Hydroponics and soilless culture
Growing crops without soil: NFT, deep water culture and substrate systems, nutrient solutions, EC and pH control, pros and cons, and a stock solution sum.
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Plants need water, nutrients, oxygen at their roots and support, but they do not strictly need soil. Hydroponics, growing plants with their roots in a nutrient solution, and soilless culture in inert substrates such as rockwool or cocopeat, supply these directly. Most greenhouse tomatoes, cucumbers, peppers and many cut flowers in northern Europe, and a growing share of lettuce and herbs worldwide, are now grown this way.
Hydroponic systems, nutrient solutions and their management are frequently examined in horticulture and protected cultivation courses.
A brief history
Plant physiologists grew plants in nutrient solutions in the nineteenth century to work out which elements are essential. In 1929, W. F. Gericke at the University of California grew tomatoes several metres tall in solution culture and coined the word hydroponics. D. R. Hoagland and D. I. Arnon published their widely used nutrient solution recipes in 1938. In the 1960s and 1970s, Allen Cooper in England developed the nutrient film technique (NFT), and rockwool slabs from Denmark became the standard greenhouse substrate.
Types of system
Solution culture (true hydroponics)
- Nutrient film technique (NFT): a thin film of solution flows down slightly sloping channels past the roots and returns to a tank. Uses little solution, but a pump failure can kill plants within hours. Common for lettuce, herbs and strawberries.
- Deep water culture (DWC) or floating raft: plants sit in rafts floating on a deep, aerated tank. The large volume buffers temperature and nutrients. Common for lettuce and leafy greens.
- Aeroponics: roots hang in the air and are misted with solution; used in some research, seed potato minituber production and vertical farms.
- Ebb and flow (flood and drain): benches or trays are periodically flooded and drained; used for pot plants and seedlings.
Substrate culture
Plants grow in an inert or organic medium fed by drip fertigation:
- Rockwool (stone wool) slabs: uniform and easy to manage, but difficult to dispose of.
- Cocopeat (coir): from coconut husks, renewable and widely available in the tropics; must be washed to remove salts.
- Perlite, vermiculite, expanded clay, sand, and bark or peat mixes.
Substrate systems may be open (drain water runs to waste) or closed (drain water is collected, disinfected, topped up and reused). Closed systems save water and fertiliser and reduce pollution, and are required in some countries.
The nutrient solution
A complete solution supplies all essential nutrients in balanced proportions, typically as two concentrated stock solutions (A and B) kept separate because calcium precipitates with sulphates and phosphates when concentrated:
- Tank A: calcium nitrate, potassium nitrate, iron chelate.
- Tank B: potassium phosphate, potassium sulphate, magnesium sulphate, micronutrients.
They are diluted together into the irrigation water, often at 1 : 100.
Two measurements control the solution:
- Electrical conductivity (EC), a measure of total dissolved salts. Typical targets are about 1.5 to 3.0 mS/cm depending on crop and stage: lower for lettuce, higher for tomatoes, where higher EC improves fruit flavour but reduces size.
- pH, usually kept at 5.5 to 6.5, where most nutrients stay available. Acid or alkali is dosed automatically.
Solution temperature (ideally about 18 to 24 °C) and dissolved oxygen matter too: warm, stagnant solution holds little oxygen and encourages root diseases such as Pythium.
Advantages
- High yields and quality, with precise control of nutrition.
- Water efficiency, especially in closed systems, which can use far less water per kilogram of produce than open-field crops.
- No soil-borne diseases, nematodes or weeds at the start, and no need for soil fumigation.
- Crops can be grown where soils are poor, salty or absent, such as deserts, rooftops and cities.
Disadvantages
- High capital cost and dependence on electricity, pumps and monitoring.
- Skills in water chemistry and system management.
- Rapid spread of root diseases in recirculating systems, needing disinfection (UV, heat, filtration or chemicals).
- Disposal of used substrates and drain water.
- Not all crops are economic; staples such as wheat and rice are grown cheaply in soil.
Hydroponic produce can be certified organic in some countries but not in others, notably the European Union, where organic production must be soil-based.
Making a nutrient stock solution
A grower wants 150 mg/L of nitrogen (150 ppm N) from calcium nitrate in the final solution. The fertiliser grade contains 15.5 per cent N and 19 per cent Ca.
- For 1,000 L of final solution: N needed = 150 mg/L × 1,000 L = 150 g.
- Calcium nitrate needed = 150 ÷ 0.155 ≈ 968 g per 1,000 L.
- This also supplies 968 × 0.19 ≈ 184 g of calcium, so 184 mg/L Ca.
Concentrated stock. With a 1 : 100 injector, each litre of stock is diluted into 100 L of irrigation water. A 100 L stock tank therefore supplies 10,000 L of solution and needs 968 × 10 = 9.68 kg of calcium nitrate dissolved in tank A.
Other N sources in the recipe, such as potassium nitrate, would reduce the calcium nitrate needed; growers balance all salts with a recipe spreadsheet and check the final EC.
Key points
- Hydroponics grows plants in nutrient solution; soilless culture uses inert or organic substrates with drip fertigation.
- Systems: NFT, deep water culture, aeroponics, ebb and flow, and substrate slabs (rockwool, cocopeat, perlite).
- Keep calcium salts (stock A) apart from sulphates and phosphates (stock B).
- Control EC (about 1.5–3.0 mS/cm) and pH (about 5.5–6.5), solution temperature and oxygen.
- Benefits: yield, quality, water efficiency, freedom from soil problems; costs: capital, energy, skills and root-disease risk in recirculation.
- Fertiliser = target ppm × volume ÷ nutrient fraction.
What the exams ask
- School-leaving and entrance papers: what hydroponics is, its advantages and limits, and crops grown.
- Degree and research-fellowship entrances: types of systems, substrates, nutrient solution formulation, EC and pH management, and closed versus open systems.
- Professional examinations: designing hydroponic units, calculating stock solutions, disinfection of recirculating water, and economics.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Do hydroponic vegetables taste worse?
Not necessarily. Flavour depends mainly on variety, light, nutrition and ripeness at harvest. Hydroponic tomatoes grown with appropriate EC and harvested ripe can taste as good as soil-grown ones.
Can I use ordinary fertiliser in hydroponics?
Field fertilisers often lack micronutrients or contain insoluble fillers. Hydroponics needs fully soluble, balanced fertilisers, and usually separate A and B stock tanks.
Why did my NFT lettuce wilt suddenly?
A pump failure or blocked channel is the most common cause: in NFT, roots dry out within hours. Root rot from warm, poorly oxygenated solution is the other usual culprit.
Sources
- Resh, H. M. Hydroponic Food Production, 7th edition. CRC Press, 2013.
- Raviv, M., Lieth, J. H. and Bar-Tal, A. (eds) Soilless Culture: Theory and Practice, 2nd edition. Elsevier, 2019.
- Hoagland, D. R. and Arnon, D. I. The Water-Culture Method for Growing Plants without Soil. California Agricultural Experiment Station Circular 347, 1938 (revised 1950).
- Cooper, A. The ABC of NFT. Grower Books, London, 1979.
- Sonneveld, C. and Voogt, W. Plant Nutrition of Greenhouse Crops. Springer, 2009.
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