Boosting agricultural productivity via small-scale vertical farming

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Zikhona Buyeye, a junior researcher at the Agricultural Research Council’s Natural Resources and Engineering Division, explores the importance of vertical farming structures for small-scale vegetable production.

Boosting agricultural productivity via small-scale vertical farming
An example of a horizontal hydroponics set-up.
Photo: Oregon State University/Flickr

A quick Internet search of the term ‘vertical farming’ produces images of seemingly futuristic buildings housing an abundance of luxuriant plant life. While these complex architectural designs and cutting-edge technology might seem far-fetched, vertical farming is, in fact, a system that can be realised sooner rather than later.

The keys to achieving this dream in South Africa may be found in simplification and downscaling. In fact, more practical and easily implementable designs could provide solutions to the country’s food security problems, as well as the rising costs of producing and transporting food.

Small-scale vertical farming structures, which predominantly employ hydroponics, offer an achievable means of improving agricultural productivity.

When plants are grown hydroponically, their roots are immersed in a nutrient solution rather than in soil. The key components of such a system are a nutrient-solution reservoir, a structure to house the plants, and a growing medium to anchor them. Optional elements can include a nutrient recirculation pump, an air pump and grow lights.

Active hydroponic systems
Hydroponic systems can be either active or passive, and vary in complexity. In an active system, a pump is used to recirculate the nutrient solution.

Common examples of active systems are the drip system, the flood-and-drain system, and the nutrient film technique (NFT).

In a drip system, the nutrient solution is pumped from the reservoir and drips onto the growing media, from where it drains down to the plants’ roots and is recirculated.
With a flood-and-drain system, the plants are kept in a reservoir that is routinely flooded with the nutrient solution. The solution is then drained, allowing fresh air to circulate around the plants’ roots. The length of the flood- drain cycle is dictated by the plants’ requirements.

In an NFT system, the plants sit in a tray and a thin film of nutrient solution is circulated constantly around their roots. As this system uses minimal amounts of water and nutrients, it is highly efficient.

Passive systems
Passive systems don’t use pumps, which means that the nutrient solution is not circulated around the plants’ roots. Instead, the roots are immersed directly in the static nutrient solution, as in the Kratky method.

Alternatively, the nutrient solution can be drawn to the plant roots without using a pump, as is the case in a wick system. Air pumps can be added to the reservoir to aid root aeration.

In small-scale vertical farming, hydroponically grown plants are often housed in structures made of PVC pipes. Designs include vertical or horizontal A-frames, columns, stacked beds, vertical towers, and stacked trays. The type of plants to be grown dictate the structure used.

Growing media
Since hydroponics and aquaponics use no soil, a substrate or growing medium is required to anchor the plants’ roots. It is important to choose an inert substrate that retains moisture and allows for root aeration.

It’s not often that these properties are found in a single growing medium, so a combination of substrates are used to achieve the desired outcome.

Expanded clay pellets, vermiculite, perlite and rockwool are commonly used growing media. Perlite and vermiculite are normally combined in a 50/50 ratio, as the former has a low water-holding capacity yet high air porosity, while the latter has a high water-holding capacity.

Choosing the right substrate is not an exact science; it is influenced by financial and technical factors, and may be based on trial and error.

Different plants require different nutrient solutions. Light-duty fountain pumps can be used to circulate the nutrient solution in small-scale systems. Manual or digital timers can be incorporated into the design to regulate water circulation according to the plants’ water and oxygen needs.

Small-scale vertical farming
Small-scale vertical farming structures can be used outdoors (provided they have suitable coverage), in tunnels, or indoors with varying levels of environmental control.

When plants are grown indoors, grow lights may be added to the system to either supplement natural light or replace it altogether. Manual or digital timers can be used to control these lights.

Various types of grow lights are available, with LED lights being the most popular with growers at present. LED grow lights can be manipulated to produce light that is only in the photosynthetically active radiation range; in other words, only the light that’s necessary for plant growth.

When compared with other grow lights, LEDs have a long lifespan, are easy to control, emit only a small amount of heat, and have very high operating efficiencies. The cost of LED grow lights is expected to decrease with the development of the vertical farming industry.

Growing plants vertically under controlled environmental conditions eliminates weather-related plant damage and allows for crop production all year round. At the same time, exposure to pests and plant diseases is reduced.

Vertical farming can be conducted in urban areas, thereby allowing the grower to be close to his or her market. This means a shorter supply chain and a lower carbon footprint.
Growing plants in hydroponic, aquaponic and aeroponic systems improves their quality and yield, and reduces growing time and water use. Doing so vertically further increases yield.

Pros and cons
The development of the vertical farming industry in South Africa could have social benefits, too, as it could lead to employment opportunities. It also stands to attract more young people to the agriculture sector.

For local farmers, the biggest challenge presented by vertical farming is the effects of the national energy crisis. A power outage can lead to wilting and drying out within a short time, depending on the design of the structure used to house the plants.

Farmers should therefore consider investing in backup power systems or using alternative sources of energy, such as solar power. The downside is that both of these options are capital-intensive.

While the risk of pests and soil-borne diseases is reduced, the circulation of the nutrient solution increases the risk of widespread plant damage due to water-borne diseases. Preventative measures such as biological control, cleanliness and sanitation therefore need to be rigorously applied.

All things considered, vertical farming has the capacity to change the course of food production in South Africa. Implementing such systems on a small scale could bridge the gap between our current struggles with conventional farming and the benefits offered by complex, large-scale vertical farms.

Conventional agriculture remains a significant contributor to climate change. At the same time, climate change is making it increasingly difficult for farmers to produce good-quality crops with fewer inputs.

We need to make drastic changes in order produce food in a more sustainable manner. Perhaps it is time for small-scale vertical farming structures to rise to the challenge.

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