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SOYBEAN

Soybean Yield Response to Reproductive Stage Soil-Applied Nitrogen and Foliar-Applied Boron

^a Virginia Polytechnic Inst. and State Univ., Tidewater Agric. Res. and Ext. Cent., 6321 Holland Rd., Suffolk, VA 23437
^b Dep. of Plant Pathology, Physiol., and Weed Sci., Virginia Polytechnic Inst. and State Univ., Blacksburg, VA 24061

* Corresponding author (dholshou{at}vt.edu)

Received for publication October 2, 2000.

	ABSTRACT

TOP ABSTRACT INTRODUCTION Nitrogen Boron Nitrogen + Boron MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Research on the effects of N and B applications on soybean [Glycine max (L.) Merr.] yield is limited. Experiments were conducted to determine the effect of (i) application rate and reproductive stage timing of N or B on soybean seed yield and (ii) cultivar, row spacing, or planting date on the response of soybean to R3-stage N and B applications. Nitrogen was applied to the soil at 0, 14, 28, 56, 84, 112, or 168 kg ha^-1, or B was applied to the foliage at 0, 0.14, 0.28, or 0.56 kg ha^-1 to either R3- or R5-stage soybean in the rate and timing experiments. Treatments for the cultivar, row-spacing, and planting-date experiments included 0 + 0, 56 + 0, 0 + 0.28, and 56 + 0.28 kg ha^-1 N + B, respectively. In yield environments ranging from 2400 to 5300 kg ha^-1, application of N or B did not increase seed yield at any rate or application stage, nor did cultivar, row spacing, or planting date alter this lack of response. Analysis of leaf tissue taken at the R2 soybean development stage and before nutrient application indicated that N and B concentrations were above the minimum level required by soybean for maximum yields not limited by N or B. Lack of response to supplemental N or B suggested that N supplied via fixation and soil organic matter mineralization and native levels of B in soils are adequate for high yields in the Mid-Atlantic Coastal Plain soybean production region.

Abbreviations: UAN, urea ammonium nitrate

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Nitrogen Boron Nitrogen + Boron MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
SOYBEAN PLANTS require 16 nutrients for plant growth and seed production (Mengel et al., 1987). However, as average yields increase, the nutrient levels that were considered adequate for lower yields may be limiting plant growth and optimum seed yields. Macronutrient research has shown that supplemental applications of N have increased seed yield in some studies (Wesley et al., 1998; Purcell and King, 1996; Syverud et al., 1980; Garcia and Hanway, 1976). Similarly, B applications have been reported to improve soybean seed yield (Reinbott and Blevins, 1995; Schon and Blevins, 1990; Touchton and Boswell, 1975; Gascho and McPherson, 1997). Although these elements have been studied in other regions, data are not available for the Mid-Atlantic USA region. Also, little data are available for combined applications of these elements.

	Nitrogen

TOP ABSTRACT INTRODUCTION Nitrogen Boron Nitrogen + Boron MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Soybean demand for N can exceed 92 g kg^-1 seed for optimum seed yield (Flannery, 1986). Soybean utilizes N from several sources, including mineralized soil organic matter, symbiotically fixed N, and N incorporated into plant tissue. Demand for seed N is highest from the R5 to R8 soybean development stage as defined by Fehr and Caviness (1977). During this period, the plant utilizes N from all sources, but in the early to mid-pod fill stages, fixation by Bradyrhizobia spp. decreases rapidly (Harper, 1987). The soybean plant compensates for this reduction in fixed N by utilizing N already incorporated in plant tissue, beginning in the R6 growth stage (Harper, 1987). As N is remobilized from older plant tissue to the developing seeds, senescence of plant tissue begins. During this period, it is possible that under certain climatic conditions such as high-yield environments, N supply could be limiting optimum seed production. Supplying N to the soybean plant during the time of peak seed demand may supplement existing N resources, prevent premature senescence, and increase seed yield (Garcia and Hanway, 1976; Nelson et al., 1984; Salado-Navarro et al., 1985; Sinclair and DeWhit, 1976).

