THE BASICS OF HYDROPONICS Water quality In soilless systems, roots cannot escape from undesirable situations like high salt concentrations or unfavourable pH. Therefore it’s very important to use pure water. Collected rainwater or demineralised water can be used without restrictions. The usability of tap water or ground water Growing plants in hydroponics or soilless culture is the opposite of normal soil, namely, roots grow in a limited volume. Consequently the volume of available water and nutrients is limited and the system is susceptible to salinity and pH fluctuations. The growing medium can be a substrate like peat, coco peat, perlite, mineral wool or even water. In fact, plants can grow in any material as long as the material provides for three basic necessities: oxygen, water and nutrients. In addition to the growing medium there are other very important aspects for hydroponics, which are explained briefly here.
Article written for Reiziger by
University & Research Center. The Netherlands.
depends on the chemicals it contains. When using pure water, the drainage from the growing system can be reused. This creates a closed system, which saves water and fertiliser and helps the environment. If the water quality is imperfect, over irrigation is necessary to avoid the accumulation of salts. The amount of extra irrigation depends on the crop and the water quality. The leachate is still a valuable source of water and nutrients and can be used to irrigate plants growing in the soil. Otherwise it should be disposed of.
QUICK TIP Rainwater is one of the best options
The usability of tap water or ground water depends on the chemicals it contains.
Water quantity Plants need water for growth and for transpiration. 90% of transpiration depends on light from the sun or from artificial lighting. As a rule of thumb the transpiration rate can be estimated at 1L/m2 for a radiation sum of 500J/cm2. To give an idea, this is the amount of light on a very cloudy day in summer, or is equal to 10 hours of light with 140W/m2 or 16.000 lux. Depending on the age of plants, the size of the growing system and the growing medium used, it will contain just enough water for one day. For example, a 5L pot with stone wool contains approx. 3L of water; a mature tomato plant requires up to 4L per plant for transpiration on sunny days. Moreover, not all the water of the substrate is available for the roots and very strong fluctuations should be avoided as well, making an adequate irrigation system necessary.
Irrigation system Irrigation is needed to refill the water uptake from the substrate. The best way to supply plants with water is drip irrigation. There is a wide variation in types of drip irrigation systems, varying from cheap capillaries, to the advanced labyrinth emitters. The most suitable system depends on the water quality and the available technical infrastructure. Capillaries are cheap, can be used with low pressure (0.50.8bar) but are prone to clogging, so these need pure water and adequate EC and pH control. The more expensive emitters require higher pressure (> 1bar) but clogging is less of a
velopment due to differences in water availability, the irrigation should ensure that the biggest plants get sufficient water. This means that smaller plants will receive too much. This is why substrates must have perfect drainage capacities and be adequately drained from the pots or containers. As a rule of thumb 10% drainage is needed to overcome the variances. For small pots and containers, one dripper is sufficient. For large pots, two or more are needed. Preferably plants should be grown together in larger units with several plants and drip nozzles, like in troughs or grow bags, which levels out differences
problem. Nevertheless, in all systems
between plants and drippers.
micro-organisms will eventually devel-
Nutrients
op in the water system and may cause clogging. This can be prevented by adding small quantities of sodium hypochlorite (bleach) max 1ppm, or hydrogen peroxide max 5ppm, to the irrigation water. Individual plants will differ in water demand, due to differences in the plant size, exposure to sunlight, the substrate and the irrigation. Even in the case of a new drip irrigation system, the supply of individual nozzles will vary. To avoid variances in plant de-
All plant species need twelve nutrients for growth and development. There are six macro elements: nitrogen (N), potassium (K), phosphorus (P), calcium (Ca), magnesium (Mg), sulphur (S) and six micro elements: iron (Fe),
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manganese (Mn), zinc (Zn), boron (B), copper (Cu) and molybdenum (Mo). Plants take up these nutrients or elements in the ionic form. These can be supplied in the form of fertiliser salts, which dissolve in water.
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Nutrient film technique irrigating a lettuce crop grown in gutters.
The total volume of water and nutrients in a soilless system is too limited to contain all the nutrients for a long period.The buffering capacity differs between substrates. Organic substrates require a base dressing with fertilisers, as the organic matter has an ‘exchange capacity’ for ions and needs to be loaded. Therefore peat or coco coir substrate can cover the plant demand for several weeks (macro elements) or even a whole cropping cycle (some micro elements). Inert substrates like stone
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wool or perlite will contain only sufficient nutrients for a couple of days, so these need to be supplied with sufficient nutrients. The most appropriate way is to supply the fertilisers with the irrigation water. This method is called ‘fertigation’ and the mixture of irrigation water and nutrients is called ‘nutrient solution’. For practical reasons the easiest way to prepare nutrient solutions is to use concentrated liquid compound fertilisers. These have a predefined composition of micro and macro-nutrients suitable for many crops. Since Ca and SO4 cannot be mixed together in concentrated form, these compounds are offered in separate A and B solutions, which need to be diluted in equal ratios in the irrigation water to compose a nutrient solution. There are several ways to arrive at the nutrient solution to be irrigated,
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either by using automatic devices like proportional dosing pumps, 2
Drip irrigation in a gerbera crop grown on stone wool.
venturi systems, diluters etc. - or manually preparing buffer tanks with the required concentration.
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Drip irrigation in a cucumber crop grown on coconut coir pith.
Electrical Conductivity
pH
Since nutrient solutions consist of
The pH plays an important part
pure ions, the total concentration of nutrients can be determined by the electrical conductivity (EC) and can be used to measure and control the dosage. At the same time, plant roots need a certain concentration of each individual nutrient for optimum growth. The sum of these specific concentrations can be expressed as EC value. So a plant needs a certain EC for optimum growth. Too low EC values will lead to depletion of the root environment for nutrients. Too high EC values will cause accumulation of nutrients and will hinder the water uptake, eventually leading to a growth reduction. For sufficient uptake of all nutrients the EC in the root environment should be kept at a certain target level. To achieve and keep this EC in the substrate, the nutrient solution supplied must always be kept lower. The target values for the EC in the root environment and the supply differ for each crop. They also depend on the age or growing stage of the plants and on environmental factors such as the climatic conditions, light, season etc. High EC levels can be used to a certain extent to manipulate the plant in a specific direction, as it reduces the stretching, stimulates the generative development and has positive effects on fruit quality. However, it requires proper control over irrigation and drainage.
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in determining the availability of nutrients in the root environment. The ideal pH is between 5.2 and 6.2, as all nutrients will be available. Below 5.2 will make Mo less available and the root development will be negatively affected. pH levels above 6.2 reduce the availability of micro-elements as well as P. For the irrigation water in particular, higher pH levels increase the risk of clogging in the drip irrigation systems, as Ca-salts will occur easily. To prevent pH problems, the irrigation water should firstly be of good quality. Hard water contains bicarbonate (HCO3) and will boost the pH. A little HCO3 (up to 125ppm) is acceptable in case of higher concentrations, acidification prior to, or together with the fertilisers. Adding extra NH4+ as a nitrogen source can counteract overly high pH levels in the root environment. However, the supply of more than 20% of the N as NH4+ should be avoided. Low pH in the root environment can be alleviated by the addition of potassium bicarbonate. Control and regulation of the pH can be carried out by the supply of acids (HNO3) or alkaline (KOH/ KHCO3).
The ideal pH is between 5.2 and 6.2, as all nutrients will be available. Below 5.2 will make molybdenum less
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