For an agricultural activity to be counted as sustainable agriculture, it should satisfy three pre-conditions: it should not upset the natural environment, while at the same time it should be something that a farmer can afford to do and it should meet society's needs. So it should be economically viable, socially responsible and ecologically sound.
To familiarize yourself with the basic issues about sustainable agriculture and for a good resource base on the subject, visit this link. This is a project by a university student and it is easy to read. It explains the concept of sustainable agriculture in simple terms.
Though we began farming 10,000 years ago, and we produce enough to feed the world, yet there are people who cannot afford to buy food and go hungry. The rate of population is growing at an alarming rate and most of this growth is in the third world or developing countries where traditional methods of agriculture are used. In these countries, many people go hungry.
In this article, the author talks about the need for sustainable agriculture for people who cannot afford very high technology methods of farming. There are different examples on how sustainable agriculture practices have doubled the yield of crops and in some cases, it has actually led to re-migration to the rural areas!
One of the ways in which we can increase the productivity of soil and also get good yield is crop rotation. This article is a good introduction to crop rotation and how it is used to improve or maintain soil fertility, reduce erosion, reduce the build-up of pests, spread the workload, reduce risk of weather damage, reduce reliance on agricultural chemicals, and increase net profits.
To get an idea of how crop rotation helps read this article. It is informative and gives you a good idea of the benefits of crop rotation. You also get a brief overview of the history of sustainable agriculture. From this article you can follow different links to know about related concepts such as Green Revolution and Organic farming.
Among different human activities that contribute to environmental damage, agriculture contributes 13%. Agricultural practices such as burning of biomass and deforestation and removal of native vegetation, widespread clearing of land and losses of organic carbon in vegetation and soils result in atmospheric increases of CO2.
For a brief overview of the harmful effects of traditional methods of farming, this page is a good reference. Visit this site to understand how agriculture can also harm the environment if we do not adapt sustainable methods of farming.