Nutrients

Generally you'll need three types of nutrient mix. Sold as liquid concentrate, these are added to water in different proportions according to the plants needs, determined by where it is in its life cycle.  The first two macronutrient mixes have a different balance of NPK - less phosphorus (P) and more nitrogen and potassium (N & K) when putting on leaves, and the reverse when in flower. The reasoning is the same as for switching from 'multi-purpose fertilizer' to 'tomato food' when chillies (and tomatoes!) are starting to flower, the relative ratios of NPK are different at the different stages.  The third bottle contains micronutrients or trace elements that the plant would get from soil or compost if it was grow in the conventional style. Sometimes trace elements are present in the first two mixes so you won't need this third bottle. Macro and micronutrients are listed below:

The nutrient mix needs to be periodically checked to make sure that there is the right amount of dissolved salts, too little will result in stunted growth, too much of one of them may make the other elements difficult to absorb. Both will result in deficiency symptoms, yellow or falling leaves or flowers, and plant failure if not corrected.  The mix is checked using an EC (Electrical Conductivity) or CF (Conductivity Factor) pen.

This electronic tester is the one piece of technology that it's difficult to do without if you intend recycling your nutrient solution. The alternative, and probably more expensive in the long run, is to regularly flush the nutrient and start over with a fresh batch. When dipped in the nutrient mix the EC pen uses a measure of the current passed to determine the amount of nutrient present as dissolved salts. The more nutrient in solution the higher the conductivity measured. Adjustments can then be made adding more nutrient or water as required. EC pens measure in millisiemens per cm², CF (conductivity factor) or PPM (parts per million). Nutrient manufacturers publish the preferred concentrations and EC readings for their products on the packaging. Usually 1.5-2.5 millisiemens or 15-25 CF is the target, less than this will result in deficiencies, more than 30 CF and the plant will also deteriorate.

Even though the EC pen gives an indication of the total amount of dissolved salts in the nutrient solution it can't tell you the ratios of NPK. For example, it's possible to have a surplus of potassium and a deficit of nitrogen. This is more likely the longer the period between complete nutrient flushes as small differences compound over time. This will lead to problems for the reasons outlined above. The risk of this can be reduced by periodic flushing the mix every fortnight, not forgetting to cycle clean water through the substrate to flush that too. The old nutrient can be further diluted and sprayed on more tolerant soil based garden plants. Short-term nutrient imbalance can be rectified using a foliar fertilizer. This should only be used as a quick fix, the underlying problem should be rectified as soon as possible.

pH is a scale used to measure a liquids acidity or alkalinity. The scale goes from 1 to 14 with a reading of 7 indicating a neutral balance, neither being acid nor alkaline. A plants ability to take up nutrient is determined by pH and temperature. Nothing new here, knowing soil pH and making adjustments in conventional gardening can help improve plant growth and tests for soil pH are commonly available.  It's the same in hydroponics although the liquid nutrient solution makes electronic testing possible with a pH pen. There are liquid tests that are initially cheaper but over time a pen is more cost effective and less messy. Different dissolved salts are taken up less effectively as pH varies from low (acid) to high (alkali) A good point to aim for is between 5.8-6.8, if pH rises much above 7.5 then problems can occur. Correction is be made if necessary using pH adjusters sold at hydroponic and aquatics stores. These acid and alkali chemicals are dilute enough to be safe. Often the mix becomes too alkaline and can be rectified with household vinegar. Gradual adjustment should be made in order to avoid pH swings, allowing sufficient time to mix and flush through any media or substrate. Small variations can be ignored if less than 0.5 from target.

The table below shows nutrient availability at different pH values. The graph plots the total nutrient available at increasing pH values. It peaks at around 6.5, both side of which the total available drops.

As mentioned above, temperature affect plant growth, this is probably not news either. However, plants benefit from a constant level of heat at the roots as well as in the air around the leaves and will sustain better growth under these conditions. In hydroponics this is easily achieved using a small aquatics heater to maintain the nutrient solution at about 24-27 deg C. The heater size is determined by the volume of nutrient, as a rule of thumb 1 watt per litre is sufficient so 100-150 watts is suitable for a 100 litre sump. Choose one of the unbreakable heaters that has a polymer or stainless steel case or one that is at least shock and shatter proof.

The nutrient tank is best insulated with polystyrene tiles or slabs to minimise nighttime heat loss, this also has the benefit of excluding light that minimises algae growth in the system. Heating is not strictly necessary, plants will still do well without, especially during the summer months, but it's particularly beneficial early in the season with small plants and seedlings.

Even if you aim to grow a single type of plant hydroponically you usually end up growing two, your favorite crop and algae. Algae are an unwelcome addition as it uses nutrient, can clog pipes and pumps, can harbor pests and looks a mess.

Algae are one of the oldest waterborne plants on the planet and it has become supremely well adapted to taking any advantage it can to grow. Hydroponic systems are perfect for algae, lots of liquid nutrient, oxygen, warmth and light. Limiting light is the best way to prevent its growth. Black plastic components should be used where possible, if not then light should be excluded by covering. Dustbin liners are good for this and when cardboard or polystyrene sheet is placed inside it make an adaptable material for boxing in nutrient sumps or covering exposed substrate.

Many systems include an air pump and air stone to aerate the nutrient solution. This action is used in simple aeroponics systems to provide a fine spray as the bubbles burst at the surface. In all other systems it's used to provide circulation of the nutrient within the holding container, helping to keep it fresh.

Oxygen is poorly adsorbed by water, the larger the surface area between the two the better. As the bubbles add very little to the total area, they add very little oxygen directly. Far greater is that added at the surface of the solution where the area is large. Bubbles help by causing a gentle flow in the solution, as they rise the surrounding liquid also rises. This flow moves the lower part of the solution to the surface where it can take up oxygen. This movement can also be used directly in some pot-based systems to move nutrient up onto the substrate through a tube, replacing the pump.

In continuous flow systems there is often no need for anything other than the returning nutrient to increase surface flow. In other systems it's possible to divert some of the flow from the pump to cause currents in the solution by splitting the outlet into two pipes using a T-piece and a valve to balance the output.

There are plenty of web and mail order sources of nutrient if you're not lucky enough to have a local supplier. Try yellow pages, you'll be surprised just how many shops there are hiding away. See what they sell, go to the manufacturers web site and read about the different offerings, then pick a brand suitable for the hydroponic system you intend using. Canna and BioBizz are very common; either is fine, there are others equally as good.

If you want to experiment you can try to mix your own. In practice this is often difficult as the sale of some fertilizer, potassium nitrate for example, is prohibited in the UK. The others are difficult to obtain in manageable pack sizes for home use.  If you do decide to brew your own then make sure that the ingredients are high quality and   completely soluble. If they are not then they will be unavailable to the plants and the build up of undissolved solids may eventually block finer bore tubing and drippers, typically resulting in a catastrophic failure, usually when you've gone away for a few days. Only consider a tried and tested recipe to provide a balanced mix of nutrients.