Field studies measuring soybean response to applied N have been conducted by several researchers. Preplant-applied N at 134 kg ha^-1 on a silt loam soil in Nebraska to irrigated soybean did not consistently alter yield (Slater et al., 1991). Similarly, on a silty clay loam in Illinois, N applications from 0 to 900 kg ha^-1 yr^-1 applied preplant and as a sidedress elicited no yield effect, except for slight yield reductions at the higher N rates (Welch et al., 1973). Two sites in Michigan generated variable results. On a Zilwaukee clay that had never produced soybean, no yield response was reported from a preplant N application of 134 kg ha^-1 while on a sandy loam, yield increases averaged 460 kg ha^-1 over the control plots (Hesterman and Isleib, 1991). Although the seeds were inoculated with Bradyrhizobia, soybean had never been produced on the land used in these experiments. Greenhouse and field studies by Yoshida (1979) reported yield increases with N applications. The greenhouse study utilized five N rates from 15 to 150 mg kg^-1 NO^-₃–N in complete hydroponic solutions and a control containing no N. Total seed yield increased from 0.9 g with the control to 81.6 g per pot with 50 mg kg^-1 N while the highest N rate, 150 mg kg^-1, yielded 76.7 g per pot. In the field study, N applications from 30 to 90 kg ha^-1 applied from seedling emergence to R1 increased yields by 720 kg ha^-1, averaged across all treatments. In field experiments, soybean was inoculated with Bradyrhizobia spp. bacteria.

However, N applications made before reproductive growth stages are reported to decrease Bradyrhizobia spp. activity, exhibited by reduced growth of nodules and lower N fixation, thus further increasing the difference between N supply and demand (Yoneyama et al., 1985; Bhangoo and Albritton, 1976; Ham et al., 1975; Yoshida, 1979). Laboratory research has demonstrated that as N applications increase, nodule fresh weight and leghemoglobin content of nodules decrease compared with a control, suggesting a lowered amount of N fixation (Harper and Cooper, 1971).

One solution for supplying needed N for plant growth without impeding the development of Bradyrhizobium spp. is N applications during reproductive stages of plant growth. Nitrogen applied to the foliage of R5 soybean at rates of 45, 90, and 135 kg ha^-1 increased yield over the control of 2640 kg ha^-1 by 123, 160, and 243 kg ha^-1, respectively (Syverud et al., 1980). Garcia and Hanway (1976) reported yield increases from foliar applications of various nutrient solutions. Yield increases of 130 and 220 kg ha^-1 over the control yields of 2290 and 2270 kg ha^-1, respectively, were observed when a total of 34 kg N ha^-1 was applied in two applications at the R4 and R5 stages of development. In a series of experiments on irrigated soybean conducted at eight sites over a 2-yr period, foliar N application rates of 22 and 44 kg ha^-1 increased yield at six of the eight sites for an average increase of 464 kg ha^-1, or 11.8% (Wesley et al., 1998). At the two sites that were nonresponsive, soybean yields averaged <3360 kg ha^-1, and the authors suggested that responses from N might only be realized under high yield potentials.

Because the above research investigated foliar applications, it could be assumed that N entered the plant through the leaves and, to a lesser extent, through the roots. However, yield benefits of up to 148 kg ha^-1 over the control of 2856 kg ha^-1 have been reported in Kentucky with soil applications (Judy and Murdock, 1998). In this work, liquid urea ammonium nitrate (UAN) was dribbled beside the rows between the R2 and R3 development stages at rates of 29 and 36 kg ha^-1 in 1996 and 1997, respectively. Therefore, positive results from reproductive-stage N applications have also been obtained when the N enters the plant solely through the roots.

