On 2 May, a pair of greenhouses in New York will reopen to the public after a winter closure. Last year, they played host to 10,000 visitors, 105 school groups and 65 journalists – numbers of which most commercial nurseries can only dream. What makes these greenhouses different is that, although they contain row upon row of healthy plants, they contain no soil whatsoever. Instead, the tomatoes, vines and lettuces are grown ‘hydroponically’ – by continuously circulating a nutrient-filled stream of water past the plant roots.
The greenhouses, constructed on a barge now moored in the Hudson river, are part of a pioneering sustainable agriculture experiment conducted by US-based New York Sun Works, known as the Science Barge. The barge is equipped with solar panels and a biodiesel generator for energy, rainwater-harvesting and desalination systems for providing water, and waste treatment equipment to make sure no excess nutrients are discharged into the river. Since it opened in May 2007, the Science Barge has successfully shown that it is possible to grow food sustainably, reliably and in an urban setting entirely without soil.
Hydroponics isn’t new. Some believe the technique was used by the Babylonians to give Nebuchadnezzar’s Hanging Gardens their spot on the Seven Wonders list, and Allied forces in the Second World War used the process to supply troops with year-round fruit and veg. As the Science Barge’s Benjamin Linsley explains, however, sustainability has rarely been high on its agenda.
‘Hydroponics is a pretty broad field, and there are some fairly dodgy practices around,’ he explains. ‘The biggest problem has always been energy – commercial growers have to heat the greenhouses and use powerful lamps to encourage rapid growth.’ To get around these problems, the Science Barge team has devised ingenious solutions. Although the prototype was heated by burning biodiesel, future incarnations – planned for school and office roofs – will use waste heat from the buildings’ air conditioning systems.
‘It turns out that this low-grade heat, which is difficult to do anything useful with, is not only warm, but also quite rich in CO 2 – which makes it perfect for pumping into greenhouses full of plants,’ says Linsley. As well as requiring heating in the spring and autumn, the plants require cooling in the summer – another significant energy demand. The cooling system on the Science Barge, however, uses an array of solar panels to drive fans mounted in the side of the greenhouses. These draw air through an array of water-soaked corrugated cardboard strips, reducing the temperature in an energy-efficient process known as ‘evaporative cooling’. Solar power is a perfect source for this technology – the brighter the sun shines, the faster the fans spin.
Behind the high-tech wizardry lies a genuine desire to tackle key problems with food’s ecological footprint. ‘The key to the system is the way in which the food is produced – without pesticides – and where it is produced – locally to where it is eaten,’ says Linsley. The project includes plans to cut out supermarkets and carbon-intensive supply chains, and to sell the produce in farmers’ markets. Linsley even sees rooftop hydroponic greenhouses fulfilling a social role.
‘Our system can work as part of a Community Supported Agriculture scheme, run by – and providing food to – residents. Installing greenhouses on top of social housing would, in effect, create high-tech allotments, at a time when land in cities is at a premium.’
On paper, there’s little doubting either the project’s sustainability credentials (lifecycle analyses show it to be significantly less CO2 intensive, as well as requiring 20 times less land and 10 times less water, than conventional agriculture) or its ambition – the team has plans for greenhouses on top of schools, shops and even integrated into office glazing.
Calculations show, in theory, that New York has enough spare flat roof space to meet the entire city’s vegetable requirements from hydroponic greenhouses. Can a plant grown in a nutrient solution really be equivalent to one grown in organically managed soil, though? Ben Raskin, supply chain and technical team manager at the Soil Association, thinks not.
‘Soil contains billions of organisms – many of which we don’t even know about – which create complex sets of relationships,’ he says. ‘I suspect that if you tried to survive purely on hydroponically grown food, you’d quickly find your body wasn’t getting everything it needs.’
Linsley counters that the Science Barge team employs a plant biologist to make sure each plant receives the exact amount of nutrients it needs, and that, by growing plants comparatively slowly, the system produces great-tasting vegetables. But he admits that the team has not yet conducted a nutrient analysis on the food, which would help show whether it contains the levels of minerals and vitamins common in organic produce.
In addition to this, Raskin believes that there are further environmental complications with hydroponic systems. ‘The nutrients for the plants have to come from somewhere. Whatever you add – nitrogen, potassium, phosphorus, copper, boron – this stuff doesn’t just “appear”. It has to be produced or mined. It’s the wrong approach in terms of sustainability.’ Nor does Raskin buy the argument that hydroponics is a solution to our land crisis. The answer, he suggests, could be a balance of different systems. ‘There’s still a lot of land in our cities that could be used for growing food – gardens and communal green spaces. I have no problem with the hydroponic system as a supplement to organic agriculture, but it should not be seen as an alternative.’
Mark Anslow is the Ecologist ’s senior reporter
This article first appeared in the Ecologist May 2008