Nutrient Management—Part 4
What is the impact of raw water quality in hydroponics and how can this be managed?
Raw water quality—general
In many hydroponic books, water has been taken to be relatively pure, which is a reasonable starting point. However, in the real world this is often not the case and the quality of your water can have a major impact upon the management of your system.
Freedom from insoluble matter and pathogens is an obvious requirement for a hydroponic water supply. Insoluble solids can cause major problems, such as blocked drippers, hence they need to be removed. They often include insolubles suspended in the water, known as colloids. To remove these they need to be settled out by treating with a coagulant, such as alum or polyelectrolyte, then held in a settling tank. Before use, the water needs to be pushed through an effective filter such as a good sand filter.
While town water should be free of plant disease pathogens, dam and stream water is almost certain to be contaminated and therefore should be sterilised. There are numerous methods of sterilisation, the most common being UV, ozone and chlorination.
Raw water quality—dissolved solids
The type and quantity of dissolved solids in raw water is just as important as pathogens and insoluble matter. When considering dissolved solids there are two basic types, those which are nutrients and those which aren’t. The nutrient ions, which are in the water, should be allowed for in your fertiliser formulations, if their concentration is significant.
There are several traps to avoid. The iron content in water supplies is much more of a problem than a benefit because it oxidises quickly to rust and so is of no use as a nutrient. Iron also helps cause dripper blockages and should be removed by aeration, followed by settling and/or filtration.
So-called hard waters, containing calcium and bicarbonate and perhaps magnesium are usually OK to use if these are the only significant dissolved solids. The bicarbonate will need to be neutralised by acid, which can be useful for buffering, but can become a problem if its level is too high.
Much more of a problem, especially in Australia, is high levels of sodium chloride. While chloride is a micronutrient, and perhaps sodium also, their required levels are far lower than 1 ppm. Individual crops have different degrees of tolerance to sodium chloride. A list of relative tolerances is given in Table 1.
Sodium chloride becomes a major problem when either the sodium and/or the chloride are present at higher levels than the plants can take up, and hence build up in the system. A similar problem in some countries, but rarely in Australia, is water with too high a boron content. In practice, the major problems occur within closed systems where the build up can continue until reaching toxic levels.
What happens is illustrated in Figure 1, where the red area is the amount of problem ions and the green area is nutrient ions. This relates to a closed hydroponic system, in which the solution strength (measured by EC—electrical conductivity) is kept constant. If there is a higher concentration of problem ions than the plants can take up, then their concentration will increase with time. This can eventually push into the toxic level and reduce the plant yield and quality. Figure 1 also shows that there is a double whammy here. At the same time, the proportion of nutrient ions is being reduced, and consequently the plants are also heading into nutrient starvation to compound the adverse effects.
Managing excessive problem ions
In an open (non-recirculating) system, the input of problem ions remains constant. If too high, they will rise through the system. The level of problem ions in the run-off depends upon the percentage of the feed being run off. As water quality gets worse, management choices become as follows:
1. Increase the percentage run off to limit the rise in sodium chloride level
2. Change from very sensitive to sensitive to tolerant crops
3. Accept some reduction in yield and quality
4. If still too poor, the only choice is to remove all ions by reverse osmosis.
Closed (recirculating) systems are more vulnerable to poor water quality because the concentration of problem ions continues to rise with time.
The management choices are the same as those above for open systems, except for #1. Here the choice becomes one of occasionally discarding the recirculating solution before the problem ion concentration gets too high. An alternative is to bleed off a proportion of the recirculating solution (often done unintentionally, through leaks).
How frequently you need to discard is influenced by numerous factors. The most important are:
• The worse the raw water quality, the more frequent the discard.
• The smaller the volume of solution in the system, the quicker the build up and hence the more frequent the discard.
• The higher the rate of water use, the quicker the build up, hence discard more frequently during periods of high growth relative to periods of low growth.
Reverse osmosis (RO) equipment uses semi-permeable membranes, which allow the passage of water but not dissolved solids. Hence, it totally removes all ions including the problem ones, giving a stream of virtually pure water.
If you are planning to use reverse osmosis, be aware that pre-treatment is usually essential to prolong the life of the membranes, which can otherwise be an expensive ongoing cost. Also, while part of the input water passes through the membranes to emerge virtually pure, all the incoming salt ends up in the drainage water, which is correspondingly stronger. For example, say the unit is operated to give a pure water flow of 50% of the input water stream. In this case, the waste stream is also 50% of the input flow, but contains all the dissolved salts. Consequently, its strength will be double that of the input raw water.
Posted 31 Apri l 2014