Georgia wheat growers harvested record high yields of up to 135 bushels per acre for the 2000 growing season in the face of drought conditions. Critical nutrient management decisions for the 2001 soft red winter wheat crop will be based on crop and moisture conditions early in 2001.
“Growers make their final nutrient applications in late winter/early spring based on winter kill, stand density and insect pressure,” notes William Segars, professor of crop and soil science, University of Georgia. “They have a very narrow window to assess crop conditions. With good stands, nitrogen rates will be increased to push for higher yields.”
Due to Georgia's thin, sandy soils, fertility recommendations for nitrogen, phosphorus, potassium and sulphur are based on crop needs rather than soil or carryover values. Sulphur is often recognized as the “fourth” major nutrient for its role in plant growth and metabolism, especially in nitrogen and protein synthesis. As a shallow-rooted plant, wheat needs nutrients in close proximity to its root zone for adequate uptake.
“We recognize that sulphur is a vital element for wheat and small grains on our soils,” says Dewey Lee, Extension grain agronomist, University of Georgia. In addition to improving nitrogen metabolism in the wheat plant, sulphur plays a key role in overall yield as well as protein content and amino acid balance of the grain.
According to Lee, it is easy for the untrained eye to confuse sulphur and nitrogen deficiencies. “If there is any doubt about what is going on in the field, we urge farmers and their Extension agents or crop specialists to get a tissue analysis of that wheat plant,” he stresses.
“We certainly recommend that wheat growers split their nitrogen and sulphur applications. This reduces the leaching potential from heavy rainfall during the winter months.”
Tissue analysis taken during the growing season provides a useful and consistent look at the level of sulphur available to the growing crop. Lee notes that wheat growers can expect results on tissue analyses within a few days from the University of Georgia. “They can have the data forwarded to them by their county agent as soon as he checks the lab's secure online site for the results,” he adds.
Current plant tissue test recommendations for nitrogen/sulphur ratios are 15:1, or 15 parts of nitrogen to one part sulphur. “At tissue ratios of 18:1 you can expect to see deficiencies due to lack of sulphur,” notes Lee.
Nitrogen utilization in the plant depends on a readily available supply of sulphur, according to Donald Messick, an agronomist with The Sulphur Institute, Washington, D.C. Sulphur availability can be affected by numerous physical and environmental conditions.
Soil compaction and pH factors can tie up sulphur in the soil making it unavailable to the growing plant. “Positionally unavailable sulphur starves the plant of sulphur because the roots can't reach or access it readily,” says Segars. “This availability problem is difficult to determine visually.”
Segars says growers can mistake symptoms of unavailable sulphur with inadequate sulphur fertility. In several reported cases, wheat growers felt that their fertilizer supplier failed to apply sulphur when in fact the sulphur had been applied, but was unavailable to the plant due to soil and environmental factors. In these types of cases, both Lee and Segars recommend tissue testing to determine the sulphur nutritional status of the plant.
“We certainly recommend that wheat growers split their nitrogen and sulphur applications,” says Lee. “This reduces the leaching potential from heavy rainfall during the winter months.”
The University of Georgia currently recommends a ratio of 5:1 for nitrogen and sulphur fertilizer applications. Most wheat acres in the state receive a 28-0-0-5 nitrogen/sulphur combination in split applications.
According to Segars, Georgia's drought-stricken soils from 2000 are susceptible to nutrient leaching from heavy rainfall during the early months of 2001. “Heavy rains have the potential to leach nutrients even under drought conditions,” he adds. Downward movement of nutrients due to rainfall removes important nutrient reserves in the soil.
“Because sulphur is inexpensive and is an integral part of nitrogen usage, most nitrogen suppliers pretty much insist that it be part of the fertility program,” notes Segars.
Adds Donald Messick, agronomist for The Sulphur Institute, “Sulphur not only increases wheat yield and protein content, improves nitrogen use efficiency, resulting in higher economic rate of return.”
“Many times in the early spring, growers will see yellow wheat and believe they are seeing nitrogen deficiency and apply more nitrogen,” says Lee. “What they're really doing is creating a greater sulphur deficiency, because plants demand more sulphur as nitrogen rates increase.” Sulphur can be carried below the root zone and needs to be added to any spring fertilizer application on sandy soils to insure adequate levels are available to the plant throughout the remainder of the growing season.
According to Lee, top-dressing sulphur along with nitrogen in the spring eliminates deficiencies that may be due to heavy rainfall. “We encourage growers to use 10-15 pounds of sulphur with their spring nitrogen application,” he adds. Potassium and phosphorus also play key roles in stem strength and disease resistance.
Research from Auburn University in Alabama shows that applying sulphur and nitrogen no later than Feeke's Growth Stage 5 (tillers formed, leaf sheaths erect, plant upright, and stem elongation beginning) results in the optimal yields.
The top-dress application including sulphur just prior to stem elongation plays an important role in yield potential. “By applying nutrients before stem elongation you increase tillering,” notes Lee. “Increased tillering means more heads are set and yield potential increases.”
Timing is critical. Wheat yields can be reduced by 10-15 percent by waiting just two weeks, or until Feeke's Growth Stage 8, to apply nitrogen and sulphur, according to field studies conducted by Auburn University.
When growers do see signs of sulphur deficiency in the spring, the deficiency can be treated. If spotted early, sulphur deficiency can be easily corrected by applying a nitrogen-sulphur product containing sulphate sulphur, such as ammonium thiosulphate (12-0-0-26S) or ammonium sulphate (21-0-0-24S), TSI's Messick says. “Other sources, such as elemental sulphur, work when applied early in the growing season,” he adds.
Lee recommends 20-30 pounds nitrogen in the fall along with 5-10 pounds of sulphur followed by 90 pounds nitrogen and an additional 10-15 pounds of sulphur in the spring.
“An 80-bushel wheat crop will take up at least 20 pounds of sulphur,” notes Messick, “making adequate sulphur fertilization necessary for optimal yields of grain and forage.”
Record high wheat yields in 2000 provide a glimpse at the efficiency of Georgia's wheat growers. “We produced more bushels of wheat on 350,000 acres in 2000 than we used to on one million acres,” says Lee.
Lee credits two factors for the yield increases: first, experienced growers committed to producing high wheat yields through nutrient management; and second, nutrient suppliers who recognize the need for sulphur in the fertility program.