Well, consider the definition of "sustainable." Agriculture that is not sustainable will certainly not be able to feed future generations. Agriculture that is not sustainable will lead to the degradation of the soil and water resources that are required for crop production. Agriculture that is not sustainable will require ever-increasing inputs of fuel, fertilizer, and pesticides to give ever-diminishing yields.

We know where the question is coming from. There is a lot of disagreement over what practices are "sustainable" versus "not sustainable." Many practitioners and promoters of sustainable agriculture think of organic agriculture as a model of sustainability because it requires no use of synthetic pesticides or fertilizers. No less an authority than Dr. Norman Borlaug, leader of the "Green Revolution," has been quoted in the popular press as saying that it is ridiculous to think that organic agriculture could feed the current population of the world. Part of the disagreement stems from differing ideas about the yield potential of organic and sustainable agriculture.

Organic agriculture has been presented by some of its critics as capable of only about 50% of the yield of conventional agriculture, at best. The critics then claim that modern, "conventional," high-input agriculture is more environmentally friendly than organic because fewer acres are needed to produce the needed yields, so more lands can remain wild. Dennis Avery of the Center for Global Food Issues proclaims that viewpoint, as in this commencement address at the University of California at Berkeley.

Klaas and Mary-Howell Martens, large-scale organic farmers from New York, have tart words for the claim of low yields from organic crops! Recent research supports the Martens' viewpoint that organic agriculture, done well, can yield as much as "conventional" high-input agriculture.  Christos Vasiliokos of the University of California at Berkeley reviews the findings of some of this research in his article, “Can Organic Farming ‘Feed the World?’” 

Of course, discussions about feeding the world have to take into account issues of poverty. High-input, high-yield conventional agriculture (including genetically engineered crops) is frequently touted as the way to feed the hungry people of the world. This was the basis of the "Green Revolution" for which Dr. Norman Borlaug received a Nobel prize.  Hunger activists point out that many of the poorest, hungriest people have no land on which to grow crops and no money with which to buy food. Poor and hungry people who do have access to land cannot afford the seed, fertilizer, and pesticide inputs required to produce the so-called "Green Revolution" crops. Reducing hunger, then, depends on reducing poverty; not necessarily on producing more quantities of food. For lots of background on this topic, see "Readings on Poverty, Hunger, and Economic Development."

Organizations such as the Food and Agriculture Organization (FAO) of the United Nations are looking to sustainable agriculture to help impoverished people feed themselves and earn money. Dr. Jules Pretty of the Centre for Environment and Society, University of Essex, United Kingdom, prepared a report titled "Reducing Food Poverty with Sustainable Agriculture." This report documents worldwide success in reducing poverty through sustainable agriculture projects that give local people the means to produce their own food. 

Finally, in any discussion about feeding the world, we need to have a word about biotechnology. Biotechnology in the form of genetic engineering of crops is often presented as part of the high-input, high-yield, conventional agriculture package that is necessary to feed the world.  Some proponents of this system have made statements that those opposing their views are "anti-science."  For example, “Listen to sound science on agricultural technology,” by C. S. Prakash and Martina Newell-McGloughlin.

There are many scientists and scientist groups, however, who support sustainable agriculture and question the use of genetically engineered crops. Dr. Charles Benbrook tracks concerns with agricultural biotechnology on Ag BioTech InfoNet.  The Union of Concerned Scientists promotes sustainable agriculture and questions the application of genetic engineering to food plants.