Annual Report Submitted To

MS-13F

ANNUAL REPORT SUBMITTED TO

FAR

Project Title: Determination of K, Mg, and S as Limiting Factors in Cotton Production on Blackland Prairie Soils

By: Dr. Jac J. Varco, Professor of Soil Fertility

Plant and Soil Sciences Dept.

MississippiStateUniversity

Mississippi State, MS39762

Although N and K fertilization continue to be the primary fertilizer nutrients applied to cotton, achieving maximum economic soil productivity may require a critical evaluation of other macronutrients such as Mg and S. In addition to improvements in yield potential of genetic lines and greater nutrient demands, changes in production practices such as elimination or minimization of tillage may alter availability of nutrients such as K, Mg, and S requiring a re-evaluation of currently accepted soil fertility and plant nutrition guidelines. The objectives of this study are 1.) Determine individual response functions of cotton leaf K, Mg, and S levels and lint yield to varying rates of K2O, Mg, and S; and 2.) Compare combined K, Mg, and S response (KMag treatment) to individual nutrient responses.

MATERIALS AND METHODS

A field experiment was conducted at the Plant Science research Center at MississippiState. The soil at the site is a Marietta fine sandy loam (fine-loamy, mixed, thermic, siliceous Aquic Fluventic Eutrochept) with the following initial average soil test levels: 0 to 6" - pH=6.53, CEC=8.3, P=178 (High+), K=264 lb/acre (High), Mg=102 lb/acre (High) and 6 to 12" - pH=6.94, CEC=20.3, P=96 lb/acre (High), K=143 lb/acre (Low) and Mg=67 lb/acre (Low). Cotton was grown using standard recommended cultural practices. The variety DPL 444 BGRR was planted on 6 May, 2004 at a rate of 4 seed/ft. Plot size was 12.67' X 30' consisting of four rows at a spacing of 38". Onsite rainfall and high and low ambient temperatures were monitored. Conventional tillage practices were used in 2004, but no-tillage practices will be adopted in subsequent years. Treatments included K2O at rates of 0, 36, 72, and 108 lb K2O/acre all with a blanket application of 18 lb Mg/acre and 36 lb S/acre; Mg at rates of 0, 9, 18, and 27 lb Mg/acre all with a blanket application of 72 lb K2O/acre and 36 lb S/acre; S at rates of 0, 18, 36, and 54 lb S/acre all with a blanket application of 72 lb K2O/acre and 18 lb Mg/acre; and KMag supplying rates of K2O-Mg-S at 0-0-0, 36-9-18, 72-18-36, and 108-27-54 all in lb/acre and 50% of the K2O was derived from 0-0-60. Sources included muriate of potash (0-0-60), ammonium sulfate (21-0-0-24S), magnesium nitrate (11-0-0-15Mg), and KMag (0-0-22-11Mg-22S). This combination of sources and blanket application of complementary nutrients was designed to avert negative yield effects caused by possible antagonistic deficiencies of non-target nutrients. Nitrogen was applied as a split application with 50% applied after planting and 50% at early square. Ammonium nitrate served as the N source at a total rate of 120 lb N/acre. In the case of treatments which included ammonium sulfate or magnesium nitrate, the rate of ammonium nitrate applied after planting was adjusted to account for the N supplied by these sources. All fertilizer materials were broadcast applied after planting. Treatments were arranged using a randomized block design involving four replications.

Soil samples at a depth of 0 to 6" and 6 to 12" were collected prior to fertilization and a routine soil test was performed. Leaf samples were obtained at early bloom by sampling 10 to 12 recently mature leaves off of the main stem. Samples were dried at 65 oC, ground to pass a 2-mm sieve and analyzed for total N, K, Ca, Mg, and S. Cotton was harvested using a two row spindle type picker. The two center rows of each plot was harvested and seedcotton samples were ginned to determine lint yield.

RESULTS

Cotton lint yield response to varying rates of K2O, Mg, and S is shown in Fig. 1. Lint yield increased from 1128 lb/acre up to 1333 lb/acre with an increase in fertilizer K2O from 0 to 72 lb/acre. Response to Mg was inconsistent, but the highest rate of 27 lb/acre yielded 1245 lb lint/acre compared to 1160 lb lint/acre for the no Mg check. Lint yield did not respond to increasing S rates from either source. Response to all three nutrients as supplied by KMag was not evident. Inconsistencies in lint yield results could partially be related to losses which were incurred by high winds and 5.47” rainfall received 12 Sept. 2004 from the remnants of hurricane Ivan. Yield loss varied from plot to plot with a range of 122 to 460 lb lint/acre estimated by ground sampling 2.7 ft2 from eight experimental plots.

Leaf tissue analysis response to varying rates of K2O, Mg, and S is shown in Fig. 2. Leaf tissue K levels increased above a critical level of 1.25% for K rates greater than 72 lb K2O/acre from muriate of potash. There was no apparent effect of KMag on leaf tissue K at early bloom. Leaf tissue Mg levels declined at Mg rates of 9 and 18 lb/acre from magnesium nitrate, but increased at the highest rate compared to the no Mg check. Leaf Mg increased up to the 18 lb /acre rate supplied by KMag. All leaf Mg levels were marginally sufficient with only the 27 lb/acre rate from magnesium nitrate resulting in tissue levels considered non-responsive. Tissue S levels increased up to 36 lb S/acre from ammonium sulfate, while with KMag maximum leaf S occurred at 18 lb S/acre.

With an increase in no-till cotton acreage in Mississippi from 10,700 acres (0.91%) in 1990 to 293,482 acres (24.7%) in 2002, it is likely that bio-availability of K, Mg, and S may be altered. For 2005, strict no-till practices will be adopted in this study to address this question.

Fig. 1. Lint yield response to varying rates of K2O, Mg, S, and KMag. Crosshair in square symbol represents the check which did not receive any fertilizer K2O, Mg, or S and served as the zero rate for determining KMag response.

Fig. 2. Cotton leaf analysis response at early bloom to varying rates of K2O, Mg, S, and KMag.

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