What is Hydroponics?

Growing plants without soil has intrigued and challenged gardeners, hobbyists and commercial growers for many years. The terms - hydroponics, water culture, sand culture, gravel culture, solution culture, mist culture, soil-less culture and more are often used describe a particular system of applying plant nutrients to the roots of the plant, and each, in its own way, is a method of substituting some other medium for soil.

In general the terms hydroponics has gained popular appeal, and is widely used to cover 'all ways of growing plants without the use of soil'.

Hydroponics the word, is derived from two Greek words,

hydro

meaning water, and

ponos

meaning to work or labour.

It is reported that hydroponic production can yield around 25% more than ground grown crops and averages four times the number of crops per year. The economic benefit of this can be enormous, however set up costs can be very high due to the infrastructure required.

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The Nature Of Hydroponics

Soil does four main things for the plants which grow in it. It provides support, supplies water and air to the roots, and provides nutrients. To be successful, hydroponics needs to cater for these four functions.

Types Of Systems

There are endless possibilities with hydroponics. The options open to you in creating a system are only limited by your imagination and your budget. Provided you can supply the plant with those things it needs achieve good growth, you can develop a system any way you wish. Systems can be very simple or very complex. They can be cheap or very expensive. On a farm it may be possible to utilise existing materials to some degree (e.g. plastic irrigation pipes, greenhouses). You might limit yourself to supplying enough product for you own use, perhaps later expanding as you gain more expertise into commercial production. If you have the expertise to grow plants commercially in the ground then it is likely that you could 'convert' to soil-less production quite readily.

The Location

This is a key factor because it influences everything else. If the system is grown under cover (e.g. in a greenhouse) then the environmental conditions are more readily controlled. The temperature may fluctuate less than outside conditions. Wind problems will be reduced and so on, however natural rainfall can't be readily utilised, unless it is captured (e.g. in tanks or a dam) for later use. If the system is outside it will be exposed to rain which may dilute the nutrient solution, and wind which can damage plants or blow debris into the system. Frost, hail, snow, etc. could also create problems. Temporary frost protection (e.g. removable covers) might be provided for outdoor systems at frost or hail danger times. Well designed windbreaks can help significantly reduce the likelihood of damage to both the plants and the hydroponic system itself.

Pest and disease problems can usually be reduced by keeping the hydroponic system isolated from those problems by growing indoors, by raising the system off ground (away from the soil which is a major reservoir of pest and disease problems), or locating your system in an isolated area away from other crops that may act as potential sources of pest and disease infestations. Outdoor systems will need to be securely fenced off from animals (e.g. cattle, sheep, in particular goats), otherwise plants may be eaten or trampled.

The Container

The plant roots, as well as the nutrient solution and medium, need to be contained by something, otherwise nutrient solutions, and any growing media would quickly run/fall away. You might, for example, have gravel, sand or perlite contained in plastic bags, in pots or in tubs. You might have rockwool fibre, sand, gravel or scoria contained in a raised bed built from timber, metal, concrete or plastic. You might use polystyrene boxes, fibreglass tanks, PVC pipes with slots in it, plastic roof guttering, square cross-sectioned PVC channel specifically designed for hydroponics, and so on. The list of possibilities is endless. The main criteria is that the container is large enough to support the particular plant/s you are growing, will contain the nutrient solution for long enough for the plant to utilise the nutrients in the solution, is stable so it won't fall over, is durable enough to last at least until the crop, etc is harvested (it will preferably last much longer), and is chemically inert (i.e doesn't release chemicals that might affect plant growth - some metal guttering, for example, corrodes over time).

Watering & Nutrients

Water quality is a critical factor in the success of hydroponic growing. Water should ideally be as pure as possible. Impurities such as sediments, salts, pathogens, etc. can block up dispensing systems (e.g. nozzles), cause toxicity problems, or affect the balance of nutrients in the solution, or rapidly spread disease. The water may need prior treatment before it is suitable for use (e.g. settling tanks to reduce sediments, filtering, disinfecting). The quality of any water you use should be regularly monitored. Reliability of supply also is critical, particularly if you are not recycling the water through the system.