Supplemental N has been thought to be beneficial only in years of adequate moisture (Keogh et al., 1979) or under irrigation at high yield potentials (Wesley et al., 1998). However, experiments have demonstrated that supplemental N will increase yields in plants under drought stress. When water stress occurs during the critical time of seed filling, C and N can be remobilized from plant leaf tissue and translocated to the seeds, resulting in faster declines in photosynthetic rates (De Souza et al., 1997). This leads to premature leaf senescence and lowered yields through reduced seed size and seed number. Early research from Illinois indicated an average yield increase of 485 kg ha^-1 over the control yield of 817 kg ha^-1 (Lyons and Earley, 1952). In this nonirrigated experiment in a dry year, ammonium nitrate (NH₄NO₃) was sidedressed and incorporated at the midbloom stage 42 d after emergence using seven N rates ranging from 0 to 672 kg ha^-1. In Arkansas, yield increases on a silt loam soil occurred where nonirrigated soybean received a broadcast application of 224 kg N ha^-1 at V6 and an additional 112 kg N ha^-1 at R2. Yield for nonirrigated soybean without N application was 2373 kg ha^-1 while yield for the nonirrigated soybean with N applications was 2798 kg ha^-1. The yield for the irrigated soybean that received no N application was 2728 kg ha^-1, and the difference between the nonirrigated soybean receiving N and the irrigated soybean not receiving N were not significantly different (Purcell and King, 1996). These researchers concluded that the application of N fertilizer alleviated the N deficiency due to poor N fixation by Bradyrhizobium spp., which was caused by low soil moisture. In an additional greenhouse experiment, Purcell and King (1996) also observed that through supplemental N fertilization, the effects of drought were partially reversed compared with plants that received full water and no additional N because the N application compensated for the low N fixation of the Bradyrhizobium spp. Therefore, supplemental N applications during reproductive stages may offer yield-increasing benefits during times of inadequate moisture as well as under optimum conditions.

	Boron

TOP ABSTRACT INTRODUCTION Nitrogen Boron Nitrogen + Boron MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Similar to the yield benefits from reproductive-stage N applications, yield enhancements have been reported from B applied during reproductive stages. Boron's widespread role within the plant includes cell wall synthesis, sugar transport, cell division, differentiation, membrane functioning, root elongation, and regulation of plant hormone levels (Marschner, 1995; Romheld and Marschner, 1991; Pilbeam and Kirkby, 1983). Boron is also recognized as one of the most commonly deficient micronutrients in agriculture, with reports of deficiencies in 132 crops in 80 countries (Shorrocks, 1997). Deficiencies typically result from B leaching in humid areas with coarse-textured soils (Mortvedt and Woodruff, 1993; Marschner, 1995; Welch et al., 1991).

Although the importance of B to crop growth is not in question, the need for B fertilization of soybean is unclear. Soil-applied B on a silt loam in Missouri at rates of 0.0, 0.28, 0.56, and 1.12 kg ha^-1 generated no significant differences in soybean yield or soybean yield components (Schon and Blevins, 1990). Likewise, there were no positive yield effects observed on a silt loam in Missouri with split foliar applications of B at rates of either 0.56 or 1.12 kg ha^-1 applied at R2 and R3 (Reinbott and Blevins, 1995). Similarly, B applied at rates up to 3.3 kg ha^-1 on a clay loam and a fine sandy loam in Virginia had no effect on soybean yield over 6 yr (Martens et al., 1974). In Georgia, no significant yield effects were observed with soil-applied B at rates up to 1.12 kg ha^-1 on a sandy loam, and yield reductions of 10% were observed at one site when a B rate of 2.24 kg ha^-1 was used (Touchton and Boswell, 1975).

In contrast to the lack of response in the above studies, Gascho and McPherson (1997) reported yield benefits from foliar B applications over the control yield on irrigated Bonifay sand in Georgia. An R3 application of 0.28 kg B ha^-1 generated yields averaging 353 kg ha^-1 higher than the control yield of 3247 kg ha^-1, averaged over five cultivars at the same site. In this study, three out of five cultivars showed significant response to B applications, leading the authors to believe that yield response to B may depend on cultivar. Other researchers from Georgia reported that B mixed with diflubenzuron [1-(4-chlorophenyl) 3-(2,6-difluorobenzoyl)urea] insecticide applied at R2 or R3 increased yield by an average of 23% at four sites (Hudson and Clarke, 1997). The yield level for the untreated check averaged 2580 kg ha^-1 while the plots treated with B + diflubenzuron yielded 3185 kg ha^-1. These yield responses were attributed to both B fertilization and less insect predation. Touchton and Boswell (1975) observed a 4% yield increase over control plots on a loamy sand testing low in hot water soluble B (0.14 mg kg^-1) when B was applied at 0.28, 0.56, and 1.12 kg ha^-1 as two split applications, each at one-half the treatment rate, with the first at R1 and the second application made 7 d later.

