Rooftop production of leafy vegetables can be profitable and less contaminated than farm-grown vegetables

Abstract

Urban agriculture may solve issues of feeding urban populations. In China, for example, densely packed mega cities will continue to expand in number and size, necessitating increasing food miles. Interestingly, it has been estimated that the total rooftop space in China is about 1 million hectares, some of which can be converted for rooftop farming. Yet, despite some favorable reports on urban farming, the Chinese commercial sector has shown little interest. This may be explained by the dearth of data comparing urban and conventional farming. Therefore, we present here a feasibility study of hydroponically grown vegetables in a rooftop screen house in Guangzhou, China. From December, 2012 to May, 2014, we tested the production of seven leafy vegetables that are easily perishable and are not well suited to long-distance transport. We calculated the production cost and measured biochemical parameters. Results show that levels of vitamin C, potassium, calcium, magnesium, iron, zinc, and crude fiber were comparable to market counterparts. None of the roof hydroponic vegetables exceeded the maximum residue limit for lead, arsenic, cadmium, chromium, mercury, or nitrate. In contrast, 5 of 98 market vegetables were contaminated by exceeding the maximum residue limit for lead. Similarly 3 were contaminated for arsenic, 23 for nitrate, and 2 for organophosphate or carbamate insecticide. Compared to high-end vegetables sold on the market, rooftop-grown vegetables were competitive in cost and quality. Given that many countries have limited arable land to feed a large population, the widespread adoption of rooftop hydroponics could help expand the total area available for food production as well as meet the rising demand for safe high-quality vegetables.