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Dep. of Agronomy, Purdue Univ., West Lafayette, IN 47907-1150
* Corresponding author (tvyn{at}purdue.edu)
Received for publication October 10, 2001.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: FD (38), fall disk with 38-cm row width • NT (38), no-till with 38-cm row width • NT (76), no-till with 76-cm row width • OCHU, Ontario Crop Heat Units • ZT (76), fall zone-till with 76-cm row width
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Vertical soil K stratification in no-till causes plant K uptake to be more dependent on soil K and root system characteristics in the surface layer. This may reduce plant K uptake, and thus increase the likelihood of K deficiency in crop tissues as well as yield loss in growing seasons when drought occurs, since soil K availability and root growth and activity in the surface layer are more vulnerable to drought than those in subsurface layers. In addition, crop residue deposit at the soil surface in no-till usually results in higher soil moisture and lower soil temperature in the surface layer, which may reduce soil K availability and restrict root growth early in the season (Barber, 1971; Fortin, 1993). The risks of reduction in plant K uptake by drought or low temperature become severe when soil K concentrations in subsurface layers are too low to optimize plant K uptake. Subsurface placement of K fertilizer, therefore, may improve applied K availability and reduce soil K stratification in no-till systems. Because the land area of no-till soybean in North America has increased rapidly since the late 1980s, new concerns have been raised about the effectiveness of applying the traditional K management systems designed for tilled soybean to soybean in no-till systems.
Information about K fertilizer management is limited for soybean production in no-till systems. Recent investigations in Iowa (Borges and Mallarino, 2000; Buah et al., 2000) demonstrated that no-till soybean yield response to K fertilization was rarely significant on optimum to very high-testing soils. Research in Illinois (Vasilas et al., 1988) also reported that no-till management did not affect K fertilizer recommendations for soybean compared with moldboard plow.
Subsurface placement of K fertilizer seems to be superior to surface broadcasting for no-till soybean on low-testing soils. Hairston et al. (1990) showed that deep banding (15-cm depth) of K fertilizer resulted in significantly higher yield of no-till soybean than surface broadcasting of K on some Mississippi soils with low K levels. Borges and Mallarino (2000) reported that both deep-banded and planter-banded K fertilizer in no-till produced slightly higher soybean yield than surface application on optimum- to very high-testing soils, and that positive yield response to banding was not related to soil-test K levels or degree of soil K stratification. However, research in Ohio (Hudak et al., 1989) demonstrated that K fertilizer placement did not affect no-till soybean yield on medium K soils. In addition, band placement of K fertilizer as a starter alone (5 cm from the row and 5 cm below the seed at planting) has not been more beneficial to soybean yield than surface-broadcast K when initial soil-test K levels were medium or higher (Buah et al., 2000; Ebelhar and Varsa, 2000).
Soybean responses to conservation tillage have been reported to vary greatly, depending on soil, weather, and various other factors (Hairston et al., 1990; Unger and McCalla, 1980; Vyn et al., 2000). Yield of no-till soybean has generally been comparable to, or even higher than, yield of moldboard-plowed soybean on coarse- and medium-textured soils; however, lower yield with no-till has frequently been observed on fine-textured, poorly drained soils (Dick and VanDoren, 1985; Vyn et al., 1998). Both fall zone-till and fall disk systems are conservation tillage alternatives to no-till that allow some degree of tillage to improve seed zone fitness, but still have soil and water conservation functions (Vyn et al., 1998). In addition, both fall zone-till and fall disk systems enable some K placement alternatives relative to surface broadcasting in no-till. Zone-till involves equipment capable of banding K fertilizer to deep depths during tillage. Fall disk could incorporate broadcast-applied K fertilizer into soil in a manner similar to moldboard plow systems, although not to the same depth as the traditional moldboard plow. The effectiveness of these K placement and tillage alternatives on soybean is largely unknown relative to surface broadcasting in no-till.
Soybean yield often increases as row width decreases in the eastern and central regions of North American soybean production due to the improved efficiency of solar interception, weed control, and (or) moisture conservation (Ablett et al., 1991; Johnson, 1987; Oplinger and Philbrook, 1992). The higher yield associated with narrow-row soybean could increase K removal from soil and thus enhance K fertilizer rate recommendations. However, little information is available on the differences in narrow-row and wide-row soybean responses to K fertilizer application since row width was not a factor in most previous K fertility studies involving soybean.