Nutrient solutions may be applied automatically at predetermined times or as required. In the case of Nutrient Film Technique (NFT), a continuous thin, stream of nutrient solution flows through a channel which the plant roots sit in. The solution could be supplied at the bottom of the growing media (e.g. sand, gravel) and allowed to move upwards through the media by capillary action, or alternatively supplied at the top of the media (e.g. through drippers), and allowed to filter downwards. 

It may be pumped onto the media, moved manually (e.g. from a watering can or bucket), or be gravity fed, such as from a tank up slope of the hydroponic system. The excess (runoff) might be collected and reused (this is known as a closed or recirculating system), which reduces waste of both water and nutrients, but needs careful management to control nutrient levels in the solution, and minimise the risk of recirculating and disease problems that may have arisen in the system (the reused water may need to be disinfected). Alternatively the nutrient solution may be lost after passing through the media (known as an open or run-to-waste system). This type of system uses more water and nutrients, but can reduce the likelihood of disease problems or salt build up in the system.    

Trellising

This is not always needed, but when growing tall or creeping plants, such as tomatoes, cucumbers, chrysanthemums, carnations, and roses, the medium may not be strong enough to support the plant. Trellising also helps control the direction of plant growth and better utilises space (more plants can be grown, with higher yields if they are grown upwards, rather than being allowed to sprawl). Trellising can be made from such things as wire (single strand or mesh), plastic mesh, strings, wooden posts or trellis, fishing line, etc.

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Types Of Systems

There are two main groups of systems. These are water, where the roots grow directly in water, or a mixture of water and air, or aggregate, where the roots are grown in a solid material, such as sand or gravel.

In water culture systems, nutrients are dissolved in water which is brought into direct contact with the roots. The water is either aerated, or the roots are allowed to contact air as well. Trellis, wire mesh or some other support is needed above the nutrient solution to provide support to the plant. Examples are NFT and mist (aeroponic) systems.

With aggregate culture, nutrients are dissolved in water which is moved into the root area. The roots are grown in a solid material which is chosen for its ability to hold sufficient moisture, but drain off any excess allowing sufficient aeration. The solid material which the roots grow in contributes to providing support to the plant, although further support (e.g. trellis) may be needed in some cases (e.g. plants that grow tall, climbing plants, plants heavily laden with fruit). Examples are beds or pots containing gravel, scoria, and sand, and beds or benches containing/supporting rockwool.

Solid Media

Media is the name given to the solid particles (e.g. sand, perlite, gravel) which the plant roots grow in hydroponics. All hydroponic media must be sterile (free of pests and diseases), and inert (free of nutrients). Other important characteristics of the medium which must be considered include its water holding capacity, its air holding capacity, its drainage, and its nutrient holding capacity.

A medium which drains well (e.g. gravel, coarse sand) is usually well aerated, but can have a low water holding and nutrient holding capacity. These types of media need to be irrigated more often, or perhaps even as a continuous flow. There is therefore a relationship between the type of media you might choose for a system and the way in which it is irrigated. A medium which holds water and nutrients better (e.g. rockwool) can usually be irrigated less frequently (it might be as infrequently as 3 days in summer, or up to 7 days in winter).

Commonly used media in hydroponics are:

Vermiculite

This is a mineral derived from mica that is commonly mined in South Africa and the USA. It needs to be mixed with other media to get the best results. Even though it retains air well (high air holding capacity), it can retain too much water for many plants. If used on its own, it can , after a year or so, turn puggy (i.e. the structure can collapse). Mixed with gravel or sand (no more than 40 - 50% vermiculite this collapse of structure is less likely to occur, drainage is improved, and it becomes a more ideal media.

Sand

Coarser granitic or silica type sands should be used. Beach sand is not suitable because of high levels of salt in it. The most ideal type of sand is a coarse, washed, granitic sand. This is the same type of sand used by nurserymen in plant propagation medias, and used in fish aquariums. Fine sands may hold too much moisture, and may be much harder to contain (i.e. will ooze out of drainage holes when wet).

Gravel

Similar properties to coarse sand, but even better drained. Be careful of using sharp edged gravel on soft tissued plants (it can abrade stems as the plants move in air currents, when knocked etc.).

