Small-scale hydroponics

Hydroponics and the future of farming

How can we produce enough food to sustain an increasing population? Growing plants without soil could be the answer, explains Rob Reddick

With the world’s population nearing 7.5 billion – and global prosperity and the desire for more resource-intensive foods rising steeply too – it’s clear that farming needs to become more productive.

One way of meeting future food needs could be hydroponics – growing plants without soil, instead using a nutrient-rich solution to deliver water and minerals to their roots. It’s already being used to increase farming outputs and grow plants in habitats that wouldn’t normally sustain them.

Despite sounding like something out of science fiction, it’s nothing new. The Aztecs built floating farms around the island city of Tenochtitlan, and the explorer Marco Polo wrote about seeing floating gardens during his travels through 13th-century China. By the 1930s, Pan American Airways had established a hydroponic farm on a remote Pacific island to allow its flights to top up with food en route to Asia.

Today farmers are slowly increasing their use of hydroponics, and researchers are looking more closely at how it could solve future food problems. In the future, some of its applications could be out of this world.

How does it work?

In conventional agriculture, soil supports a plant’s roots – helping it to remain upright – and provides it with the nutrients it needs to grow. In hydroponics, plants are artificially supported, and a solution of ionic compounds provides nutrients instead.

The thinking behind this is simple. Plant growth is often limited by environmental factors. By applying a nutrient solution directly to a plant’s roots in a controlled environment, a farmer can ensure that the plant always has an optimal supply of water and nutrients. This nutritional efficiency makes the plant more productive.

The solution can be delivered in a number of ways. A plant may be:

  • placed in an inert substance (such as the volcanic glass perlite or rock wool) and have its roots periodically flooded with solution
  • placed in an inert substance and rained on by a solution dripper
  • suspended with its roots in the air, with these then sprayed with solution mist
  • placed on a slightly sloping film that allows solution to trickle over its roots

All of these systems are mechanised in one way or another, usually using either a pump or a mister to deliver the solution from a separate store. The solution is also usually aerated to ensure that the roots are supplied with adequate oxygen. Mineral absorption requires energy, and is powered by respiration.

Is it difficult?

Running and maintaining a hydroponic system can be complex. Plants require over a dozen essential nutrients, with the optimal amount of each varying according to species, growth stage and local conditions, such as water hardness.

Plus, some nutrients get absorbed quicker than others, which can cause a build-up of positive or negative ions in the solution, affecting pH. This can then hinder the absorption of other nutrients – partly because their uptake is pH-dependent, but also because excess quantities of some will prevent the uptake of others. Too much ammonia, for instance, decreases calcium uptake. Too much calcium prevents magnesium absorption.

On top of this, some compounds react with one another to form substances that are harder to absorb, and so have to be provided separately. Hydroponic farmers have to have a firm understanding of how plants and nutrients interact, and must monitor their solutions closely and respond to any concentration changes. Their other option is to purchase expensive automated systems to do this for them.

Farmers also must protect their nutrient solutions from being contaminated with unwanted substances. Enclosing hydroponic systems inside buildings or greenhouses is a common way to do this. It also allows them to control and optimise other environmental influences on plant growth – such as temperature, light and CO2 – to further increase yields.

When people talk about hydroponics today, at least in agriculture, they usually mean not just soilless growing alone but controlling all of these factors collectively. However, this combined practice is more accurately known as controlled-environment agriculture.

Pros and cons

Hydroponic farming is complicated, but for many farmers, the benefits outweigh the downsides.

Pros Cons
Increased productivity

Using nutrient solutions, artificial lights, heaters and other pieces of equipment, plants can be made to develop faster, produce larger yields and grow all year round.
High set-up costs

Setting up a hydoponic farm requires a large amount of equipment, all of which needs to be purchased before the farm launches.
More eco-friendly

Water in a hydroponic system can be recycled – at its most efficient a hydroponic farm only uses 10% of the water a normal farm uses. Because it’s a closed system, nutrients don’t leach away – an efficient hydroponic farm may only use 25% of the fertiliser a regular farm uses. Plus, eutrophication (dense growth of aquatic plants like algae caused by run-off of fertiliser) isn’t a problem. Soil pests are non-existent, and in enclosed greenhouses natural predators can be used to control insect pests – next to no pesticide is required.
Higher running costs