The objectives of this research were to (i) evaluate soybean responses to K placement and conservation tillage alternatives on continuous no-till fields with evident soil K stratification, (ii) examine the influence of soil K stratification on plant K nutrition and seed yield of no-till soybean, and (iii) compare wide-row zone-till (or no-till) production systems reliant on deep-banded K and a narrow-row, fall disk system dependent on surface-applied but tillage-incorporated K with a narrow-row, no-till system reliant on surface broadcasting of K fertilizer.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Soybean was planted on 26 May in 1998, 19 May in 1999, and 30 May in 2000 at Kirkton and on 23 May in 1998, 4 June in 1999, and 1 June in 2000 at Strathroy. Soybean rows were positioned directly on top of the fertilizer bands in all three tillage systems in row widths of either 76 or 38 cm. Soybean ‘OAC Bayfield’ was used in 1998, and ‘First Line (FL) 2801R’ was grown in 1999 and 2000 at Kirkton. Soybean ‘NK S19-90’ was planted in 1998 and 'NK S08-80' in 1999 and 2000 at Strathroy. Maturity groups (MG) of these cultivars commonly grown in southwestern Ontario were presented as follows: OAC Bayfield, MG 0.4; FL 2801R, MG 0.8; NK S08-80, MG 0.8; and NK S19-90, MG 1.9. The target seeding rate was 400 000 seeds ha-1 in both row widths. Average soybean population for each treatment was greater than 240 000 plants ha-1 (data not presented).
Composite soil samples (10 cores per sample, 2.5 cm in diameter) were collected at four depth intervals (0–5, 5–10, 10–20, and 20–30 cm) randomly from each plot in fall before treatments were imposed in each season at both locations. After soil samples were air dried, ground to pass through a 2-mm sieve, and thoroughly mixed, 1.0 mL soil was measured by a scoop and placed into a 50-mL flask, and 10 mL of 1 M ammonium acetate (NH4OAc) solution buffered at pH of 7.0 was added, the resulting solution was shaken for 15 min and then filtered (Bates and Richards, 1993). Potassium in the filtrates was determined using atomic absorption spectroscopy. In this study, Ontario soil-test K interpretations for samples at 0–15 cm depth were used. Boundaries of soil-test K in low, medium, high, very high, and excessive categories for soybean are <61, 61 to 120, 121 to 150, 151 to 250, and >250 mg L-1 (milligrams of K per liter of soil), respectively (Ontario Ministry of Agric., Food, and Rural Affairs, 1997). Yield responses of soybean to K fertilization are expected on soils with low to medium K levels.
A soybean biomass sample was collected at initial flowering stage (R1) in mid- to late July (16 July in 1999 and 25 July in 2000 at Kirkton, 23 July in 1999 and 24 July in 2000 at Strathroy) from each plot at both locations (Fehr et al., 1971). No such sampling was conducted in 1998 because of resource limitations. Each sample consisted of all aboveground parts from a total of 2.0-m length of soybean row per plot. Biomass samples were oven dried at 65°C for at least 3 d to achieve constant moisture before weighing.
A leaf sample consisting of 20 most recently fully developed trifoliate leaves including petiole was taken from 20 plants at the initial flowering stage (R1) in mid- to late July (27 July in 1998, 16 July in 1999, and 25 July in 2000 at Kirkton; 27 July in 1998, 23 July in 1999, and 24 July in 2000 at Strathroy) from each plot in each season for the determination of tissue K concentrations. Leaf samples were dried in a forced-air oven at 65°C and ground to pass through a 1-mm sieve. Leaf K concentrations were analyzed using a dry ash method (Miller, 1998).
Soybean seed yield was determined by harvesting a 1.0-m width (two rows in the 76-cm row width, and three rows in the 38-cm row width) at the center of each plot for the entire plot length with a plot combine and adjusting to 130 g kg-1 moisture content. Daily rainfall and air temperature data were recorded at the experimental site or collected from the nearest weather station during the growing season in each season at both locations.
