Back in 1928, when research units for the Alabama Agricultural Experiment Station initially were established throughout the state, soil fertility probably was the primary issue facing cotton producers, says Charles Mitchell, Auburn University Extension agronomist.
“It's still a big issue, but we've put it on the back burner today because we think we've solved most of the problems,” says Mitchell. “But many of these original research sites remain in use today and continue to provide invaluable information. The long-term results of these studies allow researchers and producers to map the past, but also plan for the future.”
Long-term experiments conducted at outlying units of the Alabama Agricultural Experiment Station are the basis for Alabama's research-based soil test interpretations and recommendations, he says.
One of the first experiments established at the stations, says Mitchell, was the two-year rotation, which was begun in 1929. The two-year rotation tests are located at six sites in the state and always have involved cotton, corn, sorghum, soybeans or peanut rotations. Five of these have included cotton since 1992, and they involve various nitrogen, phosphorus and potassium rates.
“Those initial experiments have continued to this day. Some people ask, ‘Why keep an experiment going for 73 years — haven't you learned anything by now?’ The answer is yes — we've learned a lot, but we're continuing to learn from these old experiments,” he says.
In 1954, he adds, the soil testing laboratory was established at Auburn University. “By that time, we knew not only that our soils throughout Alabama were different, but also that each field was different because of past fertilizer practices, and we needed to use a technique to evaluate this soil fertility. This is when the soil lab was established,” notes Mitchell.
To maintain a sound research basis for its soil testing services, the Experiment Station established additional soil test calibration experiments in 1954 known as the rates of N-P-K experiments, says the agronomist. These have been continued at seven locations since their establishment, and cotton has been planted in these experiments more than any other crop. Five of the seven experiments have been planted in cotton every year since 1992.
Nitrogen, says Mitchell, is the most difficult nutrient to manage in cotton production. “It has more impact on yields, earliness and lint quality than any other primary plant nutrient. It's also the most costly plant nutrient applied per acre.”
Environmentally, potential nitrate-N leaching into groundwater is a driving force behind water quality issues and nutrient management planning policies, he says.
“Because nitrogen is a primary constituent in all protein, transforms easily into several chemical forms in the soil, is biologically active, and can be mobile in the environment, farmers have problems managing it in cotton production.”
Growers have used a variety of techniques to optimize nitrogen use efficiency in cotton, says Mitchell, including split or multiple nitrogen applications, starter fertilizers, nitrogen fertilizer placement, foliar urea applications during bloom, petiole monitoring, plant leaf analyses, water management or irrigation timing, plant growth regulators, cover crops and computer programs and plant growth models.
“Since it was founded in 1883, the Alabama Agricultural Experiment Station has been studying nitrogen on cotton in an attempt to help Alabama growers better manage this nutrient. The oldest, continuous cotton experiment in the world — Alabama's Old Rotation, begun in 1896 at Auburn University — focuses on nitrogen management by rotating cotton with other crops and by planting winter legumes.
“However, the two-year rotation and rates of N-P-K experiments also provide valuable information about nitrogen rates for cotton crops in Alabama. Alabama's current standard nitrogen recommendations were developed from these and other experiments in the 1950s and 1960s, modified in the 1970s and refined in the 1980s. Therefore, if cotton responds to nitrogen rates differently today, this might reflect the effects of improved varieties, higher yields and different management.”
To look at the effects of nitrogen rates on new cotton varieties, Bollgard and Roundup Ready varieties have been planted at most locations since 1996, says Mitchell. At all locations, the nitrogen source is ammonium nitrate (34-0-0) and nitrogen rates are split, with half applied at planting and half as a side-dress at or near early squaring.
“Results strongly indicate that the standard nitrogen recommendations still are appropriate for maximum relative yields in these tests when cotton follows cotton. Because as many as six months could elapse between soybean/peanut harvest in the fall and cotton planting the following April or May, much of the residual nitrogen may be lost from the soil, he says. Data from nitrogen-rate treatments on the two-year rotation at five locations verify the variable nature of residual nitrogen from legumes, he adds.
Promoters of fertilizer use often have espoused the concept of nitrogen recommendations based on cotton yield goals, says Mitchell. “Many commercial labs are recommending higher nitrogen rates for higher yielding cotton. And, the University of Georgia has incorporated yield goals into its soil test recommendations. This is a particularly popular and reasonable practice with grain crops such as corn, wheat and sorghum, in addition to forages. These crops remove large quantities of nitrogen in the harvested portion of the crop.
“However, Alabama's long-term experiments do not support this practice under the conditions of these experiments. In a disaster year — when cotton yields are less than a bale per acre — very little if any nitrogen fertilizer is needed. No farmer plans on a disaster year and never fertilizes for these situations. But even in outstanding production years, when yields far exceed anticipated yield goals, data from Alabama's research stations support the ‘standard’ recommendation plus or minus about 30 pounds of nitrogen per acre.”
With the boll weevil no longer a threat to Alabama producers, cotton now has the potential to set more late-season cotton bolls, thus increasing yields and the demand for more nitrogen, says Mitchell. However, data from these long-term experiments have not indicated that boll weevil eradication nor the new, genetically modified varieties have had any effect on cotton yield response to nitrogen rates since 1996. In fact, the highest yields were produced in 1992 or 1993 at all sites except for one, he adds.
Because of the rapid adoption of genetically modified cotton varieties, there have been few opportunities, says Mitchell, to evaluate their response to soil fertility variables.
“In 1996 — the first year Bollgard varieties were available to Alabama producers — the rates of N-P-K experiment on a Lucedale soil at the Prattville Experiment Field was modified to determine if any differences existed in response to soil fertility variables between two varieties of similar genetic backgrounds.”
All plots, he says, were split, and a Bollgard variety — DP 35B — was planted on half of each, and a conventional variety of similar genetics — DP 5690 — was planted on the other half of each plot. This was repeated in 1997 and 1998.
The DP 35B yielded an average of 85 pounds more lint per acre per year than the conventional variety, over all nitrogen rates. But the differences due to variety would not affect the standard nitrogen recommendation, says Mitchell. The yield difference most likely is due to sub-threshold control of bollworms and budworms by the Bollgard variety, he says.
Long-term, nitrogen rate research at several Alabama locations since 1992 supports the current standard nitrogen recommendations used on soil test reports, he says.
“While nitrogen recommendations based on a yield goal may apply for some crops, this clearly isn't the case with non-irrigated cotton in Alabama. Producers should follow the standard nitrogen recommendations on new fields and make adjustments as experience and cropping systems dictate. Nitrogen rates do not need to be adjusted for newer, genetically modified varieties.”