Rick Mascagni, Steve Harrison, Boyd Padgett and Bubba Bell
Adequate and timely fertilization is important in small grain cropping systems. Much research has been conducted on the macronutrients nitrogen (N), phosphorus (P) and potassium (K); however, less information is available on the need for the secondary nutrient sulfur (S). Sulfur deficiencies have increased because of decreased organic matter levels in the soil, less sulfur contamination in commercial fertilizers and less sulfur released into the atmosphere by industry. A cool-season crop such as wheat is more sensitive to sulfur deficiencies because of slower organic matter breakdown rates.
Deep, sandy Mississippi River alluvial soils are likely candidates for sulfur deficiencies because they have relatively low organic matter levels and, more importantly, low sulfur-adsorbing properties, which increase sulfur loss through leaching. Research indicates that the response to sulfur depends to a large extent on the depth of coarsetextured sandy soils, with yield responses increasing as the depth of sand increases.
Loessial silt loams of the Macon Ridge have shallow rooting depths and extremely low organic matter levels, which also make them susceptible to sulfur deficiencies. Information is needed on the influence of soil type on wheat’s response to sulfur fertilization on Louisiana alluvial and upland soils. Other cultural practices such as variety selection and timing of sulfur fertilization may also affect responses to sulfur fertilization.
Field experiments were conducted in the 2001-2002, 2002-2003 and 2003- 2004 growing seasons at the Macon Ridge Research Station near Winnsboro and at the Northeast Research Station near St. Joseph to determine the influence of sulfur fertilizer rate and time of application on yield performance of wheat varieties. At St. Joseph, experiments were conducted on Commerce sandy loam and Sharkey clay in 2002 and Commerce very fine sandy loam in 2003 and 2004. At Winnsboro, experiments were conducted on a Gigger silt loam each year.
Three wheat varieties, AGS 2000, USG 3209 and Pioneer/26R61 were evaluated. Sulfur, as ammonium sulfate, was applied at rates of 5, 10 and 20 pounds S per acre with spring N fertilizer. Additionally, the 20 pounds S per acre rate was applied in the fall to evaluate the effect of fall versus spring application. Spring N as ammonium nitrate was applied at 80 to 90 pounds N per acre in late February or early March. Nitrogen was adjusted for each treatment such that total N was equivalent in all the plots. At the Commerce very fine sandy loam site in 2002, only spring sulfur rates were evaluated for the wheat variety AGS 2000.
Wheat was planted in late October to mid-November at approximately 90 pounds seed per acre. Measurements included grain yield, yield components (heads per acre, kernel weight and kernels per head) and soil chemical analyses. Soil samples were collected after harvest from check plots at depths of 0 to 6, 6 to 12 and 12 to 18 inches.
At St. Joseph, soil sulfur levels were low on the sandy Commerce soil each year, with levels less than 5 ppm in 2002 and 2003 and less than 8 ppm in 2004. Sulfur level decreased with soil depth. On the Sharkey clay at St. Joseph, soil S increased from 13.6 to 21.8 ppm as sampling depth increased. At Winnsboro, sulfur increased with depth each year, averaging 8.6 ppm S per acre at the 0-to-6-inch depth, 29.8 ppm S per acre at the 6-to-12-inch depth and 39.6 ppm S per acre at the 12-to-18-inch depth.
According to AgCenter recommendations, soil sulfur is considered low at 8 ppm. Like nitrogen, sulfur is mobile in the soil, and soil test levels may vary depending on soil moisture conditions. This is particularly important in sandy soils. On heavier-textured soils, S is less mobile, and soil test levels tend to remain more stable over time.
Yields were increased by the application of S only in one of the seven experiments. On the Commerce very fine sandy loam at St. Joseph in 2002, applied S increased yields from 42.5 to 59.8 bushels per acre. Averaged across rates, S increased yields by approximately 30 percent and increased heads per acre by 28 percent; optimal yield occurred at 10 pounds S per acre. Response occurred on a soil that was very sandy down to 18 inches; the organic-matter level decreased from 0.6 percent in the top six inches to 0.2 percent at the 12-to-18-inch depth.
The lack of S responses on the other alluvial soils may have been due to a combination of higher organic matter levels and more clay at deeper levels, while on the Macon Ridge the lack of response was probably due to an accumulation of soil S as depth increased. Studies are continuing to better define soil conditions and other factors such as weather and previous crops that may affect the response of wheat to sulfur fertility.
Acknowledgment: Louisiana Soybean and Grain Research and Promotion Board
Rick Mascagni, Professor, LSU AgCenter Northeast Research Station, St. Joseph, La.; Steve Harrison, Professor, Department of Agronomy and Environmental Management, LSU AgCenter, Baton Rouge, La.; Boyd Padgett, Associate Professor, LSU AgCenter Macon Ridge Research Station, Winnsboro, La.; Bubba Bell, Research Associate, LSU AgCenter Northeast Research Station.(This article appeared in the summer 2005 issue of Louisiana Agriculture.)