The Impact Of Foliar K Fertilizer Source On Crop Response And Weed Control In A No-Till "Weed And Feed" Glyphosate-Resistant Soybean Production System

Introduction:
The incidence of K deficiency has increased in recent years due to reduced K availability under drought and in areas with soil compaction, reduced K fertilizer applications for soybean due to low commodity prices, and higher corn grain yields and increased soybean acreage in rotation with corn increasing K fertilizer requirements (Reetz and Murrell, 1998; Fixen, 2000). Soil test K data from the University of Missouri Soil and Plant Testing Lab indicated that over 50% of the soil samples tested in the low to medium range for K (Fixen, 2002).

Several studies have evaluated response of soybean to foliar fertilizer mixtures (Garcia and Hanway, 1976; Haq and Mallarino, 1998; Parker and Boswell, 1980); however, limited research has evaluated the interaction between macronutrient foliar fertilizers and weed control with postemergence herbicides. Previous research on soybean response to a foliar application of K sulfate demonstrated that soybean grain yield increased over 10 bu/acre when compared to a non-treated or MgSO₄ control (Nelson et al., 2003). The carrier volume required for an optimum foliar K application with K sulfate and the possible incompatability that the K fertilizer source may have when mixed with a glyphosate-based herbicide limits the use of this particular fertilizer source. In addition, the K source/herbicide mix must result in minimal crop injury and not affect weed control.

A single postemergence weed management treatment is commonly used in no-till glyphosate-resistant soybean production in the North Central U.S. Previous research has evaluated multiple foliar application timings in soybean; however, a single application would be the most cost-effective in a “weed and feed” management system. Limited field research has evaluated the interaction between formulated macronutrient foliar fertilizers and weed control with glyphosate. We hypothesized that the addition of K based fertilizer sources to the spray mixture of formulated K-glyphosate would not reduce weed control. The objectives of this research were to: 1) determine soybean yield response and salt injury from different foliar-applied potassium (K) fertilizer sources; 2) assess if K fertilizer source affects weed control when mixed with a glyphosate-based herbicide; and 3) evaluate the cost-effectiveness of applying K fertilization with glyphosate-based herbicides for no-till glyphosate-resistant soybean production. This research will assist farmers to make informed decisions regarding foliar weed and feed K applications based on the effects of those applications on weed control efficacy, productivity, and profitability.

Methods:
Research was conducted in 2003 and 2004 at the University of Missouri Greenley Research Center near Novelty, MO on a Putnam silt loam with a high soil test K (455 + 112 lb/a). Plots were 10 by 35 ft. ‘Asgrow 3701’ soybean was no-till planted on May 19, 2003 and May 20, 2004 in 15 in. rows at 180,000 seeds/acre. The study was arranged as a factorial randomized complete block design with four replications. Spray mixture pH was recorded prior to application of treatments. All treatments were applied with a CO2 propelled hand sprayer traveling 2.9 MPH and delivering 15 GPA at 17 psi with 8002 FF nozzles. Soybean was 6 to 8 in. tall at the V4 to V5 stage of development; common ragweed was 4 to 8 in. tall with 8 to 14 leaves; common waterhemp was 2 to 12 in. tall with 4 to 14 leaves; and common lambsquarters was 4 to 6 in. tall with 12 to 18 leaves at the time of application.

The K fertilizer additives included 3-18-18, potassium phosphate, NA-CHURS/ALPINE; 0-0-30, potassium carbonate (Double-OK), NA-CHURS/ALPINE; 0-0-25-17, potassium thiosulfate (KTS), Tessenderlo KERLEY; 5-0-20-13, potassium sulfate (Trisert K+), Tessenderlo KERLEY; 0-0-50, potassium sulfate; 0-0-62, potassium chloride (Kalium); 13.8-0-46, potassium nitrate, SQM North America; and DAS, diammonium sulfate (DAS) on plots maintained weed-free. The foliar K application rate was maximized based on the physical limitations of the K source; therefore, K application rates ranged from 2 to 55 lbs K₂O/acre. Grain was harvested and moisture adjusted to 13%. A gross margin was calculated as the [(grain yield * market price) - (cost of foliar fertilizer + fertilizer application cost in absence of herbicide)] to determine the practical use of K sources in a weed and feed soybean production system.