Many of the control systems – pumps, water purifiers, lights, heaters, etc – need to be powered, which costs money. In conventional farming, heat, light and (some) water is provided naturally for free.
Feasibility in areas not suited to traditional farming

Its high water efficiency makes hydroponic farming possible in arid environments. Hydroponic growing trays can be stacked on top of one another, and plants can be placed closer side by side than they can in soil, making it vastly more space-efficient than traditional farming. Because nearly all environmental conditions can be controlled artificially, unconventional growing spaces can be used – uninhabited buildings, disused railway tunnels, etc.
Vulnerability

Because they are mechanised, hydroponic systems are vulnerable to power failures. In systems where roots are highly exposed, unwatered plants can dry out rapidly. Nutrient and pH imbalances can build up far quicker in a solution than in soil; if something goes wrong, an entire crop can be wiped out very quickly. Likewise, water-borne diseases can spread quickly and widely, and water-borne microorganisms can contaminate solutions fairly easily.
Reduced transportation

Crops can be grown away from their natural habitats and closer to consumers, reducing transport emissions and providing people with fresher goods.
Need for monitoring

Although a hydroponic farm requires less effort overall (planting and harvesting is far less labour-intensive), hydroponic plants cannot be left to their own devices for long periods like regular fields of crops. A hydroponic farm must be regularly attended to by a farmer, or else automated.
Monoculture not a problem

Farmers don’t need to worry about exhausting their fields of certain nutrients through growing the same crop over and over – there is no need for crop rotation, so in-demand crops can be focused on.
Need for expertise

Hydroponic farmers need to understand the technique, which is complicated.

Hydroponic crops

Theoretically, hydroponics can be used to grow any crop. However, the technique is mostly used with plants that grow efficiently under hydroponic conditions, such as salad greens, cucumbers, peppers and herbs. Most commonly it is used to grow tomatoes.

Farmers tend to use hydroponics with tomato varieties that have had special characteristics bred into them, such as bearing larger fruit and growing indeterminately (meaning that they grow continually, repeatedly producing fruit along their stems).

Disease-resistant varieties are also popular as they enable plants to live for longer and bear more produce.

On the other hand, crops that are not genetically suited to hydroponics are avoided, such as wheat. Research in the USA has shown that using the system to grow enough wheat to make a loaf of bread would cost $23!

Urban farming

The vast majority of plants are still grown using soil, but hydroponics is on the rise. In 2013, Thanet Earth – the UK’s largest greenhouse complex, based in Kent – used controlled-environment agriculture to produce around 225 million tomatoes, 16m peppers and 13m cucumbers, which equated respectively to 12, 11 and 8 per cent of Britain’s entire annual production of these crops. It currently operates four greenhouses, and has plans to build another three.

Globally, it was estimated that the hydroponic farming industry was worth $21.4 billion in 2015, with its value projected to grow at 7 per cent per year. Slowly but steadily, farming appears to be changing.

But equally, there are big global changes on the horizon, and these could vastly accelerate the use of controlled-environment agriculture. By 2050, an extra 3bn people could be living on Earth, with over 80 per cent of the global population living in urban centres. We’re already using the vast majority of land suitable for raising crops, so new growing areas – particularly in arid regions – need to be found.

One much-talked-about solution is vertical urban farming – creating stacked hydroponic farms inside buildings, including tall skyscrapers. This would solve the problem of running out of available farmland, and also place farms right at the heart of where crops are needed – our densely populated cities of the future. Vertical farms are already being built in Michigan and Singapore – and even in disused bomb shelters in south London.

And, as it plans human space missions that will travel further and further from Earth, NASA is investigating whether hydroponics could be used to create space farms to feed astronauts. Working with the University of Arizona, it is seeing whether it can create a closed-loop system that feeds human waste and CO2 into a hydroponic farm to create food, oxygen and water.

Lead image:

Frank Fox/Flickr CC BY NC ND

References

Questions for discussion

  • Different hydroponic set-ups are preferred in different scenarios. Why do you think a farmer might prefer to use one system over another?
  • What other advantages and disadvantages of hydroponic farming can you think of?
  • Hydroponics is also often used to illegally grow cannabis. Why do you think this is?

About this resource

This resource was first published in ‘Plants’ in May 2016.

Topics:
Ecology and environment, Biotechnology and engineering
Issue:
Plants
Education levels:
11–14, 14–16, 16–19, Continuing professional development