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Published online 27 April 2005
Published in Agron J 97:832-838 (2005)
DOI: 10.2134/agronj2004.0241
© 2005 American Society of Agronomy
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Production Papers

Response of No-Till Soybean [Glycine max (L.) Merr.] to Timing of Preplant and Foliar Potassium Applications in a Claypan Soil

Kelly A. Nelsona,*, Peter P. Motavallib and Manjula Nathanc

a Dep. of Agron., Univ. of Missouri, Novelty, MO 63460
b Dep. of Soil, Environ., and Atmos. Sci., Univ. of Missouri, Columbia, MO 65211
c Dep. of Agron., Univ. of Missouri, Columbia, MO 65211

* Corresponding author (nelsonke{at}missouri.edu)

Received for publication September 10, 2004.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Potassium availability in agronomic crops has recently decreased due to periodic drought conditions, soil compaction, reduced K applications, lower frequency of soil testing, and higher K fertilizer requirements because of increasing corn (Zea mays L.) yields and larger soybean [Glycine max (L.) Merr] acreage. Little research has evaluated the effects of foliar K applications on low to medium soil test K claypan soils, which are susceptible to drought and surface compaction. Field research was conducted in 2001 and 2002 to determine soybean response to foliar-applied K fertilizer compared with a preplant application and evaluate the cost-effectiveness of these treatments. Potassium fertilizer (K2SO4) was either broadcast-applied at 140, 280, and 560 kg K ha–1 as a preplant application or foliar-applied at 9, 18, and 36 kg K ha–1 at the V4, R1–R2, and R3–R4 stages of soybean development. Soybean grain yield increased 727 to 834 kg ha–1 when K was foliar-applied at 36 kg ha–1 at the V4 and R1–R2 stage of development in 2001 and 2002. Foliar-applied K at the R3–R4 stage of development increased grain yield but not as much as V4 or R1–R2 application timings. Treatment cost-effectiveness ranked preplant K at 280 kg ha–1 = preplant K at 140 kg ha–1 > preplant K at 560 kg ha–1 = V4 or R1–R2 foliar-applied at 36 kg ha–1. Foliar K did not substitute for preplant K in this research; however, foliar K may be a supplemental option when climatic and soil conditions reduce nutrient uptake from the soil.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
POTASSIUM IS ESSENTIAL for crop growth and physiological functions, including regulation of water and gas exchange in plants, protein synthesis, enzyme activation, photosynthesis, and carbohydrate translocation in plants (Marschner, 1998). Potassium-deficient plants often have slow growth, poor drought resistance, weak stems, and are more susceptible to plant disease (Mills and Jones, 1996; Sinclair, 1993). Although the greatest demand for K in most crop plants is in the early phase of rapid vegetative growth, plants also require adequate K during later vegetative and reproductive growth stages (Mills and Jones, 1996; Nelson, 1968). Potassium deficiency symptoms often appear under dry and compacted soil conditions or when root growth is impaired, which reduces diffusion of K to plant roots in the soil (Barber, 1984; Sardi and Fulop, 1994).

Claypan soils in the midwestern United States cover an area of approximately 4 million ha in Missouri, Illinois, and Kansas. These soils are characterized by the presence of a subsoil horizon with an abrupt increase in clay content within a short vertical distance in the soil profile (Anderson et al., 1990). In addition, these soils have a relatively low saturated hydraulic conductivity and water-holding capacity; therefore, these soils are more susceptible to frequent drought and flooding conditions (Blanco-Canqui et al., 2002) that may reduce soil K availability. When claypan soils are wet, they are more susceptible to surface compaction (Motavalli et al., 2003), which can reduce plant uptake of K. The variable depth to the high-clay subsoil horizon in claypan soils has also been shown to affect crop response to soil test K (Kitchen et al., 2001). Soil test results indicated additional K was required for optimizing yield in shallow compared with deep topsoil according to precision agriculture research (Kitchen et al., 2001). The relatively lower K availability in soils with shallow topsoil depths was attributed to lower fertility, reduced plant-available water capacity, and less air space for root growth in the high-clay subsoil horizon (Kitchen et al., 2001).

