Life inside the visionary multi-story farms will be arranged as sustainably as possible: water circulates in closed systems, animal feed can be made from plant waste and fertilizers from animal manure or sewage. Plants will grow in soil-free material such as mineral wool or coconut fiber, since pests are less of a problem if there is no soil.

The modern greenhouses will form their own ecosystem inside the metropolis – shielded from fluctuations in water supply and climate. They will thus yield several harvests per year, and city dwellers will be able to buy fresh food from the high-rise farm around the corner whenever they wish.

“Up to now, designs for vertical farms were mostly based on futuristic architecture,” says Professor Joachim Sauerborn, an agronomist at the University of Hohenheim. “The cultivation technology required in the building has received less attention. The emphasis has also been on high-value crops such as fruit and vegetables, sometimes in combination with fish and animal production.”

In contrast, the skyfarming concept developed by the University of Hohenheim is based on the needs of plants. “Our idea is comparable to housing design: first you identify the residents’ needs and then you plan a building with the necessary facilities,” says Professor Sauerborn, explaining his strategy. Plants, like people, have differing needs – lettuce grows differently from cereals.

Rice by conveyor belt

The test crop chosen by Sauerborn’s scientists is an important and environmentally significant staple food: rice. Rice is grown over an area of 157 million hectares worldwide, or 22 percent of the global cereal production area. Rice growing requires large quantities of water – up to 30 percent of the world's fresh water resources. Also, fermentation in flooded rice paddies produces methane gas, which is far more harmful to the environment than CO2.

“It is estimated that up to 20 percent of global methane emissions are attributable to rice-growing,” says the professor of agronomy.
 It was not only agronomists who attended an initial expert workshop on the skyfarming project in early July 2010. Engineers, architects, logistics experts and economists also gathered to discuss the vision of the “high-rise paddy field.”

The scientists see the rice seedlings growing not in static fields on each floor, but on indoor terraces which are constantly in motion. The individual rice seedlings will move in stages on a conveyor belt and go on a 120-day tour of the building while they gradually ripen into plants mature enough for harvesting. The growing seedlings will obtain nutrients through their roots: they will be constantly sprayed with a fine mist containing an optimum mix of nutrients. In this “aeroponic” system, the plant roots literally dangle in mid-air in special plastic bags. At the end of their conveyor-belt trip, after approximately four months, the rice plants can be harvested and the empty spaces filled with new seedlings ready to begin their journey.

According to Professor Sauerborn, this greenhouse can produce up to three harvests per year.

 The researchers are also expecting significantly greater harvests: the aeroponic system is easy to keep free of pests and germs. And in perfect growing conditions, the plants have a higher yield. Professor Sauerborn estimates that rice has a potential yield of 14 tons per hectare – in optimum conditions. In contrast, the yield in a normal rice paddy falls to around 4 tons per hectare because of climatic stress, pests and disease.

Using the skyfarming concept, one hectare of indoor growing space is due to a better use equivalent to between five and seven hectares out in the field. The rice plants also require significantly less water: “Around one liter of water is required to produce one kilogram of rice. Current rice-growing methods use around 600-900 liters of water per kilogram,” says the agronomist.