Published in Agron. J. 96:26-33 (2004).
© American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
FERTILIZER MANAGEMENT
Comparison of Uniform- and Variable-Rate Phosphorus Fertilization for Corn–Soybean Rotations
David J. Wittry and
Antonio P. Mallarino*
Dep. of Agron., Iowa State Univ., Ames, IA 50011
* Corresponding author (apmallar{at}iastate.edu).
Received for publication May 3, 2003.
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ABSTRACT
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Variable-rate (VR) technology can be used to vary fertilization rates within a field. The objective of this study was to compare VR and uniform-rate (UR) P fertilization for corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] rotations. Grid soil sampling (0.2- to 1.7-ha cells), differential global positioning systems (DGPS), and grain yield monitors were used in strip trials established on six fields (12 site-years). Three replicated P fertilization treatments were a control (no P) and a single application of the P requirement of the 2-yr rotation based on soil-test P (STP) using UR or VR. Measurements were plant dry weight (DW), P concentration (PC), and P uptake (PU) when crops were 15 to 20 cm tall; grain yield; and STP after crop harvest. Phosphorus increased grain yield (P 
0.05) of five crops, and the fields had initial mean STP 16 mg kg–1 (Bray-P1 or Mehlich-3). Phosphorus increased plant DW, PC, and PU of five, six, and seven crops, respectively. Within each field, yield responses were observed only in areas with STP < 20 mg kg–1 and (or) areas with Clarion soil (fine-loamy, mixed, mesic, Typic Hapludoll). The responses of plant DW, PC, and PU were not related to STP or soil series. The fertilization method did not influence (P 0.05) crop responses to P. However, VR fertilization resulted in better P fertilizer management because it applied 12 to 41% less fertilizer and reduced STP variability compared with the traditional UR fertilization method.
Abbreviations: DGPS, differential global positioning systems • DW, dry weight • GIS, geographical information systems • PC, phosphorus concentration • PU, phosphorus uptake • RCBD, randomized complete block design • STP, soil-test phosphorus • UR, uniform rate • VR, variable rate
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INTRODUCTION
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PHOSPHORUS CAN BE A MAJOR yield-limiting nutrient for corn and soybean production in many regions. Studies of the spatial variability of STP and other nutrients have revealed large within-field variability in fields with contrasting soil series or long histories of fertilization (Cambardella et al., 1994; Cahn et al., 1994; Mallarino, 1996; Nolin et al., 1996; Gupta et al., 1997; Goedeken et al., 1998; Mallarino and Wittry, 2000). McGraw (1994) reported that in 86% of 392 Minnesota fields sampled using grid-sampling methods, the STP range encompassed at least four of five soil-test interpretation classes. This variability can result from both natural processes and management practices.
Use of DGPS, grain yield monitors, and geographical information systems (GIS) allows for relatively accurate recording of crop yield and soil-test values along with their location in the field. Variable-rate equipment aided by computer controllers can be used to vary fertilizer rates across a field based on map-based or sensor-based instructions (Sawyer, 1994; Wollenhaupt et al., 1994; Schnitkey et al., 1996). Variable-rate application has the potential to reduce costs in areas where UR fertilization would overapply fertilizer and to increase yield where fertilizer would be underapplied (Bullock et al., 1994; Cahn et al., 1994; McGraw, 1994; Sawyer, 1994; Franzen and Peck, 1995; Long et al., 1996; Anderson-Cook et al., 1999; Rehm and Lamb, 2000). Several authors have emphasized the potential of VR fertilization to improve water quality because no P fertilizer would be applied to field areas testing above optimum levels for crops (Mulla, 1993; Sawyer, 1994; Franzen and Peck, 1995; Mohamed et al., 1996; Schnitkey et al., 1996; Gupta et al., 1997; Schepers et al., 2000).