Perlite

Like vermiculite, this is a processed mineral. It has excellent water holding properties, but is less spongy and better drained than vermiculite. Perlite is often used by itself or in a 50/50 mix with vermiculite. It's main disadvantages are that it is fairly expensive, it is very light and can be easily blown way, it can be easily crushed or broken down with use reducing its good drainage and air holding abilities.

Expanded Plastics

These materials are inert, and in many cases relatively inexpensive (e.g. polystyrene bubbles). Their major disadvantage are that they have poor moisture of nutrient holding abilities, they are very light weight and when mixed with other materials tend to separate out and float to the top of the mix, and the provide little support for the plant.

Scoria

This is a porous volcanic rock which can be obtained in a wide variety of grades. It is fairly cheap, but it tends to hold nutrients too well, and there may be salt build ups. There may also be a problem with pH (the measure of how acid or alkaline the media is). It tends to be a bit sharp edged, and can be abrasive to soft-tissued plants.

Rockwool

This is a fibrous material, made from spun rock fibre, that was originally produced as an insulating material. Only horticultural grade rockwool should be used in hydroponic systems. Rockwool is around 97% air space, totally sterile and holds a very large amount of water, while still holding an ample amount of air for most plants. It is excellent for both propagating plants and for growing longer term plants in hydroponics. It does tend to collapse though after a few years use. Disposal of used rockwool can also be a problem. It can also cause itching and allergies when handled.

Nutrient Solutions

Studies into plant nutrition have shown that nutrient solutions must contain nitrogen, phosphorus, potassium, magnesium, calcium and perhaps sulphur to obtain reasonable growth. These are added in the form of chemical salts or compounds. In addition to these the plant also needs oxygen hydrogen and carbon, but these are provided from air and water. These aforementioned elements are commonly called the major or macro elements.

Research has also shown that a large number of other nutrients are also required, but in very small quantities (these are known as trace elements or micro nutrients). These where not picked up in early research into nutrient solutions because they where almost always supplied as impurities in the water used in the solutions (they could also have been contained in contaminants such as dust obtained from the air). These so called trace elements include iron, copper, boron, molybdenum, zinc and manganese. They are just as important for healthy plant growth as the major elements, but are only required in generally small amounts.

Though there are rough similarities between the amounts of nutrients that most plants require, to get the best out of plants you grow in hydroponics, certain balances of nutrients are needed for different plants, and at different stages of each plants growth (e.g. vegetative growth, producing fruit). Nutrients are fed to the plants in the form of a nutrient solution which contains chemical fertilizers dissolved in water.

If you want the best out of hydroponics you have to know a lot about plant nutrients, and unless you are a chemist, that takes a bit of learning. Don't be put off though. You can still grow plants in hydroponics quite successfully without getting that involved in the chemistry aspects. It is possible to buy standard nutrient mixtures from hydroponic suppliers. These may be quite good for you needs, or at least adequate. As you get more expertise you can modify these basic mixtures to get better results with specific types of plants, or even start developing your own tailor-made mixes.

Though the best results are generally obtained from tailor-made mixes, a very basic solution for the beginner can be made as follows:

6 parts of a standard liquid fertiliser such as Aquasol or Thrive (in dry form)

1 part epsom salts

5 parts gypsum

Take one teaspoon of this mixture and mix it thoroughly with 9 litres (one bucket) of water. This solution will grow most plants reasonably well.

Plant Problems In Hydroponics

There is very little difference in what can go wrong with plants in hydroponics compared with plants grown in soil. The three main problem areas are pests and diseases, nutrition and environment.

Generally pest and disease problems are lower than for plants grown in soil, particularly those diseases that are commonly soil-borne. Fungal problems are most likely to occur in moist and humid conditions. If you are not careful in your selection of a medium and the irrigation rate used, you might have more fungal problems in hydroponics than in soil. Obviously you are more likely to have problems with nutrients than you would in soil, because nutrition is totally dependent on the operator of the system. Wind can sometimes be a greater problem for outdoor hydroponic systems than in ground production because the plant does not have as strong a support system for its roots. Keeping correct moisture levels might also be a problem in some systems. Much of these problems can be reduced as you gain more expertise, and start tinkering around or modifying your system.


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