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Published in Agron. J. 96:776-785 (2004).
© American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA

PRODUCTION PAPERS

Corn Response to Starter Fertilizer and Tillage across and within Fields Having No-Till Management Histories

Manuel Bermudez and Antonio P. Mallarino*

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

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

Received for publication July 11, 2003.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Corn (Zea mays L.) early growth often is slower in no-tilled than tilled soils. Starter fertilization usually increases early plant growth but has inconsistent effects on grain yield. This study assessed (i) starter and tillage effects on corn grain yield, dry weight (DW), and N, P, and K uptake at the V5–V6 stage and (ii) the within-field variation of responses. Seven replicated strip trials were conducted on fields previously managed with no-tillage using yield monitors and global positioning systems (GPS). Treatments were no-starter and liquid starter with or without tillage and were applied in addition to farmers' normal broadcast NPK rates. Starter applied 3.9 to 27.2 kg N ha–1, 5.2 to 24.2 kg P ha–1, and 0 to 4.1 kg K ha–1 across fields. The tillage treatment was spring disking or strip tillage. Tillage increased (P ≤ 0.05) yield in four fields (210–500 kg ha–1), starter increased yield in three fields (93–522 kg ha–1), and both treatments usually increased DW and nutrient uptake. There were no treatment interactions. Tillage and starter fertilization did not influence yield variability but increased DW and nutrient uptake variability. Soil test results (P, K, pH, and organic matter) or soil series could not always identify fields where starter would increase yield. However, in two fields, starter increased yield only in areas with Bray-1 soil P < 16 mg P kg–1. On average, tillage increased yield 2.5%, starter increased yield 1.1%, and DW or nutrient uptake responses to either treatment were 20 to 30%. Large DW and nutrient uptake responses to starter did not translate into large or frequent yield responses.

Abbreviations: ANOVA, analysis of variance • DGPS, differential global positioning systems • DW, dry weight • GIS, geographical information systems • NNA, nearest-neighbor analysis • OM, organic matter • STK, soil test potassium • STP, soil test phosphorus


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
ADOPTION OF NO-TILL management in the Corn Belt increased rapidly during the early 1990s, but the adoption rate has decreased (Conserv. Technol. Inf. Cent., 2000). Reduced adoption of no-till may be attributed to lower corn yield and slower early growth when compared with corn grown in tilled soils (Mallarino et al., 1998). Advantages of no-till management include less soil erosion and better water conservation (Jones et al., 1969; Blevins et al., 1971). Slower early growth of spring-seeded crops with no-till management in humid regions may be attributed to residue cover that results in cooler and wetter soils in spring and creates conditions that can reduce early nutrient uptake and growth (Al-Darby and Lowery, 1987; Imholte and Carter, 1987; Swan et al., 1987; Kaspar et al., 1990). Residue removal along rows of no-till corn often increases early corn height and produces development rates similar to those for conventional tillage (Kaspar et al., 1990; Fortin, 1993).

Starter fertilization (usually as an NPK mixture) is a common practice used in some areas of the USA to improve nutrient uptake and early crop growth, even in soils testing high in P and K. Granulated or liquid starter mixtures are applied in bands beside and below the seeds or in the seed furrow. Although potential benefits of starter fertilization as a complement to base fertilization are well documented (Touchton, 1988; Mengel et al., 1992), there is uncertainty concerning the frequency and magnitude of yield responses. The response to starter fertilizer is more likely with reduced tillage. For example, Mengel et al. (1992) found that starter fertilization increased corn yield in only one site under conventional tillage but in eight sites under no-till management in Indiana. Wolkowski (2000) reported corn yield responses to starter fertilizer in soils testing high in P and K when corn was managed with no-till but not with conventional tillage. Vyn and Janovicek (2001) showed that corn yield increases to starter-applied K were larger with no-till than with tillage.

The response to starter fertilizer usually is attributed to N or P in the mixture (Randall and Hoeft, 1988; Ritchie et al., 1995). Rehm et al. (1988) reported that the magnitude of corn growth and yield responses to starter fertilization was larger in soils with soil test P (STP) below values considered optimum for broadcast fertilization in Minnesota (values were not specified) but also observed significant responses in high-testing soils during a cool and wet spring season. Bordoli and Mallarino (1998) found significant no-till corn yield increases to granulated P fertilizer broadcast, deep-banded (15-cm depth), and planter-banded (5 cm beside and below the seeds) in Iowa soils testing low in STP (<16 mg P kg–1, Bray-1 test) but no differences among placement methods. Only deep-band K increased yield in soils that tested optimum or higher in K according Iowa recommendations at that time (>90 mg K kg–1, ammonium acetate test). Scharf (1999) found larger responses to NP starter fertilizers compared with N-only starter in sites where STP was below levels considered optimum in Missouri (<17 mg P kg–1, Bray-1 test) but little or no response to NP starter when STP was optimum or higher. Vetsch and Randall (2002) reported that NPK starter mixtures increased corn yield across various tillage systems even in soils with P and K above levels considered optimum or higher.

