Closing the nutrient loop.

• Author: Nelson, Toni
• Position: Utilizing organic waste for food production instead of throwing them away in landfills

In an urbanizing world, growing some of our food in cities can make our communities both more sustainable and more secure.

Every day, an armada of petroleum-fueled trucks, trains, ships, and planes hauls perhaps 20,000 tons of food into New York City - a mobilization comparable in scale to that of a military invasion. In the course of the day, a large part of that cargo is converted into human energy, flesh, sweat, carbon dioxide, and heat. Most of the rest - including some 10,000 tons of organic garbage and sewage - is hauled back out of the city by a second, different, armada. The organic waste does not end up anywhere near the fields, orchards, or fisheries that produced the food and is not recycled back into the land. A large amount of it is exiled to landfills, permanently sealed off from the earth's ongoing life.

The same pattern prevails in most of the thousands of other cities on the planet, except that in many of them the waste is not hauled or piped away but dumped directly into rivers or bays, where its unnatural concentration causes such ecological disruptions as algal blooms and fish die-offs. In very few cities is any significant portion of the nutrient flow returned to the land from which it came, as an investment in future production. Unlike healthy ecosystems, in which nutrients are largely recycled, the typical urban system is a dead end. The cumulative effect is a gradual depletion of the places where the products originate, and a poisoning of the places where the wastes ultimately concentrate.

This massive shifting of nutrients from rural to urban areas has already diminished the vitality of many of the planet's most productive croplands, grazing lands, and fisheries, and the process could accelerate as more and more of the human population concentrates in cities in the coming decades. It is also creating a dilemma: how to feed the growing number of people who are far removed from their main sources of food, without unbalancing and collapsing the ecosystems - both nearby and distant - on which those people ultimately depend. Political leaders have been slow to recognize and respond to this dilemma. But in many cities residents are not waiting. Both with and without official sanction, millions of people are now producing food right where they live - in empty lots, on rooftops, and in their own backyards.

At first glance, farming may seem among the least suitable of urban activities. But, in fact, throughout much of the world, cities and farming have an ancient relationship. In the classic The Economy of Cities, Jane Jacobs argues that agriculture is actually an urban invention, developed in cities which were first founded as centers of trade. She observes, for instance, that the first medieval system of crop rotation typically centered around towns and took decades or longer to reach the most isolated rural areas.

Indeed, the integration of food production and urban life may once have been universal. Grain cultivation and domesticated animals were present in the earliest known city, Catal Huyuk in Anatolia (now Turkey), from its inception around 7000 B.C. In pre-Columbian Mexico, the highly productive system of Chinampa agriculture, consisting of raised beds surrounded by canals, provided most of the food, flowers, and fibers consumed in the Aztec capital of Tenochtitlan. And in Europe, field crops were grown both within and outside the walls of medieval cities.

In the industrialized world, urban farming largely disappeared in this century. But in the developing world it has persisted, and since the 1970s has even shown signs of a resurgence. According to the most widely accepted estimate, about 200 million urban dwellers now participate in urban farming, providing 800 million people with at least some of their food. Farmers in Accra, Ghana, supply the city with an estimated 90 percent of its vegetables, including radishes, cabbage, and cauliflower. Farmers in Singapore produce 25 percent of the city's vegetables and 80 percent of its poultry. (Much of the grain needed to feed the poultry, however, is brought in from the countryside.) In Berlin, more than 80,000 gardeners lease plots on land where buildings were destroyed by bombs in World War II.

Where in-city food production is extensive, it can also play an important role in municipal waste management. In China, human waste is treated and sold to farmers as fertilizer. In India, sewage-fed lagoons produce about one-tenth of the fish consumed by Calcutta. In the more industrialized countries, concerns about disease have put up barriers to using human waste in agriculture - barriers which contributed to the separation of farming and urban living in the first place. But the development of techniques for safely reusing urban wastes may now make it possible to bring farming back into close proximity with people.

Meeting urban food needs

The global demand for food is expected to grow at least as fast as if another Calcutta or Los Angeles were to appear on the planet every two months for the next several decades. Yet, the world's capacity to increase supply is approaching its limits. Recent rises in grain prices, combined with declining global stocks, may well be a harbinger of future food shortages. If the scarcity that many foresee becomes a reality (see "Facing Food Scarcity," November/December 1995), the impacts won't be spread evenly. It is in the fast-growing cities of the developing world where tens of millions of impoverished migrants are concentrating that the increasing competition for food will be felt most acutely. As those cities expand, the problem of food security could balloon into a widespread crisis and a dominant issue for urban policy. Today, although urban agriculture plays a relatively small role in total food production, it provides a crucial safety net for many of the people most at risk. With supportive policies, it is likely that this role could be significantly expanded.

Today's problems of hunger and malnutrition are less a function of production than of distribution, since the world's farmers still produce enough food to provide all of humanity with an adequate diet. But the poor are increasingly squeezed out of the market by the growing numbers of wealthy consumers - and the gap is growing not only between rich and poor countries but between the rich and poor within countries, and even within cities. (As wealth becomes more polarized, the ranks of both rich and poor are increasing.) Those who are newly affluent are changing their eating habits, demanding more meat. And as more grain goes to fatten beef, less is available for direct consumption (as plain rice or bread, for example), and that pushes up prices that shut out rise poor even more. Already spending the bulk of their income on food (at least 60 percent among low-income families in Bangkok and as much as 90 percent in the poor neighborhoods of Bolivian cities), some of the urban poor are being forced to look for alternative sources. What they...