Question: What are scientists doing to help us grow crops as the climate changes? Answer: There are several factors to address with your question. The first is that we are expecting to continue to feed the world on less land each year. As we continue to build towns and cities on former farm land, we expect more from the farm land that remains. Second, the world’s population is expected to reach over 9B by the year 2050. So, we’ll need to feed more people with less land. One approach to this challenge is sustainable intensification, or simply providing more food and nutrition on the same land resource while protecting the natural resource base upon which agriculture relies. The image of sustainable intensification in practice for a smallholder subsistence farmer in Africa is much different than large-scale production agriculture in the United States. However, both will need to use soil water efficiently to succeed and both will help feed the growing population. The US has experienced record droughts in recent years…making it tough to grow food on land in various regions. So, scientists are looking at other forms of water than the traditional fresh water used for irrigation. Scientists call this freshwater “blue water”. And, supplies of this water source are dwindling. Scientists are looking to tap a different resource that holds promise for agriculture – “green water”. Green water is the water in soil that is potentially available for plants to take up, use, and release to the atmosphere. To be used by plants, green water must pass through the rhizosphere, the region of soil found close to and influenced by roots.

The rhizosphere is the region of soil found close to and influenced by roots. Green water is held in this portion of soil. Photo courtesy of flickr/NRCS Soil Health.

There is a bountiful supply of green water. But, as much as 70% can be lost through evaporation or subsurface leaching. That leaves as little as 30% for transpiration—the process by which plants use the water—under some conditions. Transpiration is termed “productive green water flow” since water used for transpiration leads to plant growth. If, for example, 85% of available green water could be used for transpiration, and therefore become productive, crop yields could as much as triple in some parts of the world. Scientists know that healthy soils with plenty of nutrients and soil physical properties favorable for root growth will lead to healthy plant root systems that will take up more water for transpiration than smaller root systems. These plants will grow larger and create lush canopies that in turn provide shade and decrease evaporation from the soil. More green water is then available for transpiration that will stimulate more crop growth. Researchers are currently finding ways to capitalize on plant-soil feedbacks and optimize the community of soil microorganisms in the rhizosphere, one of the great frontiers in soil science. That work is a key ingredient in the recipe for better using green water and expanding crop production.

-Answered by Gary Pierzynski, Kansas State University

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Resources: Optimizing Green Water Use Increases Food Security https://www.soils.org/discover-soils/story/optimizing-green-water-use-increase-food-security Green Water and Global Food Security doi.org/doi:10.2136/vzj2013.02.0041 Managing Green Water in Dryland Agriculture doi:10.2134/agronj14.0038