Recently the National Research Council released a comprehensive report entitled Toward Sustainable Agricultural Systems in the 21st Century. I have grabbed a few key quotes from that report and encourage you to go purchase a copy of the almost 600 pages of information. From that conclusion chapter:
" As discussed in Chapter 1, satisfying human food, feed, and fiber needs is one of the sustainability goals in agriculture. Although practices for improving sustainability require taking some land out of production (for example, maintaining wetlands and riparian buffer strips), many farming practices for improving environmental sustainability do not compromise productivity and might even enhance yield (for example, cover cropping, crop rotations, and integrated pest management), as reported in Chapter 3. The determination of the production potential associated with various farming practices or systems at a regional or global level is actually a complex result of several interacting factors: production potentials (typical per acre crop yields or indicators of livestock feed efficiency and growth rates), land and input requirements, and biophysical resource qualities (Smil, 2000). Many studies have shown that with the right conditions and management, low-input and organic systems can have yields, productivity, and economic returns that are comparable to conventional systems (Liebman et al., 2008; Posner et al., 2008)."
Source: Committee on Twenty-First Century Systems Agriculture, "Toward Sustainable Agricultural Systems in the 21st Century", National Research Council, 2010, p. 207, http://www.nap.edu/catalog/12832.html