From the Organic Consumers Association:

Over the last forty years nitrogen fertilizer use has increased seven-fold and nearly every acre of intensively farmed, conventional cropland is treated with pesticides. A team of scientists explored the impact of pesticides and other environmental toxicants on symbiotic nitrogen fixation (SNF) brought about by Rhizobium bacteria (Fox et al., 2007). Their findings were published June 12, 2007 in the prestigious Proceedings of the National Academy of Sciences (read full study here).

Read the rest of the article, here.