Potassium Fertilization of Upland and Pima Cotton (1991-1995, a five-year project review)

Abstract

In an effort to provide information on the agronomic necessity of potassium (K) fertilization in Arizona cotton (Gossypium spp.) production, a five-year study was initiated in 1991 with a single field study located near Gila Bend. Subsequent study sites selected ranged from western (Yuma) to eastern (Safford) Arizona, which totaled 11 site-years. Both Upland (G. hirsutum L.) and American Pima (G. barbadense L.) cotton were cultivated, using soil and foliar applications of K. In 1992, study sites included the Safford Ag. Center (SAC), Maricopa Ag. Center (MAC), and a Cooperator site at Coolidge. In 1993, the experiment stations sites were maintained (SAC and MAC) and Yuma Valley was added. The 1994 study sites included only the two experiment stations (SAC and MAC). In 1995, SAC and MAC were maintained, and a third location was a farmer cooperator site at Buckeye. The results from all studies (12) indicated no lint yield increases due to K fertilization at any of the locations with either Upland or Pima cotton. However, in 1995,at the Buckeye location, the result revealed a significant yield reduction due to the K foliar treatments. There were, however, no significant differences among soil as well as the soil-plus-foliar treated plots in the 1995 study at Buckeye.

Introduction

Due to increased interest and reports of increased cotton yield as well as lint quality, from various locations across the U.S. cotton belt from potassium (K) fertilization, a five year K fertility study was initiated in 1991. Some notable studies of success on lint quality and yield improvement resulting from K input included those of Kerby and Adams (1985); Cassman et al. (1986-1992); Mullins et al. (1991); Oosterhuis et al. (1991); and Tupper et al. (1991a&b).
Potassium is an important nutrient to cotton plants for many physiological processes; but it has received considerable attention in its relation to fiber development. The development of individual fiber cells are dependant upon the maintenance of adequate turgor pressure within the cell (in the vacuole), which is controlled by a K+ malate solute system (Dhindsa et al., 1975). As a result of the role of K in both the vegetative and the reproductive structures of the plant, a study involving K fertilization on cotton yield could be valuable. The objective of this study was to assess the response of commonly grown varieties of cotton in Arizona to foliar and/or soil applied K.

Materials and Methods

1991

Beginning in 1991, a single field experiment was initiated at Paloma Ranch in western Maricopa County (Gilman loam). The treatments imposed in the field (Table 1) were arranged as split plots within a randomized complete block design (RCBD) with four replications. Soil applied treatments were arranged as mainplots, and foliar treatments as subplots. Soil applied treatments were broadcast and preplant incorporated as K₂SO₄. Mainplots consisted of 20; 36-inch rows, which extended the full length of the irrigation, run (1200 ft.). Each mainplot area was split into two, 10 row subplots, also extending the full length of the irrigation run. The field was planted to DPL 90 and watered-up on 12 April. Foliar applications of KNO₃ fertilizer were made at four dates throughout the first fruiting cycle of the crop to the entire subplot areas by use of a groundrig applicator at a carrier rate of 25 gallons per acre (Table 2). First bloom occurred on 15 June and plots were progressing into cutout based upon nodes above the top (first position) white flower (NAWF) and bloom counts by 19 August. The final irrigation was applied 31 August. The entire center eight rows of each subplot were mechanically picked on 24 October from which yield estimates were made. A preseason soil sample was taken in April 1991 to determine the concentrations of the exchangeable cations (Table 3).

1992

The locations of the three K fertility trials included the University of Arizona (UA) Safford Agriculture Center (SAC, Pima clay loam), UA Maricopa Agriculture Center (MAC, Casa Grande sandy loam), and a cooperator site located near Coolidge, AZ on a Mohall sandy loam soil series. Initial soil test results are given for each location in Table 4. The exchangeable cations including Ca, Mg, Na, and K were extracted using 1 M ammonium acetate (pH 7), and exchangeable Zn was extracted with DTPA. Phosphate (P) was extracted with sodium bicarbonate, and nitrate (NO₃-N) was determined using an ion-specific electrode (in a 1:1 soil:water extract). Soil pH was determined using a glass electrode (1:1 soil-to-water ratio).
At the SAC both Upland (DPL 90) and Pima (S-6) cotton were planted 21 April and watered-up on 23 April. Plots consisted of eight, 40-in. rows, each 34 ft. in length. Treatments included both soil and foliar K applications. All soil K application were broadcast and preplant incorporated using K₂SO₄ as the K source. Four foliar applications of KNO₃ were applied over the primary fruiting cycle, the date, HUAP, and rate of each foliar application is given in Table 5. All foliar applications were made using a ground-rig applicator with 25 gal./acre carrier. The soil and foliar K applications were combined to form a factorial arrangement of treatments with five replications as shown in Table 6. The Upland and Pima cotton had a final irrigation on 29 September and the center four rows of each plot were mechanically picked on 18 November from which yield estimates were made.