Published studies comparing yield response to P or K fertilization using UR and VR fertilization methods with VR equipment available to producers are scarce and have shown small or no differences. Anderson and Bullock (1998) found no yield differences between UR and VR methods based on a 1-ha grid sampling for P–K mixtures for corn or soybean in six Illinois fields. This result was explained by a lack of crop response to fertilization although all fields had areas with yield-limiting soil-test values according to local interpretations. Comparisons of UR and VR for mixtures of P and K fertilizers for corn, soybean, and wheat (Triticum aestivum L.) in six U.S. Midwest farms (Lowenberg-DeBoer and Aghib, 1999) showed that although VR sometimes resulted in yield increases compared with UR, it seldom increased net returns to fertilization because of increased costs of soil sampling and fertilizer application. Although all fields had areas with yield-limiting soil-test values, the mean values always were above optimum levels for the crops. Research with UR and VR for mixtures of N and P fertilizers (Yang et al., 2001) for grain sorghum [Sorghum bicolor (L.) Moench] in predominantly low-testing soils showed small yield increases from VR, but the authors concluded that increased costs of soil sampling and application (when compared with a uniform application) offset the yield benefit.
Research comparing UR and VR P fertilization methods for corn and soybean is needed for fields with variation in STP, soil series, and other production conditions. The objective of this study was to assess responses of corn and soybean grain yield, early plant growth, and early PU and STP measured after harvest to UR and VR P fertilization applied with commercial equipment commonly used in Iowa and the Corn Belt.
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MATERIALS AND METHODS
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Field response strip trials were conducted in six Iowa farmers' fields. Management practices were those used by each farmer and, thus, corn hybrids, seeding rates, planting dates, and other practices varied among fields. All fields were managed with a corn–soybean rotation and had histories of UR fertilization and chisel-plow/disk tillage. Fields were chisel-plowed only after corn harvest in the fall (usually in November), and all fields were disked in April (spring). Areas ranging from 7 to 20 ha were delineated at least 40 m away from field borders to fit experiments based on a replicated strip-trial methodology adapted to precision agriculture technologies (Long et al., 1996; Oyarzabal et al., 1996). Initial soil-test values (Table 1) were measured on composite soil samples collected using a systematic, grid-point sampling method (Wollenhaupt et al., 1994). Grid lines were spaced 130 m (1.7-ha cells) in Fields 1 through 4 (16–20 ha) and 45 m (0.2-ha cells) in Fields 5 and 6 (7 ha). Twelve cores (0- to 15-cm soil depth) were collected from areas measuring approximately 100 m2 near the center of each cell. All samples from Fields 1 through 4 were analyzed with the Bray-P1 test, and samples with pH 
7.4 (1:1 dry soil/water ratio) were analyzed with the Olsen P test. The procedures followed for both tests were those recommended for the North-Central Region by Frank et al. (1998). Iowa research (Mallarino and Blackmer, 1992; Mallarino, 1997) showed high correlations (r > 0.92) between these two tests, a 0.6 Olsen/Bray-P1 extraction ratio in soils with pH 7.3, and a reduced P extraction by the Bray-P1 test in many soils with pH > 7.3. Thus, Bray-P1 values for the few samples with pH > 7.3 (three in Field 1, one in Field 2, and one in Field 3) were adjusted accordingly. Samples from Fields 5 and 6 were analyzed with the Mehlich-3 P test (Frank et al., 1998). This test is recommended for all Iowa soils based on local field calibrations (Mallarino, 1997), and interpretations are similar to those for the Bray-P1 test (Sawyer et al., 2002). The soil P extracted with all extractants was measured colorimetrically with the Murphy and Riley (1962) method as described by Frank et al. (1998). Iowa interpretation classes (Sawyer et al., 2002) for the Bray-P1 and Mehlich-3 P tests are (units are mg P kg–1) 8 for Very Low, 9 to 15 for Low, 16 to 20 for Optimum, 21 to 30 for High, and 31 for Very High. No P fertilization is recommended for either corn or soybean when STP is High or Very High. Table 2 summarizes soil-test values and area for the two dominant soil series in each field.
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Table 1. Descriptive statistics for soil-test P and pH, and soil-test P distribution within Iowa State University soil-test P interpretation classes for six fields.
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Table 2. Predominant soil series in the experimental areas and average soil-test P and pH (0- to 15-cm depth) of six fields.
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Treatments were a control (no P) and P fertilization using either UR or VR application methods. The width of the treatment strips was 18.3 m, and the length was uniform within a field but varied among fields (370–800 m). Randomized complete block designs (RCBD) were used, and there were four replications in Fields 1 through 3, five in Field 4, and three in Fields 5 through 6. Distances were measured with a measuring tape, plastic pipes marked each trial corner, and corner coordinates were recorded with a hand-held DGPS receiver. Treatments were applied before tillage in the fall (usually in November) by broadcasting granulated monoammonium phosphate with commercial VR spreaders equipped with DGPS receivers and controllers (spinner systems in Fields 1–4 and air-delivery systems in Fields 5 and 6).
