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Production Papers

Response of Soybean Grain Oil and Protein Concentrations to Foliar and Soil Fertilization

Dep. of Agron., Iowa State Univ., Ames, IA 50011

^* Corresponding author (apmallar{at}iastate.edu)

Received for publication August 13, 2004.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Numerous studies investigated fertilization effects on soybean [Glycine max (L.) Merr.] grain yield, but few focused on oil and protein concentrations. This study determined fertilization effects on soybean grain oil and protein concentrations in 112 field trials conducted in Iowa from 1994 to 2001. Forty-two trials evaluated foliar fertilization (N–P–K mixtures with or without S, B, Fe, and Zn) at V5–V8 growth stages. Seventy trials evaluated preplant broadcast and banded P or K fertilization (35 P trials and 35 K trials). Replicated, complete block designs were used. Foliar and soil P or K fertilization increased (P < 0.05) yield in 20 trials. Foliar fertilization increased oil concentration in one trial (1 g kg^–1) and protein in one trial (5 g kg^–1) but decreased protein in two trials (6 g kg^–1). Phosphorus fertilization increased oil concentration in two trials (6 g kg^–1) and protein in five trials (5 g kg^–1) but decreased oil in five trials (4 g kg^–1) and protein in two trials (6 g kg^–1). Potassium fertilization increased oil in four trials (3 g kg^–1) and protein in two trials (9 g kg^–1) but decreased oil in two trials (4 g kg^–1) and protein in two trials (11 g kg^–1). Total oil and protein production responses to fertilization tended to follow yield responses. Fertilization increased oil production in 20 trials and protein production in 13 trials. Fertilization that increases soybean yield has infrequent, inconsistent, and small effects on oil and protein concentrations but often increases total oil and protein production.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
EXPANDING USE of soybean grain for animal or human consumption has stimulated new research on agronomic and management practices to increase oil and protein concentrations of soybean grain. In 1989, the USA Grain Standards Act was amended to include grain oil and protein concentrations (USDA-FGIS, 1989). The accepted standard for soybean grain was defined by Updaw and Nichols (1980) as 180 g kg^–1 oil and 359 g kg^–1 protein at a 130 g kg^–1 moisture concentration. Burton (1985) indicated that the significance and direction of the correlation between soybean grain oil or protein concentration and other crop traits depends on the population in which traits are measured. Furthermore, studies have shown that the correlation between grain yield and oil concentration was generally low, protein concentration often was negatively correlated with yield, and an improvement in grain yield alone has corresponded with a decrease in oil concentrations (Wilcox et al., 1979; Burton, 1985; Voldeng et al., 1997; Morrison et al., 2000). Research has shown that genetic success at increasing both yield and oil concentration has maintained protein concentration, but high protein cultivars tend to have relatively low oil concentration (Morrison et al., 1999; Westgate et al., 1999; Morrison et al., 2000).

Several environmental factors can influence protein and oil concentrations of soybean grain (Westgate et al., 1999). Fertilization with N, P, K, and other nutrients can affect yield and many physiological processes that, in turn, could influence grain yield and protein or oil concentration. Soybean has been classified as a poorer responder to N, P, and K fertilization compared with other grain crops although responses have been observed in low-testing soils (Kamprath, 1974). Large soybean responses to P or K fertilization have been reported in Iowa (Bharati et al., 1986; Mallarino et al., 1991; Webb et al., 1992; Borges and Mallarino, 2000, 2003). These responses were frequent when soils tested Very Low or Low (<130 mg K kg^–1 and <16 mg P kg^–1 by ammonium acetate and Bray-P₁ tests, respectively) according to Iowa State University interpretations (Sawyer et al., 2002).

Foliar fertilization sometimes increases soybean grain yield. Garcia and Hanway (1976) reported large yield increases from foliar application of N–P–K–S fertilizer mixtures during reproductive stages. However, other studies showed inconsistent responses and often yield decreases (Boote et al., 1978; Parker and Boswell, 1980; Sesay and Shibles, 1980; Syverud et al., 1980; Vasilas et al., 1980; Poole et al., 1983; Reinbott and Blevins, 1995; Freeborn et al., 2001). More recent studies have investigated soybean yield response to foliar fertilization at early vegetative stages (Haq and Mallarino, 1998, 2000; Mallarino et al., 2001). These researchers theorized that small amounts of nutrients applied to foliage at early growth stages could complement nutrients supplied by the soil when root uptake is limited by unfavorable soil or climatic growing conditions. These authors reported that foliar fertilization with commercial 3–8–15 (N–P–K), 3–8–15, 10–4–8, or 8–0–7 fertilizers at the V5 to V8 growth stages (Fehr and Caviness, 1977) increased soybean yield in approximately 15% of almost 100 replicated on-farm trials even though most soils tested optimum or higher in P and K according to local recommendations. At responsive sites of one study (Haq and Mallarino, 1998), soybean tissue at the V5 or R2 growth stages had lower plant P concentration, lower rainfall in late spring to midsummer, and lower N, P, and K uptake at the R2 growth stage compared with nonresponsive sites.