The K fertility trial at the MAC included only foliar K applications applied over the primary fruiting cycle to Pima cotton (S-6). Cotton was planted on 19 April (686 HU after 1 January) in plots consisting of four 40-in. rows that were 40 ft. long. The six foliar treatments (Table 7) were arranged over the experimental area in a RCBD with five replications. The experimental area received a final irrigation on 28 August and the center four rows of each plot were mechanically picked on 19 November

At Coolidge, K treatments were pre-plant, band-applied approximately 8 in. to the side of the seed row to a depth of 8 in. using two shanks per row applicator. Plots were eight 40 in. rows wide and extended the full length of the irrigation run (1200 ft.). The K source was K₂SO₄ and the rates of application are shown in Table 8. Upland cotton (STV KC311) was planted on 14 April (512 HU after 1 January) and treatments were applied in a RCBD with four replications. The last irrigation was applied on 10 September prior to mechanical picking the entire center eight rows of each plot on 29 October.

1993

Three K fertility trials were conducted in 1993, which included the SAC, MAC, and the Yuma Valley (Gadsden clay loam) locations.
At the SAC, both Upland (DPL 90) and Pima (S-7) cotton were planted 26 April (525 HU after 1 January). The experimental design was identical to that of 1992 for the site. Three foliar applications of KNO₃ were applied over the primary fruiting cycle (Tables 9 and 10). Both Upland and Pima cotton received a final irrigation on 2 August and the center four rows of each plot were mechanically picked on 11 November 1993.

At MAC, the K fertility study experimental design was also identical to the previous year for the site. Cotton (Pima, S-7) was planted on 5 April (471 HU after 1 January). Foliar treatment rates and dates (HUAP) are given in Table 11. The experimental area had a final irrigation on 17 August and the center two rows of each plot were mechanically picked on 18 October.

In the Yuma Valley, Upland (var. DPL 5409) cotton received aerial applications (10-gal carrier/acre) of foliar-KNO₃ over the course of the growing season (Table 12). Plots were 24 (40-in.) rows that extended the full length of the 640-ft irrigation run. Cotton was planted and watered-up on 11 April (773 HU after 1 January) and treatments were applied to plots arranged in a RCBD with four replications. The final irrigation was on 15 August prior to mechanical picking of the entire center eight rows of each plot on 21 September.

1994

Two K fertility studies were undertaken in 1994. The locations included MAC and SAC. At the MAC, the experimental design was identical to those of the previous two years at the site. Cotton was planted on 29 March (428 HU accumulated after January 1). Foliar treatments, rates, and dates (HUAP) are given in Table 13. The entire center two rows of all plots were mechanically picked on 29 October from which yield estimates were made.

The experimental design of the K fertility trial at SAC was also identical to those of 1992 and 1993. Pima (S-7) cotton was planted on April 7 (411 HU accumulated after January 1) and replanted on 27 April (659 HU accumulated after January 1) due to an irregular plant stand; and Upland (DLP 90) was planted on 7 April (411 HU accumulated after January 1). The date, HUAP, and the rate of each foliar application are given in Tables 14 and 15. The entire center two rows of all plots were mechanically picked on 22 November

1995

Three K fertility studies were conducted in 1995. The locations included SAC, MAC and a farmer cooperator site at Buckeye, AZ on a Laveen sandy loam soil series. The experimental design used in SAC and MAC were identical to the three preceding years.
At the SAC, the same varieties as previous years were planted on 4 April (389 HU accumulated after January 1) and replanted on 26 April (581 HU accumulated after January 1) due to an irregular plant stand. The date, HUAP, and the rate of each foliar application are given in Tables 16 and 17. Plots were mechanically picked on 31 October for the Upland (DPL 90) and 29 November for the Pima (S-7) cotton.