Direct infusions of soybean plants growing on a Mexico silt loam soil with supplemental B, using boric acid (H₃BO₃) as the source, have caused increases of 84.8 and 17.6% in the total number of branch pods per plant and the total seed weight per plant, respectively (Schon and Blevins, 1987). Seed yield in this experiment corresponded to 4170 kg ha^-1 for B-injected plants and 3540 kg ha^-1 for the control plants. In another experiment, six split applications from R1 through R8 increased both the number of pods per branch and the number of branches per plant (Schon and Blevins, 1990). Similarly, two foliar applications at R4 and R5 caused a yield increase of 356 kg ha^-1 on a Mexico silt loam in Missouri (Reinbott and Blevins, 1995).

Boron applied to the soil at 2.8 kg ha^-1 in a silty clay loam produced soybean yield increases of 11 and 13%, respectively, in the first 2 yr, with no effect in the third year after application (Reinbott and Blevins, 1995). These increases corresponded to yields of 1931 and 1934 kg ha^-1 compared with the control yields of 1736 and 1687 kg ha^-1 for Years 1 and 2, respectively. In these studies, soil-applied B increased pods per branch by 17% and number of pods per plant by 39%. A late planting date in the third year possibly contributed to the lack of response. Broadcast applications of B at 0.28 to 1.12 kg ha^-1 at three sites in Georgia generated variable results, but soybean yield was increased by 4% on a loamy sand soil with low levels of soil test B (0.14 mg kg^-1 hot water soluble B) (Touchton and Boswell, 1975). Research in Arkansas on a silt loam soil reported yield increases up to 538 kg ha^-1 over the control yield of 2861 kg ha^-1 with soil-applied B during early flowering at a rate of 3 kg ha^-1 (Al-Molla, 1985). At another site on a silt loam soil, Al-Molla (1985) reported a yield increase of 569 kg ha^-1 over the control yield of 2257 kg ha^-1 with a B application of 4 kg ha^-1 made to the soil at early flowering.

	Nitrogen + Boron

TOP ABSTRACT INTRODUCTION Nitrogen Boron Nitrogen + Boron MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Little research has been conducted with both N and B applications to soybean, except for that by Gascho (1992) on a loamy sand soil. This research utilized main plots of N, B, or N + B applied either via fertigation with a center-pivot system, spray applications utilizing a tractor, or dribble applications using a watering can. The B rate utilized was 0.45 kg ha^-1, and the N rate was 45 kg ha^-1. Subplots in the experiment consisted of five different soybean cultivars. While no significant differences were found when all treatments and cultivars were averaged together, significant treatment effects on yields did exist when cultivars were examined individually. With one cultivar, fertigation applications of N + B at R5 generated total dry matter yields 5978 kg ha^-1 higher than control yields of 9149 kg ha^-1. For another cultivar, spray applications of N and B at R5 increased seed yield by 470 kg ha^-1 over the control yield of 2889 kg ha^-1.