The influence of fertilization on soybean grain oil and protein concentrations was measured in few experiments. Jones and Lutz (1971) showed decreasing oil concentration with increasing P and K fertilization rates (a 18 g kg^–1 decrease) and inconsistent protein concentration responses. However, because of the grain yield response to fertilization, both oil and protein yield were higher with fertilization. Ham et al. (1973) reported significantly different soybean grain yield response to various P and K rates applied to soil using various placement methods but no protein or oil concentration response. Poole et al. (1983) showed increased grain N and protein concentrations and reduced oil concentration as a result of N–P–K–S foliar fertilization at reproductive stages. Wesley et al. (1998) showed that foliar N fertilization increased oil concentration slightly (by 4 g kg^–1 on average) at three of eight sites and increased protein concentration at four sites (by 10 g kg^–1 on average) although N consistently increased total protein and oil production because of yield increases.

Further investigation of effects of fertilization on soybean oil and protein concentration and production is needed because published research is scarce and results are inconsistent. The objective of this study was to determine fertilization effects on soybean grain oil and protein concentrations and total production.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Sites and Fertilization Treatments
Foliar Fertilization Trials
Soybean grain samples for oil and protein analyses were collected from selected treatments of 42 foliar fertilization trials conducted in Iowa farmers' fields from 1994 through 1998. Table 1 shows summarized information about the sites. All crop and soil management practices (except foliar fertilization) were those used by the farmers. Plots at each trial measured 12.2 m in length and 4.5 to 5.5 m in width, depending on the row spacing (19 to 91 cm) and planter width used. A randomized, complete block experimental design with four replications was used for all trials. In 1994, grain was sampled from three treatments of 10 trials (from a total of six treatments). Treatments were a control, a single application of 28 L ha^–1 of a 3–8–15 (N–P–K) fertilizer at the V5 growth stage (Fehr and Caviness, 1977), and 38 L ha^–1 of the same fertilizer applied one-half at the V5 growth stage and one-half 8 to 9 d later. In 1995 and 1996, grain was sampled from four selected treatments of 14 trials (and a total of six treatments). Three treatments were similar to those sampled in 1994, and the other consisted of 56 L ha^–1 of a 10–4–8 fertilizer. The nutrient rates applied for these 24 trials were 1.2 to 7.1 kg N ha^–1, 3.1 to 4.2 kg P ha^–1, and 5.9 to 8.0 kg K ha^–1. In 1997 and 1998, grain samples were collected from all plots of 18 trials having a similar design. Treatments (six) were a control and a single application of 28 L ha^–1 of five commercial fluid fertilizers at the V5 growth stage. The fertilizers were 3–8–15, 10–4–8, 3–8–15–1 (N–P–K–S), 10–4–8–1, and 10–4–8–1 plus smaller amounts of B, Fe, and Zn. The nutrient rates applied for these 18 trials (in kg ha^–1) were 1.2 to 3.5 N, 1.5 to 3.1 P, 3.0 to 5.9 K, 0.32 S, 0.13 Fe, 0.02 Zn, and 0.03 B. Procedures for fluid fertilizer application simulated procedures commonly used by producers. The fertilizers were diluted into 100 L ha^–1 of water and were applied with a hand-held CO₂–powered sprayer adjusted to a constant pressure of 0.17 MPa.

Soil Sampling and Analyses
Composite soil samples (12 to 16 cores, 2-cm diam. each) were collected before planting soybean from a 15-cm soil depth. Samples were dried at 35 to 40°C and crushed to pass a 2-mm sieve. Soil P and K were determined following procedures recommended for the North Central Region based on the Bray-P₁ test (Frank et al., 1998) and the ammonium acetate test for K (Warncke and Brown, 1998). Soil test P and K values are shown in Tables 1 and 2. Iowa State University soil test interpretation classes (Sawyer et al., 2002) are referred to in this study. Boundaries for the soil test P classes are Very Low, Low, Optimum, High, and Very High, corresponding to 8, 16, 20, and 30 mg P kg^–1, respectively, and boundaries for similar soil test K classes are 90, 130, 170, and 200 mg K kg^–1, respectively.