At the MAC, cotton was planted on 4 April (517 HU accumulated after 1 January). Foliar treatments, rates and dates (HUAP) are given in Table 18. Plots were mechanically picked on 21 November

The Buckeye site study was conducted with a farmer-cooperator. Pima (S-7) cotton was planted on 3 March (363 HU accumulated after 1 January) and received one foliar K treatment in split plots that had received all combinations of soil applied K rates (Table 19). Each plot was 8, 38 in. rows wide with a run of 1216 ft., and was split into two 4-row subplots to accommodate the in-season foliar treatment which was applied on 2 August. The K sources for the foliar and soil applications were KNO₃ and K₂SO₄ respectively. The entire center two rows of all plots were harvested on 18 November.

For each site-year of the project (1991-1995), all crop inputs (irrigation, pest control, and nitrogen (N) fertilization, etc.) were applied as needed throughout the season. Routine plant measurements were obtained every two weeks for all plots, including plant height, number of mainstem nodes, bloom counts, number of nodes above the topmost white flower to the terminal (NAWF), and percent canopy closure. Heat unit data was collected from AZMET weather stations at each location and heat units accumulated after plant planting (HUAP) was determined using 86/55 oF maximum and minimum thresholds, respectively. Statistical analyses of all resultant data (12 site-years) collected in this experiment were performed in accordance to procedures outlined by the SAS Institute (1985).

Results and Discussion

There were no apparent symptoms that would indicate any K nutritional problems at any of the 12 site-years utilized in this project. None of the plant measurements collected in this project revealed any differences due to treatments. Fruit retention (FR) levels were favorable for all treatments, as was the vegetative/reproductive balance (height to node ratio, HNR) maintained by the crop (data not shown in this report). The observed significance level (OSL) for the soil/foliar interaction studies for all locations (Tables 1-18) revealed no differences among K fertilization treatments for both Pima (S-7) and Upland (DPL 90). Generally, the OSLs were about an order of magnitude greater than an alpha value ("=0.05) commonly used for declaring statistically significant differences in the testing of hypotheses. At the Buckeye location (Table 20), the OSL indicated a significant difference associated with the foliar treated plots. The results indicated a reduction in yield associated with the soil-plus-foliar treated plots which may have resulted from an "osmotic inhibition". A high "solubility salt" such as KNO₃ that was used for the foliar application may have had an inhibitory effect on the crop. Bolt and Bruggenwert (1976) found that soluble salts deposited on the plant exert high osmotic pressure (e. g. in excess of 5-10 bars), which may have a detrimental effect on subsequent crop growth and development.

Foliar K applications at Buckeye in 1995 were undertaken at the time (2 August) when plant nutrient uptake may be receding and just prior to an irrigation event when soil and plant water levels were low. The combination of the above factors may have been responsible for the reduction in yield of the foliar treated plots. The OSLs (Table 19) for the soil-only and soil-plus-foliar treated plots indicated no yield response due to K fertilization. The results (Table 21) of the soil samples taken at two dates at the Buckeye location revealed that the exchangeable K for the two upper horizons at the study site was barely higher than the minimum threshold of 150 ppm critical level currently used in UA cotton fertility guidelines to support cotton production in Arizona soils (Silvertooth, 1998).

Also, in 1992, a physical and chemical characterization of southern Arizona representative agricultural soils was undertaken (Table 22). The result of these analyses (Table 22) revealed mixed clay mineral composition (Unruh et al., 1993). None of the soils contained less than 150-ppm exchangeable K except the Superstition soil. An important feature of these analyses is that all soils contain K bearing mica, and none contain significant amounts of vermiculite, which possesses a high capacity for K fixation.

These results provide the basis for two important observations; 1) the 150 ppm soil K threshold for cotton does not appear to be too low in that yields were not improved with K fertilization (Silvertooth, 1998); and 2) in view of the outcome at the Buckeye study (1995), caution should be exercised in the application of foliar fertilizers, particularly just prior to an irrigation.

Acknowledgment

The valuable technical assistance and cooperation provided by Eddie W. Carpenter, Lee J. Clark, McD Hartman, Dan Gladden, J. S. Stephens & Sons (Paloma Ranch), The Barkley Co. of Arizona (Yuma Valley), Prechel Farms (Coolidge), and H-4 Farms (Buckeye) are greatly appreciated.

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