The Lessons of History: Past Green Revolutions.

In the long perspective, there have been at least three major Green Revolutions in history each of which influenced the course of human history. Each of these offers insight about the basic requirement for substantial advances in crop yield.  The first Green Revolution took place in the valley of the Euphrates River about 5000 years ago.  Sumerians built an impressive irrigation scheme that brought both water and nutrients to their fields.  Sufficient nitrogen was supplied to their grain crops to achieve yields of 2 t ha-1 or greater. For nearly all of succeeding history, however, nitrogen input to fields was only sufficient to support yields of 1 t ha-1.  Not until the early 1700s was there a second Green Revolution when the Norfolk crop rotation was introduced in England.  This rotation included at least two seasons of a legume/grass pasture that not only fed animals but it enriched the soil with nitrogen. Grain yields doubled to 2 t ha-1 freeing people to work in the factories of the Industrial Revolution. The third Green Revolution began in the U.S. following World War II when there was a steady increase in the application of manufactured nitrogen fertilizer to crop fields. As a result there was a steady increase in crop yields that allowed nearly all people to engage in activities other than producing food.  An extension of the third Green Revolution was taking nitrogen fertilizer technology to developing regions of the world resulting in dramatic increases in rice and wheat yields, improving the well being of people in these regions. New cultivars were developed to flourish in fields with additional nitrogen. The common feature of all Green Revolutions has been technology to increase nitrogen availability and use by crops. This is an important lesson for a future Green Revolution. Those regions of the world which still suffer low yields cannot obtain higher yields without providing the required amounts of nitrogen for increased plant growth and yield. Also, developed regions cannot sustain high crop yields in “green” cropping systems without dealing with the continuing challenges of adding large amounts of nitrogen to the environment. Enhancing the role of legumes and symbiotic nitrogen fixation appears to be an attractive approach in both less-developed and developed regions to achieve the next Green Revolution.