Grain Harvest, Sampling, and Analyses
Soybean grain was harvested from two or three central rows of each plot, and a sample of approximately 0.75 kg was collected for moisture, oil, and protein determinations. Grain yields were corrected to 130 g kg^–1 moisture concentration. Yields from foliar fertilization trials conducted from 1994 to 1996 were published before (Haq and Mallarino, 1998, 2000) but are summarized here because only selected sites and treatments were sampled for this study and, therefore, statistical interpretations of yield response within sites and across sites differed. Grain yields from the 18 foliar fertilization trials conducted from 1997 and 1998 were published before by Mallarino et al. (2001) and are not shown because all treatments and replications were sampled. Yield data for all soil P and K fertilization trials are summarized here because only grain of selected sites and treatments was sampled (1995 through 1997) for this study or were not published (1998 through 2001). Oil and protein concentrations were determined in whole-grain samples by near-infrared spectroscopy at the Iowa State University Grain Quality Laboratory following the laboratory standard procedures and local calibrations in an Infratec 1225 Seed Analyzer (Foss North America, Eden Prairie, MN). The calibration process was described by Rippke et al. (1996) and was subsequently made into a standard method of the American Association of Cereal Chemistry (1999). The present calibrations are based on chemical reference values for 2320 calibration samples (crop years 1989–2003) that were obtained from Woodson-Tenant Labs (a Division of Eurofins Scientific, Des Moines, IA). Both oil and protein concentrations were adjusted to 130 g kg^–1 grain moisture concentration. Total oil and total protein production on a unit area basis were calculated from grain yield and the concentrations of oil and protein.

Statistical Analyses
Treatment effects on grain yield, oil and protein concentrations, and total oil and protein production were assessed by analysis of variance for a randomized, complete block design by site and across sites within a year using the General Linear Models procedure of SAS (SAS Inst., 2000). A site was classified as responsive when the treatment main effect or an orthogonal contrast of the control versus the mean of all fertilized treatments was significant at P 0.05. Treatment differences at responsive foliar fertilization trials were tested with LSD. Treatment differences at responsive P or K soil fertilization trials were assessed with rate, placement, and interaction components of the factorial combinations of two fertilizer rates and two placement methods. Relationships between grain yield and oil or protein concentrations were analyzed with correlation and regression procedures of SAS.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Grain Yield
Foliar Fertilization Study
All or some foliar fertilization treatments influenced soybean yield (P 0.05) at three trials conducted from 1994 through 1996 (Table 3). At Site 2, the 19-L rate applied twice decreased yield, and the single 28-L rate increased yield compared with the control. This result has no logical explanation. At Site 7, both treatments increased yield. At Site 11, only the 19-L fertilization rate sprayed twice increased yield. Yield responses in foliar fertilization trials conducted from 1997 and 1998 are not shown because results were published before (Mallarino et al., 2001) and fertilization influenced yield only at Site 35 (in 1998, Table 1). At this site, there was a statistically similar yield response to application of fertilizers 3–8–15, 10–4–8–1, and 10–4–8–1 with micronutrients (232 kg ha ^–1 on average). The response at this site and cumulative effects of small responsive trends at other sites (not shown) explained a small mean yield response to 3–8–15 and 10–4–8–1 fertilizers (92 kg ha ^–1) across all 1998 trials.

Soil Phosphorus and Potassium Fertilization Study
Only means of the two soil P and K fertilization rates applied with each placement method are shown and discussed because study of all treatment effects indicated that the high fertilizer rates increased (P 0.05) grain yield and oil or protein concentration further than the low rate only at three sites. Also, the interaction between fertilizer rate and placement method never was significant although the placement methods sometimes differed.

Phosphorus fertilization increased (P 0.05) soybean grain yield in seven sites (Table 4). The P placement method did not affect yield at any site. Soil test P was Very Low or Low in all responsive sites (Table 2). Analyses of year means indicated a significant average effect of P in 1996, 2000, and 2001. Potassium fertilization influenced yield in 10 sites (Table 4). At six sites (Sites 3, 10, 18, 21, 27, and 28), K increased yield, and the placement methods (when evaluated) did not differ. At Sites 32 and 33, band K increased yield more than broadcast K. At Site 34, broadcast K increased yield more than band K. Fertilizer K applied with both placement methods decreased yield at Site 12, which we believe was a random result unrelated to the treatments applied. Soil test K was below Optimum at Sites 3, 12, 27, and 32; Optimum at Sites 10, 18, 28, and 33; and Very High at Sites 21 and 34 (Table 2). Analyses of annual yield means indicated a significant effect of K in 1998 and 2001 but no differences between placement methods.