Several studies have evaluated soybean response to foliar fertilizer applied at early vegetative growth stages (Haq and Mallarino, 1998, 2000) or during late reproductive growth stages (Garcia and Hanway, 1976; Parker and Boswell, 1980; Vasilas et al., 1980). These studies have reported variable and inconsistent results for foliar fertilizer application. However, these studies evaluated mixed N, P, K, and sometimes S fertilizer sources (Garcia and Hanway, 1976; Haq and Mallarino, 1998; Parker and Boswell, 1980; Poole et al., 1983), and several studies were conducted under optimal soil test fertility levels (Haq and Mallarino, 1998, 2000; Parker and Boswell, 1980). Most of the reported responses to foliar fertilizer applications did not justify the application expense (Boote et al., 1978; Haq and Mallarino, 1998; Parker and Boswell, 1980; Poole et al., 1983).

The incidence of K deficiency in agronomic crops has increased in recent years in Missouri and other Midwestern states due to the effects of drought conditions and soil compaction on decreasing K availability, reduced amounts of applied K fertilizer, lower frequency of soil testing by producers due to low commodity prices, and higher K fertilizer requirements because of increasing corn yields and larger soybean acreage (Fixen, 2000; Reetz and Murrell, 1998). As a result of these factors, over 50% of the soils tested by the University of Missouri Soil Testing Laboratory during the fall of 2000 and the spring of 2001 had low to medium levels of soil test K (Fixen, 2002).

Postemergence application of foliar K fertilizer would have the potential advantage of increased flexibility for growers to respond to observed K deficiency due to the effects of variable soil properties, management practices, or climatic conditions. In addition, the increased area sprayed with postemergence glyphosate [N-(phosphonomethyl)glycine]-based herbicides in soybean may allow for a more economical method of applying foliar K. The objectives of this research were (i) to determine soybean response to foliar-applied K fertilizer applied at several growth stages compared with a preplant K fertilizer application and (ii) to evaluate the cost-effectiveness of these different timings and methods of K fertilizer applications for soybean growth in claypan soils.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Field research was conducted in a cooperator's field (40°02' N, 92°14' W) near the University of Missouri Greenley Research Center at Novelty, MO, in 2001 and 2002 at adjacent areas in the field each year. Figure 1 shows the daily and cumulative precipitation over the 2001 and 2002 growing seasons and the timing of K fertilizer applications. The site was a Mexico silt loam (fine, smectitic, mesic Aeric Vertic Epiaqualfs) that had been in continuous soybean. Initial soil test characteristics are presented in Table 1. The initial exchangeable soil test K levels were in the low to medium range based on University of Missouri soil test interpretations (Buchholz, 1992).



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  		Fig. 1. Daily and cumulative precipitation in 2001 and 2002. Arrows indicate timing of K fertilizer applications during the growing season and when soybean was harvested.

 

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  		Table 1. Selected initial soil properties before K treatments were applied in 2001 and 2002.{dagger}

 
The experiment was arranged as a randomized complete block design with four replications in plots 3 by 15.2 m. ‘Asgrow 3701’ soybean was no-till planted on 13 June 2001 and 2 June 2002 in 19-cm rows at 494000 seeds ha–1. Potassium fertilizer was either preplant broadcast-applied at 140, 280, and 560 kg K ha–1 (as K2SO4) or foliar-applied at 9, 18, and 36 kg K ha–1 (as K2SO4) at the V4, R1–R2, and R3–R4 stages of soybean development (Fehr and Caviness, 1977). Environmental conditions, leaf K, and soybean height at the time of application are in Table 2. The K fertilizer source selected was K2SO4 because it had a low salt index and minimal crop injury was expected (Rader et al., 1943). Application of MgSO4 was made at 15 kg ha–1 at V4, R1–R2, and R3–R4 stages of development as a foliar control. Foliar treatments were applied with a CO2–propelled hand-boom calibrated to deliver 39, 78, 156 L ha–1 for the 9, 18, and 36 kg K ha–1 rates, respectively, due to the solubility of K2SO4. The sprayer was calibrated at 124 kPa and equipped with 8003 flat-fan nozzles (Spraying Systems Co., Wheaton, IL) spaced 51 cm apart and 48 cm above the soybean canopy. The entire field was fertilized with 81 and 33 kg P ha–1 (as triple superphosphate) in 2001 and 2002, respectively, based on University of Missouri fertilizer recommendations for soybean (Buchholz, 1992). A burndown application of glyphosate (formulated as Roundup Ultra, Monsanto Co., St. Louis, MO) at 840 g a.e. ha–1 followed by two postemergence applications maintained a weed-free plot area throughout the season.


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  		Table 2. Environmental conditions, soil K, leaf K, and height at the time of application in 2001 and 2002.

