(This first appeared in the Wyoming Livestock Roundup in January, 2015)
Farmers are aware of low organic matter levels that hamper productivity and become lower each time the soil is disturbed. Under furrow-irrigation, declining soil organic matter (SOM) leads to loss of soil structure that causes sealing and erosion, as well as loss of nutrients and increasing fertilizer needs. Reversing those trends is difficult because furrow irrigation requires tillage to level and furrow the soil, and root crops require soil disturbance for harvest. Conserving surface residues can impede water flow and reduce irrigation efficiency. Research is sparse but shows that there are ways to improve soil quality in furrow-irrigated cropping systems. Recent research in Latin America, Australia, and Asia emphasizes advantages of permanent raised beds in furrow irrigated production. This article summarizes information from several key studies about different approaches to soil conservation under furrow irrigation.
Reduced tillage. While disturbance is necessary for making clean furrows and harvesting root crops, tillage operations can be strategically reduced to conserve SOM. A study in Montana showed that strip till yielded better than conventional tillage and no till in furrow-irrigated sugarbeet. No till yielded the same as conventional till. A similar Nebraska panhandle study showed that sugarbeet was the weak link for soil erosion control: the soil had almost no cover for over a year and a half from early spring furrowing for the beet crop to harvest of the following corn crop. They recommend an alternative beet harvesting technique that would place residues on the soil surface.
Other studies on fine-textured soils suggest that no till can lead to decreased porosity and increased runoff compared with conventional and reduced tillage. In a long-term study of a sugarbeet-wheat rotation in Germany, no till resulted in much denser soil, 4% lower wheat yields, and 15% lower beet yields than conventional tillage. Lower production costs made up for the lower yields in wheat, but not in sugarbeet. Their conservation tillage treatment (loosening and mulching), however, produced equivalent yields, and higher profits, compared with conventional plowing.
None of these authors discuss problems with residue blocking water flow, and farmers report mixed results using reduced- or no-till systems with furrow irrigation. The scale of this problem may be related to individual farmers’ tolerance of trash in their furrows, and their willingness to occasionally walk the furrows, but generally, residue-conservation is not embraced by furrow irrigators.
Cover crops. Some producers in western Wyoming use cover crops after barley and dry beans. There is not a great deal of information in the scientific literature about cover crops in furrow-irrigated systems, but one study reported much less runoff and greater infiltration when they combined a wheat cover crop with standard tillage (including several 8-inch-deep passes with a disk plow) compared with no cover crop and with no till in a California sunflower-Sudan grass-corn rotation. Fast-growing grain, grass, legume, and other crops can be planted following small grains and dry beans to create winter soil cover, contribute nutrients, and add considerable organic matter to the soil. A University of Wyoming team is currently studying cover crops following barley at our Powell Research & Extension Center.
Longer rotations. A study in the Big Horn Basin showed that the longer the rotation, the more soil organic matter and the higher the sugarbeet yields, with four-year alfalfa-alfalfa-sugarbeet-sugarbeet rotations outperforming sugarbeet-bean-barley and sugarbeet-barley rotations. Long-term studies in Nebraska, Montana, and South Dakota established that sugarbeet production improved when alfalfa was in the rotation, and kept improving with longer rotations up to one beet crop every six years.
Combining principles: Permanent raised beds. In this practice two or three rows of crops between furrows are managed with zero or minimum tillage, and furrows are maintained as needed (Figure 1). This has received a great deal of attention in recent research in Asia and Australia. Furrows double as wheel tracks for cultivating, spraying, and other operations. Studies that compare permanent-raised-bed furrow systems with typical furrow irrigation and with no till management report marked improvements in soil quality, including lower density, warmer soil temperatures, better water productivity, and increased corn and wheat yields by 5 to 42%. Root crops like sugarbeet would require destruction and rebedding each time beets are harvested, but four- to six-year rotations with grain or forage crops would allow recovery of soil quality as observed in other regions. Perennial crops like alfalfa in the rotations, and cover crops following early-harvested crops like barley and dry beans would speed soil quality improvements.
Soil degradation and erosion are causing heartburn for furrow-irrigators concerned about soil quality and sustainability. We hope to begin some research and demonstration projects on the practice of permanent raised beds in Wyoming conditions during the next growing season. I would like to hear what producers think about the practice.
Figure 1. Bed planting with two and three rows spaced ‘a’ inches apart and furrow gaps ‘b’ inches wide (left), and direct-seeded wheat on permanent beds. From Roth et al. (2005) Evaluation and performance of permanent raised bed cropping systems in Asia, Australia and Mexico. ACIAR Proceedings No. 21, Canberra.