Soil Phosphorus and Potassium Fertilization Effects
Mean grain oil concentrations across all soil P and K fertilization sites ranged from 156 to 221 g kg^–1 (Table 9), and mean protein concentrations ranged from 312 to 411 g kg^–1 (Table 10). These values encompass mean values reported from Iowa surveys (Brumm and Hurburgh, 2002, 2003).

Grain protein concentration responded inconsistently to preplant P or K fertilization (Table 10). Phosphorus increased protein concentration at five sites (Sites 5, 9, 18, 22, and 29) but decreased it at two sites (Sites 7 and 32). The P placement methods did not differ at any site. Potassium fertilization decreased grain protein concentration at Sites 27 and 30 but increased it at Sites 31 and 32. The K placement methods differed at Sites 31 and 32, but responses were inconsistent. Only broadcast K increased protein concentration at Site 31, and there was a larger response to the band method at Site 32.

Phosphorus or K fertilization seldom influenced oil and protein concentrations at the same site. Phosphorus influenced both oil and protein concentrations at Sites 7, 22, 29, and 32 (Tables 9 and 10), and increasing or decreasing effects were always the opposite for these traits. Potassium influenced both oil and protein concentrations only at Site 32 (Tables 9 and 10) where it decreased oil concentration and increased protein concentration.

Grain Oil and Protein Production
Data for total oil and protein production per unit area are not presented because amounts can be calculated from grain yield and oil or protein concentrations. Because foliar fertilization effects on oil and protein concentrations were infrequent and small (negative or positive), fertilization effects on oil and protein production were explained mainly by grain yield responses. Foliar fertilization increased (P 0.05) oil production at Sites 7, 9, 11, 14, and 35, and the average response at each site ranged from 21 to 87 kg oil ha^–1. Foliar fertilization increased protein production at Sites 7 and 11, and the average response was 140 and 191 kg protein ha^–1, respectively. At Site 37, some treatments increased protein production, others decreased it, and on average, fertilized treatments did not differ from the control.

Effects of preplant P and K fertilization on oil and protein production also tended to follow results for yield. Responses were more frequent and larger than for the foliar fertilization study because grain yield responses were more frequent and larger. Fertilization increased total oil production at seven P sites and eight K sites (not necessarily at the same locations), and the average response ranged from 15 to 152 kg oil ha^–1. Fertilization increased protein production at seven P sites and four K sites, and the average response ranged from 46 to 393 kg protein ha^–1. Potassium fertilization decreased protein production at Site 12 (by 35 kg protein ha^–1) where yield of plots receiving K fertilizer was lower than yield of the control treatment due to unknown reasons probably unrelated to the treatments.

Relationships between Yield and Oil and Protein Concentration
Differences in grain oil and protein concentrations across sites and years were larger than any difference due to treatments. This observation confirms previous reports of complex environmental effects on soybean oil and protein concentrations (Westgate et al., 1999; Brumm and Hurburgh, 2002, 2003). The infrequent and small response of oil and protein concentrations to fertilization, lack of a significant interaction between fertilization and sites, and the fact that no cultivar was planted in more than two or three locations did not allow for a meaningful study of fertilization effects for specific cultivars at different sites. Relationships between grain yield and oil or protein concentration across sites were very poor and not significant (even at P 0.10) for fertilized or nonfertilized treatments (Table 11). Analyses across sites within each year (not shown) resulted in a similar lack of correlation.

View this table: [in this window] [in a new window]   Table 11. Simple correlations between soybean grain yield and oil or protein concentration across 42 foliar fertilizer trials, 35 P soil fertilization trials, and 35 K soil fertilization trials.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Responses of soybean grain oil and protein concentrations to fertilization were infrequent, small, inconsistent (positive or negative), and unrelated to more frequent and usually positive grain yield responses. Therefore, a grain yield response to fertilization usually resulted in increased total oil and protein production per unit area. The large diversity of Iowa growing conditions, soybean cultivars, and years included in this study indicates that these results can be reasonably extrapolated to large areas of the North Central Region. The most significant implications of this study are that although increases of grain oil and protein concentrations from fertilization are unlikely, a yield response to fertilization will not result in significant concentration decreases and will increase total oil and protein production.

	ACKNOWLEDGMENTS

 
We thank Dr. Charles R. Hurburgh, Jr., director of the Iowa State University Grain Quality Laboratory, for his valuable help with grain oil and protein analyses for this study.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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