Hydroponics is the science of growing plants without using soil, feeding them on mineral nutrient salts dissolved in water. There has been renewed interest in this method following the broader installation of living walls, roof gardens and other growing situations where conventional potting media could be lighter, more manageable and more lightweight.
Suitable for…
Any plant can be grown hydroponically, but the method is most widely used to produce greenhouse crops; cucumbers, peppers and tomatoes. Florists’ roses are an example of cut flower production.
All plant roots need oxygen (air), and the root zone in hydroponic cultivation must be moist, nutrient-rich, and very well-aerated. The seeds of most plants will die in stagnant water.
Hydroponic techniques
Hydroponic systems can be divided into two basic types:
- Those requiring aggregates or other porous rooting media (substrate) to support the plant roots – sometimes referred to as hydroculture.
- Those are not needing aggregates to support the plant roots – the most widely used term being nutrient film technique (NFT).
At its simplest, hydroponics can be a plant growing in a pot of inert aggregate (such as perlite) and given water containing a liquid fertiliser from a watering can sufficiently often that it does not dry out. The spaces between the aggregate have aired so that roots do not suffocate.
In practice, hydroponics is more sophisticated. Many plants are watered via a drip system fed with a pump from a stock tank of nutrient solution. The nutrient solution can run to waste but is less expensive and less potentially polluting to recirculate the nutrient solution via a trough below the plants that run back to the stock tank.
Commercial tomatoes, for example, are grown in ‘pillows’ of rockwool drip fed with nutrients, and the nutrient solution is re-used. A variant of this is ebb and flow, where the plant containers (filled with rooting media) are stood in a trough or basin that is periodically flooded with nutrient solution, which is then allowed to drain back to a stock tank.
The nutrient solution is a balanced mix of major nutrients (nitrogen, phosphorus, potassium, magnesium and sulphur) and a low concentration of minor nutrients or trace elements; copper, boron, iron, manganese, molybdenum and zinc.
The pH is crucial, and a pH meter or test papers are required. A pH of 5.8-6.2 is usually aimed for. Citric acid is used to lower pH, and baking soda is used to raise pH, although proprietary materials are offered.
Nutrient solutions can be made from straight chemicals such as potassium nitrate and magnesium sulphate or bought ready-made. As plants remove water, nutrient solutions become more robust and become so concentrated or ‘salty’ that roots can be damaged. This can be assessed by testing the electrical conductivity of the solution with a conductivity meter and adding water as required. However, even with careful monitoring, the nutrient balance changes and every two weeks or so, it is advisable to discard the solution and make up fresh. The discarded resolution can be watered onto garden soil as a fertiliser.
Dispensing with the growing media is entirely possible and cuts the costs and labour of growing.
One of the first ways of doing this was the nutrient film technique. Here the nutrient solution is allowed to trickle along gently sloping covered gutters. Plants are grown in small pots or cubes of rock wool and placed at intervals in the trenches. Their roots form a thick mass in the humid, well-aerated channels, and robust plant growth can result. The surplus nutrient solution trickles back to a stock tank, and a pump is used to recirculate it. If the pump stops, the plants quickly die as there is no supporting medium to moisture to allow plants to survive until the nutrient solution circulation can be restored.
Again, as above, pH and conductivity must be monitored and maintained, and nutrient solutions must be renewed occasionally.
Although now little used in commercial horticulture, the nutrient film technique is still used for teaching and home gardening.
A variant of this is aeroponics, where the roots grow in a chamber below the plants where a drizzle of nutrient solution is periodically applied by a suitable pump and spray nozzle(s). As with the nutrient film technique, any pump failure can quickly damage plants.