The P rates used (Table 3) followed Iowa guidelines for a single fertilizer application in a 2-yr crop rotation (Sawyer et al., 2002) based on STP and expected P removal in harvested grain. No P fertilizer is recommended for corn or soybean in fields with STP High or Very High. In Fields 1, 3, and 4, however, the VR rates were similar for the Very Low and Low classes because the cooperating farmers thought Very Low areas were small and corresponded to isolated cells. The median STP for each experimental area was used to define the UR rate. Interpolated STP maps were used to define the VR rates by using an inverse-distance interpolation method with a distance-weighting exponent of 2 (Wollenhaupt et al., 1994). Field-average yield goals were used for both fertilizer application methods. No corrective N rate was used to offset the small amount of N supplied with the P fertilizer, but 150 kg N ha–1 of additional N was applied across all treatments before planting corn. Potassium fertilizer at rates of 110 to 130 kg K ha–1 was applied across the experimental areas when the P treatments were applied. Trials in Fields 1 and 2 were evaluated only in 1996 (corn in Field 1 and soybean in Field 2) because the fields were sold. The trial in Field 3 was evaluated in 1997 and 1998 (corn–soybean). The trial in Field 4 was evaluated in 1997 and 1998, and treatments were reapplied (based on new STP data from the UR and VR strips) for a second rotation cycle in 1999 and 2000 (soybean–corn–soybean–corn). Trials in Fields 5 and 6 were evaluated in 1998 and 1999 (corn–soybean in Field 5 and soybean–corn in Field 6). Thus, the study included six corn crops and six soybean crops (12 site-years). The field-crop code for Fields 3 to 6 includes the suffix a or b to indicate the first and second crop of the rotation and, only for Field 4, the suffix a2 or b2 to indicate crops of a second rotation cycle.
Grain yield was harvested with farm combines equipped with commercial impact flow-rate yield monitors and DGPS receivers. The differential correction was obtained through the U.S. Coast Guard AM signal. The monitors were calibrated by weighing grain harvested along at least four combine passes over the field lengths outside of the trial area. Grain moisture was determined by a sensor located in the augers that fill the combine grain tank. Yield was adjusted to 155 g kg–1 H2O for corn and 130 g kg–1 H2O for soybean. Yield data were unaffected by borders because the experimental areas were at least 40 m away from border rows, and data from combine passes that may have included rows from neighboring treatments were not used. Two 4.57- or 7.62-m-wide combine passes were used from each soybean strip, and two to four 4.57-m-wide combine passes were used from each corn strip. Yield monitor data were imported to ArcView GIS (Environ. Syst. Res. Inst., Redlands, CA) and were analyzed for common yield monitor errors, such as effects of waterways and incorrect setting for grain-path time lag through the combine. Affected data were corrected (such as grain-path lags) or deleted (such as inaccurate yield points).
The aboveground portions of 10 plants were collected when plants were 15 to 20 cm tall from 100-m2 areas located at the center of cells defined by the width of each treatment strip and the separation distance of the soil-sampling grids along crop rows. Samples were dried at 65°C, weighed, and ground to pass a 1-mm sieve. Total P was analyzed by digesting samples with concentrated H2SO4 and H2O2 (Digesdahl Analysis System, Hach, Denver, CO) and measuring P in the extracts with the Murphy and Riley (1962) colorimetric method. Plant DW and PU are reported on a per-plant basis. Treatment effects on STP were assessed only in Fields 3 to 6 by collecting soil samples (12 cores, 0- to 15-cm depth) at the end of the 2-yr crop rotation cycle (2 yr after the single P application for two crops) from the same areas the plant samples were collected.
Treatment effects on grain yield, DW, PC, PU, and STP of samples collected after harvest were analyzed with an ANOVA for a RCBD design assuming fixed treatment and block effects (SAS Inst., 2000). The data inputs were means of all observations within each treatment strip. The treatment sums of squares were partitioned into orthogonal comparisons of the control vs. the mean of the two fertilization methods and the difference between the two fertilization methods.