These variable results demonstrate the need for further research on N and B applications to soybean. Proper application timing, rate of application, and the conditions necessary for yield responses have not been fully determined for the Mid-Atlantic USA. Also, more research is needed on the effect of N and B combinations. The objectives of this research were to determine the effect of (i) application rate and reproductive-stage timing of N or B on soybean seed yield and (ii) cultivar, row spacing, or planting date on the response of soybean to R3-stage N and B applications.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION Nitrogen Boron Nitrogen + Boron MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Nitrogen and Boron Application Rate and Timing
Deltapine brand soybean cultivar DP 3478 was planted at a population of 263000 and 272 000 plants ha^-1 on 23 May 1997 and 17 May 1998, respectively, on a Nansemond fine sandy loam (coarse-loamy, siliceous, thermic, Aquic Hapludult) in Suffolk, VA. In 1999, soybean was planted on 14 June at a population of 432250 plants ha^-1 on a State fine sandy loam (fine loamy, mixed, semiactive, thermic Typic Hapludult) near Mt. Holly, VA, following barley (Hordeum vulgare L.) harvest. The higher plant population at the Mt. Holly location follows Virginia Cooperative Extension recommendations for late planting after small-grain harvest (Holshouser, 1998). In all years, soybean seed was inoculated with Bradyrhizobia spp. bacteria within 24 h before planting. Plot size was four rows 46 cm wide by 7.3 m long. To minimize moisture stress, experiments were irrigated using subsurface microdrip irrigation in 1997 and 1998 and an overhead center-pivot system in 1999. Cotton (Gossypium hirsutum L.) was grown the previous years in the 1997 and 1998 studies, and corn (Zea mays L.) was grown in the year before the 1999 study. Both cotton and corn crops were fertilized with recommended rates of N and other nutrients. In all years, soil samples were taken at planting from depths of 0 to 15, 15 to 31, 31 to 61, and 61 to 91 cm and tested for available nutrients (Table 1). Plant tissue samples were taken randomly from the uppermost fully expanded leaves at the R2 soybean development stage and analyzed for nutrient content. In 1997, both phosphoric oxide (P₂O₅) and potassium oxide (K₂O) were applied at 45 kg ha^-1 before planting. No fertilizer was applied in 1998 because nutrient levels were adequate for high yields. In 1999, 67.2 and 112 kg ha^-1 P₂O₅ and K₂O, respectively, were applied before barley planting. Weeds were controlled chemically and hand-weeded as necessary.

Experimental design for the B study was a randomized complete block with four replications. Treatments were a factorial arrangement of four B rates and two application timings. The B source for this experiment was soluble disodium octaborate tetrahydrate, sold under the commercial trade name Solubor (U.S. Borax, Valencia, CA), applied broadcast at rates of 0, 0.14, 0.28, or 0.56 kg ha^-1 to R3 or R5 soybean. Foliar applications were made utilizing a CO₂ backpack with a 2-m boom, having four 8004VS flat-fan nozzles spaced 46 cm apart and calibrated to deliver 234 L ha^-1.

The center two rows were harvested with a small-plot combine equipped with a scale, moisture tester, and data logger. Seed yield was adjusted to 130 g kg^-1 moisture for all plots. Subsamples of each plot were obtained to determine seed weight. Data were subjected to analysis of variance using the standard least squares procedures of JMP v. 3.2.1 by SAS, and means were separated using Tukey's studentized range (HSD) procedures (SAS Inst., 1996). A probability level of 0.10 was used for mean separation.

Cultivar, Row-Spacing, and Planting-Date Effects
In experiments to evaluate the effect of cultivar and row spacing, soybean cultivars Graham and Hutcheson were planted on 23 May 1997 at a population of 260000 plants ha^-1 on a Nansemond fine sandy loam in Suffolk, VA. In 1998, soybean cultivars Terra brand TS 415, Deltapine brand DP 3478, and Holladay were planted on 17 May at a population of 272000 plants ha^-1 on a Nansemond fine sandy loam in Suffolk, VA. In 1999, the same cultivars used 1998 were planted at a population of 422000 plants ha^-1 near Mount Holly, VA on 14 June, following barley harvest on a State fine sandy loam. The cultivar TS 415 represents an early Maturity Group IV cultivar; DP 3478 represents a late Maturity Group IV cultivar; and Holladay, Graham, and Hutcheson represent Maturity Group V cultivars. Soybean seed was inoculated with Bradyrhizobia spp. bacteria within 24 h before soybean planting in all experiments. Plot size was four rows 46 cm wide by 7.3 m long or seven rows 23 cm wide by 7.3 m long. To minimize moisture stress, experiments were irrigated using subsurface microdrip irrigation in 1997 and 1998 and an overhead center-pivot system in 1999. Soil samples were taken at planting as described in the application rate and timing experiments (Table 2). Plant tissue samples were taken randomly from the uppermost fully expanded leaves at the R2 soybean development stage and analyzed for nutrient content. Fertilizer applications were the same as in the application rate and timing experiments. Weeds were controlled chemically and by hand-weeding as necessary.

