Farming hydroponically is fairly costly and needs close attention, but it’s a method that has a place among those who supply specific markets.
- Hydroponics is the production of crops in the absence of soil, or “soilless culture”.
- If no growth medium is used, it’s called liquid hydroponics.
- If an artificial medium is used to support the plant, the technique is called aggregate hydroponics.
- Various media can be used as a substitute for soil.
- These include gravel, sand, rockwool, perlite, sawdust, etc.
Hydroponic systems enable producers to manipulate some of the characteristics of plants to satisfy consumer demand – product quality, for example, and out-of-season production. All the nutrients are available to the plants in the correct ratios throughout the growing season and crop rotation is not necessary since no soil is used.
Start-up costs are fairly high, however. Hydroponic systems also require a high management level. Another drawback is that not all vegetable crops can be grown profitably using hydroponic systems.
The following hydroponic systems are commonly used commercially in South Africa (SA), which is used an example here.
OPEN BAG SYSTEMS (OBS)
- These systems do not recirculate the nutrient solution after it has passed the plant’s roots (drain to waste).
- An open bag system is most commonly used for the production of tomatoes, cucumbers and green peppers.
- Plants are grown in containers (usually plastic bags) filled with a growth medium such as sawdust.
- The containers are placed directly on the greenhouse floor, which is generally covered with white poly¬ethylene sheeting to isolate the system from soil-borne diseases and to prevent the plant roots growing into the soil.
In SA, sawdust is generally used as substrate or growth medium. It is usually placed in 15 litre polyethylene bags that have drainage holes near the base. One or more plants are planted in each bag or container and an irrigation system is installed to supply the nutrient solution to each bag.
A drip irrigation system is generally used for these systems. Plants are fed individually, using a trickle feed line placed on the bag and into the substrate.
- The risk of root rot spreading is less than for recirculating systems.
- The system provides good lateral movement of the nutrient solution throughout the root zone.
- Some types of sawdust contain chemicals that are toxic to plants, so chemical analyses are necessary before using the medium.
- Management and maintenance levels are high.
Take care, during the growing season, that salts in the nutrient solution don’t accumulate in the medium, as this can hamper plant growth.
In a closed (recirculating) system, the excess nutrient solution is recovered, filtered, replenished and recirculated.
In most of these systems the water supply consists of a thin layer (1 mm to 3 mm) in hydrolines, pipes or channels to ensure there’s enough air, water and nutrient solution at root level. This system is mainly used for the production of leafy vegetable crops such as lettuce.
There are two choices of technique if you are using a closed/recirculating system:
Nutrient Film Technique (NFT)
- The NFT system consists of a thin layer of nutrient solution flowing, by gravitation, through plastic channels (gullies) which contain the plant roots.
- There is no solid growth medium.
- During the growing season a thick root mat develops and some of the roots are in direct contact with the nutrient film layer.
- Avoid submerging all the roots in the water solution, as this will create similar conditions to water-logged soil and have detrimental results for the plants.
The key requirements for managing a nutrient film system successfully include ensuring that the gradient at which the water flows is uniform and not subjected to localised depressions. The base of the channel should be flat and gullies shouldn’t be longer than 30 m to 40 m to ensure even water flow.
Gravel Film Technique (GFT)
- GFT is used widely in SA and is based on the NFT system.
- Gravel is used for the sole purpose of supporting plants and blocking out sunlight at root level.
- The nutrient solution is pumped to the top of the gravel beds, from where a thin layer flows down by gravitation.
- There’s a good balance between oxygen and water, and the nutrient solution is collected at the bottom and pumped to the top again.
- The ideal slope for the GFT is between 1% and 3%.
Although many vegetable crops can be grown successfully in a GFT, the system is ideal for growing leafy vegetables such as lettuce, and results in high quality and a short production cycle.
- GFT can be used for many crops.
- There’s good aeration of plant roots.
- Watering and feeding of plants is uniform.
- Water and nutrients are used efficiently through the recycling system.
- GFT is suitable for non-arable areas.
- The system is fairly costly to build, maintain and repair.
- Thick root mats may slow down water flow and puddling may occur.
- Root rot can spread rapidly through cyclic systems.
- This article was written by Silence Chiloane and first appeared in Farming SA.