 
Soybean injury from 0 (no visual crop injury) to 100% (complete crop death) was evaluated 3 and 7 d after treatment. Reproductive development of soybean was classified according to pod and seed development (Fehr and Caviness, 1977). Soil samples (0- to 15-cm depth) were collected on 12 July 2001, 19 June 2002, and 15 October 2002. Exchangeable soil test K was determined by extraction with 1 M NH4OAc, and K in the extractant was measured using atomic absorption/emission spectrometry (Warncke and Brown, 1998). A composite sample of the most recently mature trifoliolate from 20 plants in each plot was removed before the V4, R1–R2, and R3–R4 application timings, and K in the leaf was determined using atomic absorption/emission spectrometry after dry ashing and extraction with 6 M HCl (Jones et al., 1991). Soybean grain was harvested with a Massey 10 (Kincaid Equipment Manufacturing, Haven, KS) small-plot combine and moisture adjusted to 13%. Grain samples were collected, and K in the grain was determined with the same procedure used for leaf tissue analysis (Jones et al., 1991).

An economic analysis evaluated gross margins for the foliar treatments. The gross margin was calculated as the difference between the gross receipts and foliar K fertilizer plus application cost. Foliar fertilizer and application costs were estimated at $0.08 kg–1 and $12.30 ha–1, respectively, while dry fertilizer application cost was estimated at $9.88 ha–1 (Plain et al., 2000). Gross receipts were the product of crop grain yield and market price of $0.04 kg–1.

An analysis of variance was conducted using PROC ANOVA (SAS Inst., 1999). Data were combined over years when interactions were not observed. Grain yield was presented separately due to weather differences between years (Fig. 1). Individual treatment differences were determined using Fisher's Protected LSD (P ≤ 0.05). Linear and quadratic regression analysis was performed using best-fit analysis determined with SigmaPlot (Ver. 8.02, SPSS Inc., Chicago, IL). Significance was determined using PROC REG (SAS Inst., 1999).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
Salt injury is common with foliar fertilizer applications (Rader et al., 1943). However, no foliar crop injury was observed 3 or 7 d after the V4, R1–R2, and R3–R4 application timings of K2SO4 or the foliar control (MgSO4) during 2001 and 2002 (data not presented). Treatments were applied midday under high air temperature conditions (Table 2).

Soil test K increased 15, 23, and 43 mg kg–1 compared with the nontreated control 69 d after a preplant application of 140, 280, and 560 kg K ha–1, respectively (data not presented). Reproductive development (Table 3), height, grain K, leaf K (Fig. 2A), grain yield (Fig. 3A and 3B), and gross margins (Fig. 4) were higher when soil-applied K was compared with the untreated control for all K application rates. Foliar-applied MgSO4 did not affect soybean leaf K, reproductive development, grain K, or grain yield when compared with the nontreated control (data not presented). Garcia and Hanway (1976) reported that S was essential for a foliar fertilizer solution; however, this research indicated that S alone did not affect soybean grain yield when soils were low to medium in soil test K.


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  		Table 3. Effects of application method, time, and rate of K on selected soybean growth characteristics averaged over 2001 and 2002.

 


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  		Fig. 2. Leaf K at the R3 to R4 stage of development in response to (A) different preplant and foliar timings and rates of K fertilizer applications and (B) different rates of foliar K fertilizer applied at the V4 stage of growth over time in 2001 and 2002. Leaf samples for the R3 to R4 application timing were taken before foliar K fertilizer application. The vertical line represents the LSD at p ≤ 0.05.

 


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  		Fig. 3. Soybean grain yield response to preplant and foliar application timings and K fertilizer rates in (A) 2001 and (B) 2002. The vertical line represents the LSD at p ≤ 0.05.

 


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  		Fig. 4. Economic impact of different preplant and foliar timings and rates of K fertilizer applications on gross margins of soybean production. The gross margin was the difference between the gross receipts and foliar K fertilizer plus application cost. The vertical line represents the LSD at p ≤ 0.05.