The center two 46-cm rows or the center three 23-cm rows were harvested with a small-plot combine equipped with a scale, moisture tester, and data logger. Yields were adjusted to 130 g kg^-1 moisture for all plots. Subsamples of each plot were obtained to determine seed weight. Data were subjected to analysis of variance using the standard least squares procedures of JMP v. 3.2.1 by SAS, and means were separated using Tukey's studentized range (HSD) procedures (SAS Inst., 1996). A probability level of 0.10 was used for mean separation.

In another series of experiments located at the Tidewater Agricultural Research and Extension Center, Suffolk, VA, N and B applications were evaluated at early and late planting dates. In 1998, Deltapine brand soybean cultivar DP 3478 was planted on 14 May at a population of 296000 plants ha^-1 on a Eunola fine sandy loam (fine loamy, siliceous, semiactive, thermic Aquic Hapludult). The same cultivar was planted in a separate, adjacent experiment within the same field on 16 June at a population of 346000 plants ha^-1. In 1999, the same cultivar was planted on the same soil type in two separate, adjacent experiments on 19 May and on 6 July following wheat (Triticum aestivum L.) harvest at populations of 252000 and 467000 plants ha^-1, respectively. Plot size was eight rows 46 cm wide by 9.1 m long. To minimize moisture stress, experiments were irrigated using subsurface microdrip irrigation. In 1998 and 1999, soil samples were taken to a depth of 20 cm during the previous December and tested for available nutrients before planting and fertilizer application (Table 3). Plant tissue samples were taken randomly from the uppermost fully expanded leaves at the R2 soybean development stage and analyzed for nutrient content. In 1998, 49 kg ha^-1 P₂O₅, 100 kg ha^-1 K₂O, 49 kg ha^-1 SO^-₄, and 25 kg ha^-1 Mg²⁺ were applied before planting. In 1999, 60 and 121 kg ha^-1 P₂O₅ and K₂O, respectively, were applied before planting. Weed control was provided with herbicides and hand weeding.

View this table: [in this window] [in a new window]   Table 3. Selected chemical properties for soils utilized in the planting-date experiments, 1998–1999.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION Nitrogen Boron Nitrogen + Boron MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Nitrogen and Boron Application Rate and Timing
Barlett's test for homogeneity rejected the hypothesis that the N or B means between years were equal, indicating heterogeneity of variance (P > 0.01). Correspondingly, a Welch analysis of variance testing that year means were equal was rejected for both N and B experiments (P < 0.0001); therefore, the data are presented by year. Within each year, analysis of variance indicated no significant difference between any N rates at any stage of application (Table 4). The mean soybean yields in the N experiments for 1997, 1998, and 1999 were 3600 (SE = 10.5), 5160 (SE = 156.2), and 2450 (SE = 72.4) kg ha^-1, respectively. Although the data show a slight trend towards increased yields with increasing N rates, especially at the R3 stage of application, yields were not significantly different using a probability level of 0.10 (Table 5).

