by Dr Lynette Morgan
The hydroponic nutrient solution is not just a mix of fertiliser salts and water, there are a number of organisms and compounds commonly found in our hydroponic systems that we need to be aware of. One of the most important of these is dissolved oxygen which is vital for the health and strength of the root system as well as being necessary for nutrient uptake.
Most growers are familiar with the need to have some form of aeration in their nutrient solution - whether they be in a recirculating or a media based system. In NFT systems, this is often accomplished with the use of an air pump or by allowing the nutrient to fall back into the reservoir thus introducing oxygen. However, the effect of temperature of the solution on the dissolved oxygen levels and on root respiration rates also needs to be taken into account. As the temperature of your nutrient solution increases, the ability of that solution to `hold' dissolved oxygen decreases. For example, the oxygen content of a fully aerated solution at 10C (50 F) is about 13ppm, but as the solution warms up to 20 C (68 F) the ability of the liquid to `hold' oxygen drops to 9 - 10ppm, by the time the solution has reached 30 C (86 F), then it's only 7ppm.
While this may not seem like a huge drop in the amount of dissolved oxygen, we have to remember that as the temperature of the root system warms, the rate of respiration of the root tissue also increases and more oxygen is required by the plant. For example, the respiration rate of the roots will double for each 10C rise in temperature up to 30C (86 F). So the situation can develop where the solution temperature increases from 20 - 30C (68 - 86 F) during the day, with a mature crop and a large root system, then the requirement for oxygen will double while the oxygen carrying capacity of the solution will drop by over 25%. This means that the dissolved oxygen in solution will be much more rapidly depleted and the plants can suffer from oxygen starvation for a period of time.
The symptoms of oxygen starvation which can occur in both NFT and media based systems can be difficult to pick up as they are very general signs. Media based plants are just as prone to oxygen starvation in hydroponic systems as those grown in solution culture, but here we must also take into account the `air filled porosity' of the media used. This is simply how much air can permeate between the particles in the substrate and selection of a free draining media which won't break down will ensure that maximum aeration is going to reach the root zone. Injury from low (or no) oxygen in the root zone can take several forms and these will differ in severity between species. Often the first sign of inadequate oxygen supply to the roots is wilting of the plant during the warmest part of the day when temperature and light levels are highest. Insufficient oxygen reduces the permeability of roots to water and there will be the accumulation of toxins, thus both water and minerals cannot be absorbed in sufficient quantities to support plant growth particularly under stress conditions. This wilting is accompanied by slower rates of photosynthesis and carbohydrate transfer, so that over time, plant growth is reduced and yields will be affected. If oxygen starvation continues, mineral deficiencies will begin to show, roots will die back and plants will become stunted. Under continuing anaerobic conditions, plants produce a stress hormone - ethylene which accumulates in the roots and causes collapse of the root cells. Once root deterioratation caused by anaerobic conditions has begun, opportunist pathogens such as Pythium can easily take hold and rapidly destroy the plant.
Another more visible and longer term effect of oxygen starvation which also occurs in waterlogged crops is leaf `epinasty'. Epinasty is a downward curvature of the plant leaves, resulting in plants which look wilted. If the oxygen starvation continues and is severe, then eventually leaf chlorosis yellowing, premature leaf and flower abscission will occur.
There are a number of things we can do to make sure our nutrient solution is carrying sufficient dissolved oxygen, and this is important when we consider that many of the root diseases encountered in hydroponics have occurred because the root system was damaged in some way, with anaerobic conditions being a major factor in many situations. The first most important factor to remember with oxygen is that the best way to introduce this gas into the nutrient is to have the solution fall back into the reservoir, and the greater the drop height, the better the aeration effect. Breaking the flow up into a fine shower also assists by introducing more air bubbles into the tank. Secondly, while nutrient EC does reduce the oxygen carrying capacity of the solution, the effect is very small and temperature has a much greater influence on oxygenation. Reducing excessive solution temperatures will ensure more oxygen can be held by the solution and the rate of respiration by the roots will be kept down to optimal levels. Thirdly factors such as nutrient flow rate, channel width, length and slope have a large effect on oxygen levels- faster flow rates, greater slopes and shorter channel lengths all assist with prevention of oxygen starvation.
Perhaps one of the commonest problems in hydroponic systems is the Pythium pathogen and what many growers don't realise is that Pythium being an `opportunist' fungi, often takes advantage of plants which have been stressed by a combination of high temperatures and oxygen starvation in the root zone. Pythium is usually described as a `secondary infection' meaning that the Pythium spores which are actually common in just about all hydroponic systems, don't actually attack the plant until it has been damaged in some way. Even very clean hydroponic systems and grow rooms which are isolated from the outdoor environment will have some Pythium present as these fungal spores are naturally present everywhere on a world wide scale - in the water, soil, vegetation, carried in the air and in dust, so its difficult to eliminate the source of this disease. However, one way we can reduce the `spore load' is to sterilise any water supply which may be contaminated with high levels of pythium - water from dams, and streams should always be sterilised before use for this reason if Pythium is a problem