Foliar salt injury was rated 3, 7, 14, and 21 days after application on a scale of 0 (no effect) to 100 (complete crop or weed death). Weed control for individual weed species was recorded 14, 28, and 56 days after treatment. A biomass harvest of soybean and weeds was collected 28 days after application to determine total weed control. Percent dry weight reduction was calculated as 100[1-(total weed dry weight/untreated weed dry weight)].

Data were subjected to ANOVA and means separated using Fisher’s Protected LSD (P<0.05). All data were combined over years and main effects were presented in the absence of interactions. Weed control data were presented separately each year due to differences in the primary weed species present each year.

Summary:
Since fertilizer sources can vary, a compatibility test was conducted and observations were recorded. This study evaluated the highest rate of foliar fertilizer that could be applied with glyphosate; therefore, the liquid formulations were utilized as the carrier. A slight precipitate was formed when 3-18-18 was tank mixed with glyphosate while KTS formed a viscous solid with glyphosate that was removed prior to the tank mixture application. A poor spray pattern was observed when 0-0-30 was applied with glyphosate and the spray boom height was adjusted to compensate.

Visual injury was primarily necrosis of leaves exposed to foliar application. All treatments except 0-0-30 had less than 10% injury 7 and 14 days after treatment with almost complete recovery by 21 days after treatment (Table 1). Potassium chloride, nitrate, and sulfate tank mixed with glyphosate injured soybeans similar to glyphosate plus diammonium sulfate (DAS). The adjuvants present in the glyphosate formulation probably increased uptake of foliar fertilizers and increased crop injury.

Potassium chloride, nitrate, sulfate, or phosphate plus glyphosate controlled common lambsquarters, common ragweed, common waterhemp, and giant foxtail similar to glyphosate plus DAS (Table 2). Other potassium sources antagonized weed control with glyphosate at the rates evaluated in this research. In a weed-free environment, soybean grain yield was similar among K fertilizer source treatments (Table 3). Soybean treated with potassium phosphate, chloride, sulfate, or nitrate tank mixed with glyphosate had grain yields similar to glyphosate plus DAS. Potassium chloride, sulfate, and nitrate applied alone or tank mixed with glyphosate had gross margins similar to glyphosate plus DAS at the rates evaluated in this research. This research indicated that some K fertilizer additives maintained weed control, soybean grain yield, and were as costeffective as DAS while serving as a fertilizer source for soybean.

The authors would like to extend their appreciation to the Fluid Fertilizer Foundation and PPI for their support of this research.

References:
Fixen, P. 2000. A national perspective on nutrient management guidelines and regulations. Symposium on the Status and Basis for Mandating Nutrient Management Guidelines, ASA Annual Meetings, Nov. 6, 2000, Amer. Soc. Agron., Madison, WI.

Fixen, P.E. 2002. Soil test levels in North America. Better Crops with Plant Food 86:12-15.

Garcia, R.L. and J.J. Hanway. 1976. Foliar fertilization of soybeans during the seed-filling period. Agron. J. 68:653-657.

Haq, M.U. and A.P. Mallarino. 1998. Foliar fertilization of soybean at early vegetative stages. Agron. J. 90:763-769.

Nelson, K.A., P.P. Motavalli, and M. Nathan. 2003. Response of no-till soybean to timing of pre-plant and foliar potassium applications in a claypan soil. Agron. Abstr., Am. Soc. Agron., Madison, WI.(CDROM).

Parker, M.B. and F.C. Boswell. 1980. Foliage injury, nutrient intake, and yield of soybean as influenced by foliar fertilization. Agron. J. 72:110-113.