The variation in the yields between years can be attributed to soil type, environmental differences, and cropping system. With respect to moisture in 1997 and 1998, the soil profile was brought up to field capacity after planting and maintained there via daily irrigation using the subsurface microdrip irrigation system. Total amount of water delivered to the field per day approximated amounts being used by the soybean crop. During the early growing season, the amount delivered was equivalent to approximately 1 mm of rainfall per day, but the amount was gradually increased to the equivalent of approximately 6 mm of rainfall per day during pod and seed development stages (R3 through R6). After R6, the amount of irrigation per day decreased gradually until irrigation was ended at the R7 stage. If natural rainfall occurred, irrigation was ceased for a period of 3 to 5 d. However, in 1997, due to limited irrigation reserves and prolonged lack of rainfall, the field was only maintained at field capacity until the R6 development stage, probably causing some of the reduction in yields compared with 1998. The reduction in yield was most likely attributed to a lower number of pods because seed weights were higher than in 1998. In contrast, the lowest average yields were experienced in 1999. This was primarily due to a completely dry soil profile to a 91-cm depth at planting that was not fully recharged with natural rainfall plus four center-pivot irrigations of 2.5 cm. In this year, seed weights were much lower than the previous years, indicating that seed size as well as a lower number of pods contributed to the lower yields. These less-than-optimum moisture levels in 1997 and 1999 are most likely responsible for the lower yields compared with 1998 when optimum water was maintained at all times.

With respect to cultural practices, planting date in 1999 was later than the planting dates in either 1997 or 1998. However, the late planting date is not likely the sole reason for the lower yield; planting date in 1999 was only about 20 d later than the other years, and plant populations were adjusted upwards. Regardless, the differences in yield levels between years allowed for three different yield potentials in which to test the effect of N and B fertilizer application on soybean yields.

Leaf tissue analysis indicated 53.4 and 53.9 g N kg^-1 in 1998 and 1999, respectively, and 45, 63, and 67 mg B kg^-1 in 1997, 1998, and 1999, respectively (Table 8). These concentrations fall within the sufficiency ranges of 40 to 55 g N kg^-1 and above the minimum concentration of 10 mg B kg^-1 required by soybean for maximum, non-N- or non-B-limited yields (Mills and Jones, 1996; Gupta, 1993). All other nutrient concentrations were also in the sufficient range (Table 8).

Cultivar, Row-Spacing, and Planting-Date Effects
The 1997 data for the cultivar and row-spacing experiments is presented separately from the 1998 and 1999 data due to a change in cultivars and number of cultivars planted. The mean soybean yield for 1997 was 3440 (SE = 28.4) kg ha^-1. Within this year, analysis of variance indicated no effect of any N or B treatment but did indicate significant cultivar effects and a significant cultivar x row spacing interaction. Yield of Hutcheson increased as row spacing decreased from 46 to 23 cm, but Graham exhibited no yield response to row spacing (Table 9). Analysis of variance of the 1998 and 1999 data indicated that the years differed significantly. The mean soybean yields for 1998 and 1999 were 5120 (SE = 75.8) and 2356 (SE = 75.7) kg ha^-1, respectively. In both years, there was no significant effect of N or B treatments. In 1998, there were significant effects of cultivar and a cultivar x row spacing interaction while in 1999, cultivar was the only significant factor (P < 0.0001) (Table 9). Seed weight also differed between years and between varieties, but there were no differences due to N and B treatments within any year. In 1997, seed sizes averaged 16.04 (SE = 0.078) and 18.77 (SE = 0.08) g per 100 seeds for cultivars Graham and Hutcheson, respectively. In 1998, seed weight averages for cultivars TS 415, DP 3478, and Holladay were 15.0 (SE = 0.108), 16.21 (SE = 0.111), and 15.71 (SE = 0.515) g per 100 seeds, respectively. In 1999, seed weights were 13.41 (SE = 0.132), 14.09 (SE = 0.162), and 12.37 (SE = 0.138) g per 100 seeds for TS 415, DP 3478, and Holladay, respectively. The variation in yields and seed weights over years was previously discussed.