The resulting data were analyzed using a general linear model appropriate for a randomized complete block design. The K treatment sums of squares were partitioned into orthogonal contrasts in each tillage and row width production system (the mean of all three K-fertilized treatments vs. zero K, the average of fall band and spring band vs. surface broadcast, and fall band vs. spring band in ZT (76) and NT (76); the mean of spring band and surface broadcast vs. zero K, and spring band vs. surface broadcast with FD (38); and fall band vs. zero K in NT (76)). To compare ZT (76) with NT (76), FD (38) with NT (38), and 76-cm row width with 38-cm row width, nonorthogonal contrasts were conducted over means of treatment combinations. Soil K stratification coefficient (defined as the quotient of soil K concentrations in the 0- to 5-cm layer divided by K levels at the 10- to 20-cm depth) was used to describe the degree of vertical soil K stratification. Pearson product-moment correlation coefficients were calculated to examine the relationships of leaf K concentrations at the initial flowering stage with initial soil K levels and K stratification coefficients based on the 16 zero K plots in each season at both locations.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The frequent increases in leaf K concentrations on medium K soils in this study was in agreement with a previous study (Hudak et al., 1989), which showed that leaf K concentrations of soybean during the early to mid-flowering stage could be greatly increased by both deep-banded and surface-applied K in various tillage systems on medium-testing soils. However, soybean leaf K concentrations are generally not increased by K fertilizer application on high- to very high-testing soils (Buah et al., 2000; Rehm, 1995).
According to the recommended adequate K concentrations for soybean ranging from 17.1 to 25.0 g kg-1 in trofoliate leaves during the initial flowering stage (Small and Ohlrogge, 1973), leaf K concentrations in both K-fertilized and zero K plots in this study were within or above the listed sufficiency range in all seasons except the 1999 season at Strathroy. Soybean from zero K plots in the latter experiment had leaf K levels below the adequate range. Leaf K concentrations from the K-fertilized treatments at Kirkton in 1999 and 2000, and at Strathroy in 1998 and 2000 were even equal to or higher than 25.0 g kg-1 (the upper limit of the sufficiency range), indicating that plant K nutrition had been adequate in these four site-years at this growth stage.
The results at Kirkton showed that soil-test K levels, although stratified and in the medium range, were sufficient to meet the K requirements of soybean. The increases in leaf K concentrations with K application were much greater in 1999 than in 1998 and 2000 at both locations. The high leaf K concentrations (i.e., >24.0 g kg-1) from all zero K plots at Strathroy in 2000 were unexpected, since the initial soil-test K (0–15 cm depth) was only 96 mg L-1 (Table 1). The possible explanation was that the rainfall in June 2000 was three times higher than normal, which may have greatly increased near-surface root proliferation and soil K availability.
Tillage Effects
At Kirkton, ZT (76) did not significantly increase leaf K concentrations compared with NT (76) averaged over fall band and zero K in any of the three seasons (Table 4). Fall disking resulted in similar or even lower leaf K concentrations relative to NT (38) in all three seasons where similar K treatment was combined. Similar tendencies were observed at Strathroy (Table 4). All these results demonstrated that the availability of either soil K or applied K was not improved by fall zone-till, even though it loosened soil in the row area, reduced soil K stratification, and caused some incorporation of surface-applied K in the tilled strips (Yin and Vyn, 2002). Shifting from no-till to fall disk also did not significantly enhance the availability of soil K or surface-applied K.
Row Width Effects
When soybean was NT planted, leaf K concentrations were not affected by row width at either Kirkton or Strathroy when the data were averaged over fall band and zero K treatments (Table 4). This may be due, in part, to similar plant populations in 76- and 38-cm row widths, or the fact that root proliferation (not measured in this study) in zones of higher soil-test K availability was not substantially affected by row width. Leaf K concentrations were similar despite the fact that midseason biomass yield of no-till soybean on an area basis was approximately 50% higher in 38-cm row widths than those in 76-cm rows (data not presented). At the same seeding rate, narrow rows did not result in higher leaf K concentrations just because of more uniform root distribution compared with wide rows on the medium to very high K soils in our experiment.