Reetz, H.F. and T.S. Murrell. 1998. Negligence of potassium in corn/soybean systems: Are you guilty? News & Views, December issue, Potash & Phosphate Institute, Norcross, GA.

Taylor, J.B., J.A. Koscleny, J.J. Sandbrink, D.C. Heering, and P.G. Ratliff. 2002. Characterization and performance of an alternate salt formulation of glyphosate in Midwestern environment. Proc. North Cent. Weed Sci. Soc. 57:(CD-ROM).

Table 1. Soybean injury 7 and 14 DAT and fresh weights 28 DAT with K sources in a weed-free environment and tank mixed with glyphosate in 2003 and 2004 with a high soil test K^a.

^aAbbreviations: DAS, diammonium sulfate; DAT, days after treatment; and LSD, least significant difference.
^bK sources: 3-18-18, potassium phosphate, NA-CHURS/ALPINE; 0-0-30, potassium carbonate (Double-OK) NA-CHURS/ALPINE; 0-0-25-17, potassium thiosulfate (KTS) Tessenderlo KERLEY; 5-0-20-13, potassium sulfate (Trisert K+) Tessenderlo KERLEY; 0-0-50, potassium sulfate; 0-0-62, potassium chloride (Kalium); 13.8-0-46, potassium nitrate SQM North America; and DAS, diammonium sulfate (DAS).

Table 2. Total weed dry weight reduction 28 DAT (days after treatment) in a weed-free environment and tank mixed with glyphosate, and control of common lambsquarters in 2003, common ragweed in 2003, common waterhemp in 2003 and 2004, and giant foxtail in 2004 with glyphosate tankmixed with K fertilizer sources on a high soil test K^a.

^aAbbreviations: DAS, diammonium sulfate; DAT, days after treatment.
^bWeeds included common lambsquarters in 2003, common ragweed in 2003, common waterhemp in 2003 and 2004, and giant foxtail in 2004. Total dry weight reduction was calculated as 100[1-(total weed dry weight/untreated weed dry weight)].
^cK sources: 3-18-18, potassium phosphate, NA-CHURS/ALPINE; 0-0-30, potassium carbonate (Double-OK) NA-CHURS/ALPINE; 0-0-25-17, potassium thiosulfate (KTS) Tessenderlo KERLEY; 5-0-20-13, potassium sulfate (Trisert K+) Tessenderlo KERLEY; 0-0- 50, potassium sulfate; 0-0-62, potassium chloride (Kalium); and 13.8-0-46, potassium nitrate SQM North America.

Table 3. The effect of fertilizer additive on grain yield and gross margin of K sources in a weed-free environment and tank mixed with glyphosate in 2003 and 2004 with a high soil test K.^a

^aAbbreviations: DAS, diammonium sulfate; DAT, days after treatment.
^bK sources: 3-18-18, potassium phosphate, NA-CHURS/ALPINE; 0-0-30, potassium carbonate (Double-OK) NA-CHURS/ALPINE; 0-0-25-17, potassium thiosulfate (KTS) Tessenderlo KERLEY; 5-0-20-13, potassium sulfate (Trisert K+) Tessenderlo KERLEY; 0-0-50, potassium sulfate; 0-0-62, potassium chloride (Kalium); and 13.8-0-46, potassium nitrate SQM North America.
^cCalculated as the [(grain yield * market price) - (cost of foliar fertilizer + fertilizer application cost in absence of herbicide)]. Fertilizer cost estimated for 3-18-18 at $3.00/gallon, 0-0-30 at $2.75/gallon, 0-0-25-17 at $2.75/gallon, 5-0-20-13 at $4.25/gallon, 0-0-50 at $0.22/lb K₂O, 0-0-62 at $0.22/lb K₂O, 13.8-0-46 at $0.57/lb K₂O.
^dNo additive was included.