Treatment effects on crop measurements for field areas with different initial STP interpretation classes or soil series were assessed by two procedures. One procedure assessed treatment effects for each STP class or soil series with a method previously used for strip trials harvested with yield monitors (Bianchini and Mallarino, 2002; Bermudez and Mallarino, 2002). The F test from a one-way ANOVA in which sources of variation were replications (blocks) and P treatments was used to estimate the consistency of treatment effects across STP classes and soil series. Data inputs for the analysis by STP classes were treatment means for areas defined by the width of each strip and the separation distance of the soil-sampling grid lines along crop rows. Data inputs for the analysis by soil series were treatment means for each soil series. The data inputs for these analyses were calculated using ArcView GIS by overlaying yield maps, initial STP maps, experimental layouts, and digitized (scale 1:12000) soil survey maps (ICSS, 2001). Figure 1 shows an example of the GIS maps used. This analysis was not performed for STP classes or soil series in which all treatments were not represented in at least two replications of the experimental design. When this happened for the two extreme STP classes (Very Low and Very High), the data were combined with the neighboring class (Low or High, as appropriate). Because of lack of replication, the analysis by soil series was done only for the two or three dominant soil series in each field.
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Fig. 1. Example (Field 6) of soil series map, treatment strips, initial soil-test P (means for cells defined by the width of each replication), and sampling points and cells for each treatment.
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The second procedure used regression to study the relationship between crop response to P and initial STP within each field. The General Linear Models (GLM) and Nonlinear Models (NLIN) procedures of SAS (SAS Inst., 2000) were used to fit linear, quadratic, and quadratic-plateau models. The data inputs were the values used for the first procedure. Relative crop response was calculated for each initial STP value by subtracting the mean of the control from the mean of the two fertilized treatments, dividing the result by the mean of the control, and multiplying by 100.
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RESULTS AND DISCUSSION
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Whole-Field Responses
Phosphorus fertilization increased (P 0.05) corn yield in Fields 3a, 4b2, and 5a and soybean yield in Fields 5b and 6a (Table 4). The fertilization methods did not affect crop yield (P 0.05) in any field or year. Comparisons of yield responses and initial STP values (Table 1) suggest that the observed yield increases are reasonable because the responsive fields had initial mean STP 16 mg kg–1 (Very Low or Low classes) and large areas testing below this value. The probability of grain yield response to P is high (>20%) only in soils testing below the Optimum class (16–20 mg kg–1) according to Iowa STP interpretations and previous research based on conventional small-plot methods (Mallarino and Blackmer, 1992; Mallarino, 1997). Studies conducted in other regions of the Corn Belt with P–K fertilizer mixtures for corn or soybean (Anderson and Bullock, 1998; Lowenberg-DeBoer and Aghib, 1999) also showed no yield differences between UR and VR fertilization.
Phosphorus fertilization increased (P 0.05) plant DW in five site-years (Table 5), PC in six site-years (Table 6), and PU in seven site-years (Table 7). Most fields where these plant measurements responded to P also showed a grain yield response. The fertilization method never affected DW or PU. There was a slightly higher PC response to P for the UR method compared with the VR method only in Field 5a (corn). The results showed that PU responded to P in fields with STP optimum or higher and where yield did not respond to added P. Previous research with corn and soybean (Mallarino et al., 1999; Borges and Mallarino, 2000) showed that optimal STP levels were higher for early plant PU than for grain yield. Only Fields 1 and 2, which had the highest initial STP, showed no P fertilization effects on any plant measurement.
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Table 6. Corn and soybean early plant P concentration (PC) as affected by P fertilization with two application methods.
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Responses in Field Areas with Different Soil-Test Phosphorus Values or Soil Series
Analyses of corn yield response for field areas that initially tested within different Iowa STP interpretation classes (Table 8) showed significant responses to P fertilizer (P 0.05) only for field areas testing Low in Fields 3a, 5a, and 6b. The two fertilization methods differed only for the Low STP class of Field 3a where the VR method increased yield more than the UR method. The whole-field results (Table 4) showed corn response to P in Fields 3a, 4b2, and 5a but no response in Field 6b and no difference between the two fertilization methods. Similar analyses for soybean (Table 9) showed a significant yield response to P only for areas testing Low or Optimum in Field 6a. The two application methods differed for areas that initially tested Optimum where VR increased yield more than UR. This difference cannot be explained satisfactorily because in this field, less P was applied with VR than with UR in areas testing Optimum. The whole-field results (Table 4) showed a soybean yield response to P in Fields 5b and 6a and no difference between the two fertilization methods.
