It's the System for Saving Water!

Resource, May 2008 by Hawkins, Gary L, Gaskin, Julia, Risse, L Mark

The drought in the southeastern United States over the past two years has made national news. In some areas of Alabama, Georgia, and Florida, surface and ground water sources are still stressed, and farmers are being asked to cut their usage of irrigation water. One approach to conserving water is conservation tillage systems.

Conservation tillage systems plant the summer cash crop directly into a killed winter cover crop with minimal or no tillage. Research has shown the combination of reduced tillage and the use of cover crops increases infiltration rates by as much as 35 to 40 percent compared to conventional tillage for loamy sand and sandy loam soils.

Projected potential savings

Based on 2004 data, potential water savings for Georgia at the current adoption rate of conservation tillage systems (approximately 30 percent) are 16.7 billion L (4.4 billion gal) annually in the 0.5 million ha (1.3 million acres) of cotton planted. Similar estimates for corn and peanuts acres are 1.2 and 3.8 billion L (0.33 and 1.0 billion gal) annually. This amount of water is equivalent to three months of the water use in Atlanta, Georgia, (based on average daily consumption). These potential savings calculations were based on work done by USDA-ARS indicating an additional two to three days of water was available for crops in the soil profile of fields in a conservation tillage system compared to conventional tillage.

The higher infiltration rates on conservation tillage system fields make better use of the natural rainfall and also typically reduce the number of passes needed by irrigation equipment. This represents an additional savings in energy costs for the farmer.

A win-win saving situation

Conservation tillage systems affect infiltration rates by increasing crop residue and soil organic matter at the soil surface. The loamy sands and sandy loams of the southeastern U.S. Coastal Plain are low in soil organic matter and prone to crusting with conventional tillage. The increased residue reduces the energy of raindrops hitting the soil surface and helps prevent detachment of small soil particles that plug soil pores at the surface. The increase in soil organic matter promotes the formation of water stable aggregates that resist the impact of raindrops and keep surface soil pores open.

Research has shown that in the hot and humid climate of the southeastern United States reducing tillage and using cover crops are important for increasing soil organic matter. Tillage adds oxygen to the soil and increases microbial activity, which accelerates the decomposition of soil organic matter. Cover crops add organic matter to the soil, both in the belowground root biomass and as residue aboveground. The residue acts as a mulch and slowly breaks down over the growing season. The key is adding more organic matter than is decomposed over the year. Studies have shown that about 1.3 tons/ha (1.5 tons/ acre) of biomass is needed to maintain soil organic matter and 3.6 to 4.5 tons/ha (4 to 5 tons/acre) of biomass is needed to increase soil organic matter in the Southeast.

One caveat for water use is that an actively growing cover crop can deplete soil moisture during dry spring seasons. Farmers need to monitor soil moisture closely and terminate the cover crop at least three weeks before planting to give rains a chance to replenish soil moisture.

Saving for a rainy day

As annual precipitation in the southeast has been below average the past couple years, farmers are being asked to conserve water. In addition to other irrigation conservation measures, conservation tillage systems can save water by increasing the infiltration rate and water holding capacity, providing good stewardship of both soil and water resources.

In Georgia, the legislature has adopted a new statewide water plan. This plan calls on agriculture to invest in irrigation conservation measures and to adopt conservation tillage systems as a water conservation measure. This proactive approach of encouraging conservation of soil and water resources could prove useful in other states with similar resource concerns.

For further reference, see: Hawkins, G. L.; D. Sullivan; C. Truman. 2007. Water Savings through Conservation Tillage. University of Georgia - CAES Extension Circular Number C916. http://pubs.caes.uga.edu/caespubs/pubcd/C916/C916.htm.

ASABE member Gary L. Hawkins is an agricultural pollution prevention specialist, row crops, in the University of Georgia Biological and Agricultural Engineering Department, Tifton, Georgia, USA; ghawkins@uga.edu. Julia Gaskin is the sustainable agriculture coordinator for the University of Georgia College of Agriculture and Environmental Science, Athens, Georgia, USA; jgaskin@engr.uga.edu. ASABE member L. Mark Risse is a University of Georgia Biological and Agricultural Engineering Department professor, Athens, Georgia, USA; mrisse@engr.uga.edu.