Cotton Response to Surface Applications of Potassium Fertilizer: A 10‐Year Summary

Abstract

Field studies were conducted for 10 yr on a Limestone Valley soil (Dewey silt loam, a fine, kaolinitic, thermic Typic Paleudults) in north Alabama to evaluate cotton ( Gossypium hirsutum L.) yield and lint quality response to long‐term surface applications of K. Potassium rates were 0,60,120, and 180 lb K 2 O/acre for the first 3 yr of the study. Fertilizer was broadcast in the spring or split applied in fall and spring. No K fertilizer was applied during the fourth year (1990). After the fourth year, half of the treatments were left in residual and remaining treatments continued to receive annual applications of K (1991‐1996). The experiment was a split plot arrangement of a randomized complete block. From 1987 to 1989, two varieties were compared, which served as whole plots. From 1991 through 1996, one variety was grown and annual vs. residual K fertilization served as whole plots. Potassium treatments were used as subplots throughout the test. From 1987 to 1989 there were no differences between ‘Stoneville 825’ and ‘Deltapine 50’. Lint yields were increased significantly by K fertilization in 6 of 9 yr that cotton was grown on this low K soil. Potassium fertilization increased lint yields by as much as 450 lb/acre in responsive years. From 1991 through 1996, residual K produced lesser yields than where K was applied annually. There were no differences in lint yield among split (fall‐spring), fall, or spring applied K. In responsive years, micronaire increased with K rate. Our results support the calibration currently used by the Auburn University Soil Testing Lab for Limestone Valley soils. Research Question This field study was initiated in the fall of 1986 on a Tennessee Valley soil in north Alabama that had been in alfalfa production for 5 yr. The objectives of the study were: (i) evaluate cotton varietal differences to K fertilization; (ii) evaluate cotton response to long‐term applications of K fertilizer; (iii) evaluate cotton response to annual and residual K. fertilization; (iv) determine cotton yield response to spring, fall, and split (fall/spring) application of K. fertilizer; and (v) evaluate lint quality as affected by K fertilization. Literature Summary Late season K deficiency has been observed across the U.S. Cotton Belt leading to renewed interest in soil test calibration for K in many states. In recent years, cotton response to K fertilizer rates and placement as well as foliar feeding has been evaluated. Responses of cotton to K fertilization in many of these relatively short‐term studies have been inconsistent. Previous work in Alabama has shown that cotton yields are more affected by inadequate K than soybean and grain crops. Limited data are available to describe the response of cotton to fall vs. spring applications of K. There are also limited data available to describe varietal differences in cotton response to K fertilization. Study Description A 10 yr (cotton grown 1987–1989 and 1991–1996) field test was conducted on a Limestone Valley soil (Dewey silt loam) in north Alabama. The test was initiated in the fall of 1986 and K was applied at rates of 0, 60, 120, and 180 lb K 2 0/acre for the first 3 yr of the study. Fertilizer was broadcast in the spring or split applied in the fall and spring. No K fertilizer was applied during the fourth year of the study (1990). After the fourth year of the test, half of the treatments were left in residual without any further additions of K, and the remaining treatments continued to receive annual applications of K fertilizer (1991–1996). The experiment had a split plot design with four replications. From 1987 to 1989, two varieties were compared (Stoneville 825 and Deltapine 50) and these served as whole plots. From 1991 to 1996, only one variety was evaluated and the comparison of annual vs. residual K fertilization served as whole plots. Potassium treatments were used as subplots throughout the test. At maturity, seed cotton yields, lint yields and lint fiber quality were determined. Soil test K levels in the plow layer were determined in selected years of the test. Applied Questions What rate of fertilizer K is sufficient for cotton production on this Limestone Valley soil? On a Limestone Valley soil with a ‘low’ soil test rating for K at the initiation of the test in the fall of 1986, a positive yield response to K fertilization was obtained during 6 of 9 yr that cotton was grown. Based on regression analysis, lint yields generally peaked at the highest rate of K fertilization (180 lb K 2 0/acre), but in many years the yield difference between the 60 and 180 lb K 2 0/acre rates was not significant. Plotting lint yields vs. soil test K levels showed that lint yields peaked at approximately 240 to 250 lb/acre of Mehlich 1 extractable K, which agrees with the calibration currently used by the Auburn University Soil Testing Lab. From this 10‐yr study, it was calculated that following the recommendations of the Auburn University Soil Testing Lab (90 lb K 2 0/acre) resulted in an average increase of 2.7 lb lint/acre for every pound of applied K 2 0 if K is applied to this soil with a low soil test rating. When the recommendations were followed for this soil having a ‘medium’ soil test rating, the average return over a 6‐yr period was 2.0 lint for every lb of applied K 2 0. Do some cotton varieties respond differently to K fertilization? During the first 3 yr of this study (1987‐1989) two cotton varieties: (i) Deltapine 50 and (ii) Stoneville 825 were compared. Stoneville 825 is a longer season verticillium wilt susceptible variety while Deltapine 50 is a short season variety with some resistance to verticillium wilt. Throughout this test we did not see any visual evidence of verticillium wilt. Based on our results we conclude that there were no differences among these two varieties in terms of their response to K fertilization when produced under these growing and climatic conditions. Does timing (fall vs. spring) and split (fall/spring) applications of K fertilizer affect cotton yields or fiber quality? In this test, 120 and 180 lb K 2 0/acre was applied in the spring, in the fall (120 lb rate), or split applied in the fall and spring (120 and 180 lb acre rates). There were no significant differences between split (fall‐spring), fall, or spring applied K with respect to lint yields or fiber quality. Does residual K fertility on Limestone Valley soils contribute to cotton production? In this test annual K rates were applied for 3 yr (1987–1989) and no K fertilizer was applied during the fourth year of the study (1990). After the fourth year of the test, half of the plots were left in residual while the remaining plots continued to receive the annual K rates (1991–1996). The residual effects of applying K fertilizer during the first 3 yr of the test did influence cotton yields from 1991 to 1996. However, optimum yields were obtained when K was applied every year of the test. Our results suggest that, for these soils, optimum yields will be expected if recommended rates of K are added every year until the residual soil test K level has been raised to a “high” rating. Did K fertilization of a Limestone Valley soil affect fiber quality? Fiber quality as determined by HVI was affected by K fertilization. Micronaire was the only lint quality parameter consistently affected by K. Significant differences in micronaire were observed primarily during those growing seasons where lint yields were affected by K treatments. Micronaire increased with increasing K rate and the level of soil test K. Recommendation Cotton produced on a silt loam, Tennessee Valley soil in north Alabama with an initial low soil test rating for K responded to K fertilization during 6 of the 9 yr that cotton was grown in this study. These results showed that cotton producers on similar soils should follow the recommendations of the Auburn University Soil Testing Lab (i.e., 90 lb K 2 0/acre for soils testing low and 60 lb K 2 0/acre for soils testing medium). Applying K fertilizer in the fall or split applications of K in the fall and spring did not result in any advantages compared with applying all of the K in the spring as a preplant broadcast. Residual soil K increased from previous fertilization was shown to increase cotton yields, however, for these soils optimum yields will be expected if recommended rates of K are added every year until the residual soil test K level has been raised to a high rating.