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Table 8. Corn yield for field areas testing within different soil-test P interpretation classes as affected by P fertilization with two application methods.
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Table 9. Soybean yield for field areas testing within different soil-test P interpretation classes as affected by P fertilization with two application methods.
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Analyses of plant DW, PC, and PU responses for field areas that initially tested within different STP interpretation classes showed few statistically supported (P 0.05) responses to P within a field. Because of this general result, and because of the numerous treatment means involved (three measurements, 12 site-years, and usually three STP interpretation classes), data are not shown. The plant DW response to P varied across STP classes only in Field 2 where (contrary to expectations) there was a large soybean DW response to P in areas testing high but not in areas testing Optimum or Low. Both PC and PU often responded positively to P (which confirmed results of whole-field analyses), and with the exception of Fields 1 and 2, responses usually were observed for all STP interpretation classes. Others (Mallarino et al., 1999; Borges and Mallarino, 2000) have observed significant early DW and PU responses to P in high-testing soils.
Regression analyses of relationships between relative crop response to P and initial STP values within each field showed statistically significant trends in few instances, and detailed results are not shown. The yield response to P fertilization decreased linearly from low- to high-testing areas in Fields 5a, 5b, and 6b, but R2 values were very low (0.32, 0.12, and 0.24, respectively). Curvilinear trends (quadratic terms) were not statistically significant (P 0.05). These fields contained larger low-testing areas than the others, and the initial soil sampling was done with a smaller grid size (0.2 ha) than for the other fields (1.7 ha). No frequent or consistent relationships were observed between STP and the response of DW, PC, or PU across fields. The response to P of each plant measurement was significantly related to STP only in two fields, but the responsive fields often did not coincide across the measurements. Only for one field (Field 5a) did the response of all measurements (including yield) decrease with increasing STP.
Grain yield, DW, PC, and PU often varied among soil series at each field, which was an expected result. However, except for grain yield and PU, responses to P fertilizer and fertilizer application methods were similar (P 0.05) for all soil series within a field. Because of this result and the numerous treatment means involved, Table 10 shows only grain yield data for the five site-years in which PU and grain yield responses differed across soils. Both PU and grain yield responded to P fertilization only in field areas with Clarion soil. The application methods differed in three site-years, but differences were small and inconsistent (greater yield for VR in two fields and greater for UR in one field), which suggests random differences. In Fields 5 and 6, larger responses for the Clarion soil could be explained by lower initial STP (a difference of 4–6 mg kg–1 that changed the interpretation class) and lower pH compared with other soils (Table 2). However, STP and pH were similar (within 2 mg P kg–1 and 0.3 pH units) for the soils in Fields 3 and 4. Study of yield levels for each soil, which could have influenced a response to P, indicated no consistent differences. These soil series differ in other properties whose potential impact in responses to P cannot be assessed with the methods used. For example, all soils were formed on loamy glacial till, but the Clarion soil occupies higher and steeper landscape positions and is better drained than the other soils (ICSS, 2001). Also, the particle-size composition of the plow layer is similar for the Clarion and Nicollet soils (loam) but is coarser than for the Canisteo and Webster soils (silty clay loam).
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Table 10. Grain yield as affected by P fertilization with two application methods for field areas with different soil series for fields in which responses among soils differed.
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Treatment Effects on Soil-Test Phosphorus
Study of STP after crop harvest in Fields 3, 4, 5, and 6 (Fields 1 and 2 were not sampled after harvest) indicated that P fertilization increased STP (P 0.05) compared with the control treatment, except for Field 3 (Table 11). Soil-test P values for the VR treatment tended to be lower than for the UR treatment in all fields although the difference was statistically significant only in Field 6. This result agrees with lower amounts of P fertilizer applied with VR in all fields. Calculations from data in Table 3 show that VR applied 12, 19, 41, and 31% less P than UR in Fields 3 to 6, respectively. Another important result was that standard deviations of STP were lower for VR than for UR, which indicates that VR reduced STP variability compared with UR. This result is reasonable because the VR method was designed to apply higher P fertilizer rates to low-testing areas and no fertilizer to high-testing areas.
