it down to 200 ppm or so.
Water-culture systems are like driving a Ferrari, fantastic performance is possible if you know what you’re doing but you can also get into trouble really fast if you don’t! You need to ensure your temperatures remain within strict ranges and your water is excellent – this less forgiving environment is not the way to learn unless you are ready for some challenges. Keep in mind that water-culture systems require very high levels of dissolved oxygen to work well. Oxygen solubility, like all gasses, is directly related to liquid temperature. Cold nutrient holds a lot of dissolved oxygen, warm nutrient much less – just as a cold beer is full of carbon dioxide and a warm beer is flat. Temperature controls gas solubility. For reference 82 °F (28 °C) water can only hold about half as much dissolved oxygen as 58 °F (14 °C) water. This is a very steep solubility curve.
Hydroponic Nutrient Nasties
The problem of pathogens, or disease, in the nutrient solution can be a serious one. It is not uncommon for this to be a regional and seasonal problem. For example, in Holland during the winter, fungi thrive in the cool and damp environment – the air is full of spores. All kinds of soil and airborne diseases become endemic so growers have to work smart to avoid infestations. One of the reasons Dutch growers adopted hydroponics so readily was to avoid soil-borne diseases.
Nonetheless, hydroponics is susceptible to a host of diseases when things go wrong. If your nutrient solution becomes too warm, its ability to hold dissolved oxygen is drastically reduced. This, in turn, leads to oxygen deficiencies in the nutrient which invites many pathogens – these can quickly gain an advantage over the plant. Pythium can lead to slimy dark roots. Sick roots cannot supply a plant with nutrients and moisture and soon the part of the plant above the ground, or hydroponic system, stops growing and becomes sickly. Fungus gnats often precede pythium. The larvae of the fungus gnats crawl over the roots eating holes into them, then the pythium fungus enters the holes in the roots and the disease begins. If the nutrient is warm the pythium gets worse until the plants keel over and die! So remember: dark slimy roots = dead plants = heartache!
Nutrient temperature is one of the most crucial factors in plant health when growing hydroponically. Simply speaking, it is best if the nutrient solution is like a cool mountain stream bathing the roots. It is dangerous when nutrient becomes hot, does not flow fast enough to maintain oxygen saturation, or is contaminated by the presence of a sick plant or pathogen. Warm nutrient is where inoculation of roots and nutrient solution with beneficial organisms really pays off. Generally speaking, the warmer the nutrient, the more important aeration is.
Cleanliness is Next to Crop Success
Keep your growing area clean! Some growers I know place a sponge-mat soaked with disinfectant at the doorway of their greenhouse and demand that everyone who enters clean their shoes on the mat before entering! It’s a very wise move. For similar reasons, don’t allow pets into your growing area. Dogs, cats, mice and rats are magnets for insects and spores and can easily bring them in from the outside, introducing them to your growing environment can introduce a disaster – take spider mites for example! Experienced growers attach a fine filter to their inflow fans to exclude insects and spores. The filtered inflow also gives the greenhouse or indoor garden a positive pressure charge of fresh air. This is an effective and practical way to prevent insects and disease organisms from entering the greenhouse and endangering the crop. Commercial and research growers know these tricks and apply them routinely.
Finding an infected plant in your hydroponic system is always a shock but don’t panic just yet. If you move quickly you can reduce the risk of the disease racing through the entire crop. However, if you are not so vigilant, by the time you notice a problem it could already be way out of control. Plant diseases are a huge subject, so the best and most concise advice is to avoid problems by working clean, planting only healthy, disease-free plants, perfecting the growing environment and closely monitoring the crop. In many cases the infection can start with an infected donor plant from which cuttings are taken. The donor may look ok, just weakened by the disease, but the cuttings taken from the infected donor will carry the disease into the hydroponic system where it can spread. Fungi, like powdery mildew, can enter a crop and spread this way. It is systemic within the donor plant and continues through the cutting. A good method is to drench, or immerse cuttings in a specialized solution to kill the disease prior to dipping into a rooting stimulant. This is a very sensitive subject since the regulations prevent manufacturers from discussing pathogens and disease management on their labels and in advertising. Look for products that provide a ‘shield’ or protection for your plants or products containing ‘neem’ or its active compound ‘azadirachtin’. Neem is an excellent natural pesticide that can be sprayed onto plants to control insects or used as a ‘drench’ to act systemically within the plant. Put some into a watering can and give your plants a drench periodically to prevent insect and fungi attack. Note that these disease-controlling products can challenge beneficial organisms in the root zone, not a problem with topical or spray application, but definitely an issue when added to nutrient or used as a drench. A few days after the drench, renew your beneficials with a ‘chaser’ or drench with beneficials a few days after using the ‘shield’, or neem.
If you see evidence of disease in a single plant, don’t miss a beat – remove and destroy it quickly before the disease spreads. Watch the crop closely and remove any other plants that show signs of disease. It is better to lose a few sick plants than to risk an entire crop. Use a product that contains beneficial inoculants mixed with fresh water in a watering can to give each plant’s roots a periodic drenching and establish and maintain a protective colony of beneficials.
If you do encounter disease problems it is a good idea to completely drain and renew your nutrient after removing the sick plants. If it is possible there is nothing better than to flush the system by running fresh water without nutrient for a day. Use one of the many flushing solutions available to really clean up your media and system, the good ones contain ingredients that will strip away impurities like salt accumulations while maintaining good osmotic pressure for the plants. Flushing between every two to four nutrient changes can help maintain cleanliness in the root zone and in the hydroponic system. Periodic flushing is especially helpful for gravel systems and for soils and ‘soilless mixes’ to remove salt accumulation in the medium. More frequently if your have hard water, less frequently if you have good soft water. Flushing products are also used to clear irrigation sprayers and drippers from salt blockages.