Seed Yield
Potassium Effects
Significant yield increases ranging from 5 to 13% were observed in response to K application at Strathroy in NT (38) in 1999 and in ZT (76) and NT (76) in 2000 (Table 5). Yield responses to K were not observed at Strathroy in 1998 or at Kirkton in any of the three seasons. Yield responses, or lack of them, appeared to have little association with initial soil-test levels at both locations. However, leaf K concentrations at the initial flowering stage were indicative of soybean yield responses to K application at Kirkton in all three seasons, and at Strathroy in 1999. Increases in leaf K concentrations at the initial flowering stage in response to K fertilization did not guarantee final seed yield increases.
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It is evident in this study that subsurface banding of K fertilizer (averaged over fall banding and spring banding) often resulted in similar yield to surface broadcast in all tillage systems. Buah et al. (2000) concluded that broadcast application was equally effective to starter-band placement on medium to high-K soils. However, Hudak et al. (1989) reported that banded K resulted in higher seed yield than surface broadcast at low rates of K fertilizer on medium-testing soils. More apparent yield benefits associated with banding in our research might have resulted if we had applied lower K fertilizer rates.
Tillage Effects
At Kirkton, soybean yield in ZT (76) was never higher than in NT (76) in any season when results in fall band and zero K treatments were combined (Table 5). Fall disking resulted in similar or even lower yield compared with NT (38) when the results were averaged over spring band, surface broadcast, and zero K. No significant yield differences were observed between ZT (76) and NT (76) or between FD (38) and NT (38) in any season at Strathroy.
Row Width Effects
At Kirkton, the yield of no-till soybean planted in 38-cm rows was 17 and 38% greater in 1998 and 2000, respectively, than those in 76-cm rows when averaged across similar K treatments (Table 5). At Strathroy, soybean in 38-cm row widths had similar yield as those in 76-cm rows in 1998, but resulted in significant yield increases of 17% in 1999 and 7% in 2000, relative to those in 76-cm rows (Table 5). The yield responses to narrow row width were frequently associated with the increased midseason biomass yield (data not presented). Positive soybean yield responses to narrow row width have often been observed compared with wide-row width, particularly in high yielding environments (Bullock et al., 1998; Devlin et al., 1995).
Comparison of Production Systems
One of the typical soybean production systems currently used in Ontario is the narrow-row no-till system in which K fertilizer is surface broadcast applied. The main disadvantage with this system is the vertical soil K stratification. Three possible alternate production systems to overcome soil K stratification in no-till are wide-row zone-till with banded K, narrow-row fall disk with surface-applied but tillage-incorporated K, and wide-row no-till with banded K. Our results showed that none of the three production systems significantly increased soybean biomass (data not presented), leaf K concentrations, or seed yield compared with the narrow-row no-till system at either location (Tables 4 and 5). This suggests that leaf K concentrations did not benefit from the soil mixing with zone-till and the concurrent change from surface broadcasting of K fertilizer to deep banding on fields with medium and high K levels. Therefore, the zone-till production system probably did not enhance either applied K or soil K availability. Overall, there was no yield benefit associated with the wide-row zone-till system compared with narrow-row no-till, even though zone-till offers opportunities to place K fertilizer in deep bands. Our research also indicates that disking soil in fall did not increase either applied K or soil K availability, although it loosened the soil to 10 cm deep and incorporated K fertilizer into soil.
It is clear that soybean growers who are currently surface broadcasting K fertilizer in a narrow-row no-till system, have no need to switch to wide-row zone-till or no-till systems to take advantage of a deep banding opportunity for K fertilizer, or to shift to a narrow-row fall disk system to achieve incorporation of surface-applied K on medium to high K soils. Thus, broadcasting is not only simple, but also appropriate when K fertilizer is not incorporated under continuous no-till production as long as soil K fertility is generally maintained at the medium or higher levels.
Correlations of Leaf Potassium with Soil Potassium Concentrations and Stratification
Leaf K concentrations were not significantly correlated with initial soil-test K levels (0–20 cm depth) in five of six site-years (Table 6). This indicates that soil K concentrations seemed not to be a limiting factor to plant K nutrition at these nonresponsive sites. Contrary to expectations, no significant negative correlations between leaf K concentrations and soil K stratification coefficients were observed in any season at either location (Table 6). Indeed, a significant positive correlation between leaf K concentrations and soil K stratification coefficients was observed at Strathroy in 1998, which was the only site-year with very high soil K levels in this study. Thus, the degree of soil K stratification was never detrimental to plant K nutrition at the initial flowering stage on these medium- to high-testing soils.
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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