To backyard gardeners who love to muck around in real dirt, growing plants in water somehow seems other-worldly but the science of doing so, known as hydroponics, is a well established technology. Soil in natural conditions serves as a reservoir for water and plant nutrients, but plants don’t actually require soil to survive. Nutrients are absorbed by plants through roots as inorganic mineral ions dissolved in water. So long as plant roots have access to water containing essential minerals, plants can survive without soil. Since plant survival also requires access to oxygen through roots, roots cannot be completely and perpetually immersed in water unless it is adequately aerated or else the plants will drown. Plants can be grown hydroponically in solution using something as simple as a water in a Mason jar, but more frequently growers use an inert medium in which to grow the plants such as perlite, gravel, mineral wool, or coconut husk.
Hydroponic gardening has virtues that offer special advantages for growing plants in an arid climate like Cairo’s. In soil-based farming, applying the right amount of water is a tricky business. Too much watering causes plants to die from a lack of oxygen, and too little leads to plant starvation. For hydroponic farming, plant roots can be continuously or frequently exposed to nutrient-laden water and the plants can absorb as much or as little as they want. Unused water can be drained away and recycled keeping water use to a bare minimum. The key challenge for the hydroponic approach is to get the balance of needed mineral in the water just right, including macronutrients such as nitrates, calcium, phosphate, and magnesium, and micronutrients such as iron, copper, zinc, boron, chlorine and nickel. In addition to an appropriate balance of nutrients, care must be taken to not let the water’s pH get out of whack or salts to build up excessively. Any interruption in water flows can be catastrophic, and water must be stored in light-free tanks to prevent the formation of algae. A successful hydroponic system can achieve high levels of productivity with a modest water and nutrient input.
The other central ingredient in plant growth is sunshine, something that the rooftops of Cairo have in abundance year around. The city’s low income residents have long used rooftops to raise chickens and goats but not to grow crops. Looking out over Cairo’s rooftops from a minaret or any other high vantage points, one sees mostly accumulated debris, satellite dishes, and virtually nothing green. The dream of social entrepreneur, Sherif Hosny, is to create 325 hydroponic rooftop gardens in Cairo by 2013. Hosny named the business, Schaduf, after a simple traditional Egyptian tool for raising water to higher ground for crop irrigation. Sherif’s brother, Tarek, will join the business after completing his college education in the U.S.