Interpretive Summary Discussion
The study did not show larger grain yield response to P fertilizer applied with VR compared with UR. This result could be explained by several reasons. One possible reason is that fertilization rates applied to low-testing areas with both application methods were sufficient to produce maximum crop yield. The P fertilizer recommendations used in Iowa and the Corn Belt for low-testing soils often include a buildup component or recognize that application of recommended rates will result in significant STP buildup (Sawyer et al., 2002). Sufficient P fertilization to maximize yield is even more likely for the first crop of the rotation when the 2-yr P recommendation is applied once. Mallarino and Blackmer (1992) showed that corn yield responses in several low-testing Iowa soils were similar for P rates ranging from 25 to 75 kg P ha–1. The criteria used for selecting rates in this study are used frequently in production agriculture, and the lack of yield differences between VR and UR fertilization methods coincides with results of other research with P–K fertilizer mixtures (Anderson and Bullock, 1998; Lowenberg-DeBoer and Aghib, 1999). Insufficient precision of commercially available VR technology, including controller's response or uniformity of application across the spreading width, could also explain the results of this study and other studies.
Lack of yield differences between UR and VR, poor relationships between STP and crop response, and some unexpected results for STP changes after fertilization could also be explained by high small-scale STP variability and (or) inadequacy of the soil-sampling methods used. Previous research based on denser sampling methods showed very high small-scale variability in STP in Iowa fields (Cambardella et al., 1994; Mallarino, 1996; Borges and Mallarino, 1998; Mallarino and Wittry, 2000). Poor relationships between crop response and initial STP even in Fields 5 and 6 where a small cell size (0.2 ha) was used for the initial soil sampling allude to the problem created by high small-scale STP variation. Although grid-point soil sampling is the sampling method used most frequently in the Corn Belt for VR fertilization, the results suggest that this method may need to be re-examined. For example, Borges and Mallarino (1998), Mallarino and Wittry (2000), and Schepers et al. (2000) showed that collecting samples from different parts of a grid cell often results in very different soil-test values and suggested that soil sampling based on grid distances even smaller than those that are economically affordable to farmers may not provide reliable information.
Calculations from data in Table 3 show that VR applied less P than UR in all fields (23, 19, 12, 19, 41, and 31% less P in Fields 1–6, respectively). Thus, this study shows that use of VR could result in fertilizer savings although these results cannot be directly extrapolated to other fields because differences in P applied are affected by STP levels and within-field STP variation. Also, this advantage for VR could be offset by increased application costs and likely increased soil-sampling costs as others have shown (Anderson and Bullock, 1998; Lowenberg-DeBoer and Aghib, 1999; Yang et al., 2001). However, the results of our study showed that reduced P fertilizer application to high-testing field areas and lower STP variability from VR compared with UR improves P fertilizer management. A cutback in P application to high-testing field areas could reduce P loss to surface water resources because research has shown that P loss with surface runoff increases with increasing STP (Sharpley, 1995; Pote et al., 1999; Klatt et al., 2003).
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CONCLUSIONS
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Phosphorus fertilization increased crop grain yield only (but not always) in fields or field areas with STP below optimum for corn and soybean production according to Iowa interpretations (16–20 mg kg–1). The Clarion soil series was more responsive to P than other soils in four fields, a difference that may have resulted from lower STP and pH in two fields but that could not be explained by soil testing in other fields. The method of fertilizer application did not influence crop responses to P fertilization. Use of current P fertilizer recommendations that encourage STP buildup in low-testing soils combined with high small-scale STP variation may explain the lack of difference between fertilization methods. Use of the VR fertilization method resulted in better P fertilizer management because it applied 12 to 41% less P and reduced STP variability compared with the traditional UR fertilization method and could reduce P loss to surface water resources.
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NOTES
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Iowa Agric. Home Econ. Exp. Stn. Journal Paper no. J-19876. Project 4062. Project supported in part by the Iowa Soybean Promotion Board and the Leopold Center for Sustainable Agriculture.
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ABSTRACT
INTRODUCTION