In the experiments evaluating N and B applications at early and late planting dates, analysis of variance over years and planting date revealed differences in soybean seed yield between years and planting dates. However, there was no significant effect of any N or B treatment regardless of whether the analysis was performed within year and planting date or combined over years and planting date. In 1998, yields of the first and second planting dates were 4530 (SE = 88.7) and 3880 (SE = 83.7) kg ha^-1, respectively. In 1999, the first planting date had an average yield of 3350 (SE = 80.8) kg ha^-1, and the second planting date had an average yield of 2690 (SE = 27.6) kg ha^-1. Seed weight was significantly different between years and planting date, but there were no differences due to N and B treatments within any year. In 1998, seed size averages for the first and second planting dates were 15.66 (SE = 0.284) and 15.71 (SE = 0.380) g per 100 seeds, respectively. In 1999, seed sizes were 14.49 (SE = 0.121) and 15.69 (SE = 0.025) g per 100 seeds for the first and second planting dates, respectively. The lower yields and seed weights in 1999 may be due to low light conditions and nearly 60 cm of rainfall from three hurricanes passing over the area during the months of September and October. Once again, leaf tissue analysis (Table 11) indicated adequate levels of N and B. Soil samples were not taken at planting in these experiments; therefore, soil N levels were unknown. However, soybean plants appeared to be well nodulated and as tissue analysis revealed, no N stress was occurring at the time of N and B applications.

The lack of response to B in our experiments is in agreement with the research of Reinbott and Blevins (1995), who reported no response to foliar B applications totaling 1.12 kg B ha^-1 applied at four times over the growing season, each time at a rate of 0.28 kg B ha^-1. Their applications were made at R2, R3, R4, and R5 stages of plant growth. The researchers noted leaf burning as a possible cause for eliminating yield benefits of B with four applications. Yield benefits observed by Reinbott and Blevins (1995) were seen when 0.56 kg B ha^-1 was applied in two split applications at the R4 and R5 stages. However, we observed no leaf burning at any B rate and had no yield response to B even with average soil-test B levels seven times lower than those of Reinbott and Blevins (1995). Touchton and Boswell (1975) also reported no response to foliar applications of B at first bloom and 1 wk after bloom at one of two sites. At the second site, these researchers found a 4% yield increase, but this increase may have been due to a low initial soil B concentration (0.14 mg kg^-1) determined by the hot water soluble method (curcumin method). Lack of response to B applications could originate from high B levels in the soil or incorrect timing of B applications. However, in our experiments, soil test B levels were relatively low, and B application timing was similar to work by various authors where yield increases from B application were observed (Schon and Blevins, 1987; Schon and Blevins, 1990; Hudson and Clarke, 1997) as well as that suggested by industry (U.S. Borax, 1997).

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION Nitrogen Boron Nitrogen + Boron MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Previous research with N or B applications to soybean was inconsistent and variable. However, our data revealed a consistent lack of soybean yield response to reproductive-stage N or B applications. This research encompassed three significantly different yield potentials over 3 yr, and no response to N or B fertilization was observed for any N or B rate, time of application, or yield level. In addition, no response to N or B fertilization was observed for any cultivar, row spacing, or planting date tested.

Mineralization of soil organic matter and previous crop residues resulted in relatively high residual N levels in 2 of the 3 yr tested. This, in combination with a well-nodulated crop, may have prevented any response from reproductive-stage N applications. But, there was not a response to N applications when residual soil levels were lower. Regardless, N from N fixation and soil reserves appeared to be adequate and elicited no response from R3- or R5-stage N applications in the yield environments of these experiments. Likewise, native soil B levels of 0.1 to 0.2 mg kg^-1 as determined by the Mehlich I or hot water soluble extraction methods appeared to be adequate to achieve high yields in non-drought-stressed soybean production systems in the Coastal Plain soils of the Mid-Atlantic growing region. These conclusions were validated by tissue analysis showing N and B content within or above the sufficiency ranges established for soybean production (Gupta, 1993; Mills and Jones, 1996). In summary, N or B fertilization of soybean had no significant yield effect at yield environments ranging from 2400 to 5300 kg ha^-1.

	ACKNOWLEDGMENTS

 
The authors acknowledge the Foundation for Agronomic Research, the United Soybean Board, the Virginia Soybean Board, the Virginia Agricultural Council, and U.S. Borax for financial support. Appreciation is also given to Mr. Phil Wynn and Mr. Bruce Beahm for their cooperation in providing land and irrigation for this research.

	REFERENCES

TOP ABSTRACT INTRODUCTION Nitrogen Boron Nitrogen + Boron MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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