 
Potassium is essential for soybean growth and reproductive development. All treatments increased soybean reproductive development when compared with the untreated control except K applied at the R3–R4 stage of development at 9 kg K ha–1 (Table 3). However, reproductive development for the foliar-applied K treatments was delayed compared with preplant K applications. Leaf K concentration was 0.11% higher at the V4 application timing at 36 kg ha–1 and 0.10 to 0.26% higher at the R1–R2 application timing at 18 and 36 kg ha–1 when compared with the untreated control (Fig. 2A). Similarly, Boote et al. (1978) reported that a foliar fertilizer application increased upper leaf K concentrations 34 to 44%; however, none of the foliar treatments had leaf K values similar to a preplant application. Leaf K concentration was highest 18 d after the V4 application timing for all rates (Fig. 2B). Haq and Mallarino (1998) reported no difference in K concentrations in leaves treated at the R2 stage of development when compared with untreated soybeans. Soybean plants were 7 to 16 cm taller than the untreated control when K was applied at the V4 or R1–R2 stage of development (Table 3). All foliar application treatments were shorter than preplant treatments except foliar K at 36 kg ha–1 applied at the V4 stage of development. Foliar-applied K at the V4 timing was probably utilized for vegetative growth that may have diluted the K levels in the upper leaves over time.

Soybean grain K for all foliar applications was similar to the untreated control except at the R3–R4 timing at 36 kg K ha–1 (Table 3). This late K application timing probably had increased K translocated to soybean seed, which contributed to the high seed K level. Preplant treatments at 140 and 280 kg K ha–1 had grain K levels greater than foliar applications except the R1–R2 and R3–R4 application timing at 36 kg K ha–1 while preplant K at 560 kg K ha–1 had greater grain K levels than all foliar applications.

Grain yields were generally higher in 2001 (Fig. 3A) compared with 2002 (Fig. 3B), which was probably due to a better rainfall distribution pattern throughout the growing season in 2001 (Fig. 1A and 1B). Soybean grain yields in both 2001 and 2002 were higher with preplant K compared with foliar-applied treatments. Foliar-applied K at 18 or 36 kg K ha–1 increased average grain yield 422 to 648 kg ha–1 across all foliar application dates when compared with the untreated control in 2001 (Fig. 3A). However, soybean yields were more responsive to foliar K applications from 9 to 36 kg K ha–1 under relatively drier conditions in 2002, increasing average grain yields across application timings from 563 to 720 kg ha–1 when compared with the untreated control (Fig. 3B). Soybean grain yield was maximized at a foliar rate of 36 kg K ha–1 applied at the V4 or R1–R2 stages of development in 2001 when drought-stressed conditions were minimal. A foliar K application may be more effective when applied from V4 to the R1–R2 stages of development to obtain optimal yields in years with good rainfall distribution. However, yield was increased at low K application rates when compared with the untreated control. Differences in soybean response to foliar K may be affected by climate since lower soil water content may reduce K uptake through the roots and thereby increase the relative crop response to foliar applications. Other research conducted in Iowa has indicated that foliar-applied N–P–K with K rates at 5.7 and 7.8 kg ha–1, at the R5 stage of development, and soil test K of 89 and 90 mg kg–1 had no significant effect on grain yield (Haq and Mallarino, 1998) although average grain yield increased 171 to 276 kg ha–1 (Haq and Mallarino, 2000). Initial soil test K was lower at 71 to 74 mg kg–1 (Table 1), and foliar K fertilizer application rates were higher in this research.

The cost-effectiveness of treatments evaluated in this study ranked preplant K at 280 kg ha–1 = preplant K at 140 kg ha–1 > preplant K at 560 kg ha–1 = foliar K applied at the V4 and R1–R2 at 36 kg ha–1 followed by additional foliar treatments and the untreated control (Fig. 4). All treatments except the V4 timing at 9 kg ha–1 and R3–R4 timing at 9, 18, and 36 kg ha–1 increased gross margins when compared with the untreated control.

Although optimal soybean grain yield and gross margin were obtained with a preplant K fertilizer application, foliar K applications may be a management tool to mitigate K deficiency during the growing season and minimize yield loss. In addition, foliar K may be a supplemental nutrient management practice when conditions reduce plant K uptake from soil. These research results indicate that early foliar applications (V4–R2) at high rates on a low to medium soil test K field were more effective in increasing grain yield compared with a late (R3–R4) foliar application. Carrier volumes required for foliar application of K2SO4 at rates shown to be effective in this research are generally impractical for most farm operations, and additional research needs to evaluate crop response from more soluble K fertilizer sources with relatively low salt indexes. With increasing cropland planted with glyphosate-resistant soybean, additional research may need to determine whether foliar K applications may be compatible with postemergence weed management systems that may allow foliar K applications to be more cost-effective.


   ACKNOWLEDGMENTS
 
This research was partially supported by the Potash and Phosphate Institute. We greatly appreciate the technical support provided by Randall Smoot, Dana Harder, and Matthew Jones of the Greenley Research Center and additional assistance provided by Josh IntVeld, James Waddell, and John Dodds.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 





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