Within-field yield variability is large in most fields. Factors producing yield variability include variations in nutrient levels and many other soil chemical or physical properties. These factors also may influence the response to fertilization. Yield monitors, differential global positioning systems (DGPS), and geographical information systems (GIS) are essential to describe yield and yield response variability over the landscape. Bermudez and Mallarino (2002) used these tools to study within-field variation of no-till corn response to starter fertilization in Iowa. They showed that yield responses to starter fertilization were larger and more frequent when STP was below optimum for corn (<16 mg P kg–1, Bray-1 test) and that responses often were due to starter N even though N fertilizer was applied across all treatments at recommended rates. Wittry and Mallarino (2003) used similar methods to study the within-field variation of corn and soybean [Glycine max (L.) Merr.] early growth and yield response to broadcast P fertilization in several Iowa fields. They reported that the procedures were successful in identifying larger yield response to P in low-testing field areas, that yield responses often were different across soil series, and that early growth responses to P fertilization were unrelated to STP or soil series variation.

Methods based on precision-agriculture technologies such as those used by Bermudez and Mallarino (2002) and Wittry and Mallarino (2003) can be used to study crop response to both tillage and starter fertilization and the within-field variation of the responses. The objectives of this study were to assess (i) starter and tillage effects on corn grain yield, early DW, and early N, P, and K uptake in fields having histories of no-till management and (ii) the variation in yield and early growth responses for field areas with different soil test levels and soil series.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Seven strip trials were conducted in Iowa during 1998 and 1999 to evaluate corn yield, early growth, and nutrient uptake responses to starter fertilizer and spring tillage. Trials were established on farmers' fields with 8- to 14-yr histories of no-till management and corn–soybean rotations. Table 1 provides information about field locations and the two predominant soil series in each field. Approximately 12 to 20 ha located at least 40 m from field borders were used to establish replicated strip trials. Each area was divided across future corn rows into blocks that ranged from 60 to 90 m in width, which were further subdivided into four strips to fit two tillage treatments and two starter fertilization treatments. These blocks corresponded to the replicates (three replicates in Field 4 and four in other fields) of a split-plot design, in which the tillage treatments were in large plots and the starter fertilization treatments were in the subplots (strips). Strip lengths were uniform within each field but ranged from 270 to 600 m across fields. Strip widths were also uniform within each field but ranged from 12 to 24 m across fields to accommodate different planter widths. A 16-row planter set for a 76-cm row spacing was used for Fields 1, 2, 4, 5, and 7. An eight-row planter set for a 96-cm row spacing was used for Fields 3 and 6. Measurements were made with a measuring tape or wheel, and georeferences were recorded with a hand-held DGPS receiver.


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  		Table 1. Field locations, predominant soils series, and selected mean soil test values for seven field trials.

 
Crop management practices, including broadcast N, P, and K fertilization, were those normally used by each farmer and varied among fields. Except for Field 4, which received no P or K fertilization since 1996, farmers broadcasted P and K fertilizers over the entire field 1 to 4 mo before applying the tillage treatments. Rates varied across fields from 35 to 70 kg P ha–1 and 90 to 120 kg K ha–1. At Fields 1, 2, 3, 4, and 6, no preplant N fertilizer was applied, and farmers injected 28% urea ammonium nitrate solution or anhydrous ammonia across all treatments when corn was 20 to 25 cm tall at rates that were 100 to 145 kg N ha–1 across fields. At Fields 5 and 7, anhydrous ammonia was injected into the soil in November of the previous year at a rate of 170 kg N ha–1.

The two tillage treatments were no-till and spring tillage. In Fields 1, 2, 4, 5, and 7, the tillage treatment was applied 1 to 2 wk before planting by disking to a depth of 10 to 15 cm. In Fields 3 and 6, the tillage was done 4 wk before planting with a strip-tillage tool that tilled zones 15 to 18 cm in width to a 10- to 15-cm depth at 96-cm spacings. No fertilizer was applied with the tillage operations. Corn was planted into the tilled zone. The starter treatments were no starter and starter. The starter mixtures (fluid commercial fertilizers) varied across fields. Table 2 provides information about corn hybrids, seeding rates, and starter fertilizer rates for each field. The starter was applied into the seed furrow in Fields 1, 2, 3, 5, and 6 and 5 cm beside and below the seeds in Fields 4 and 7.


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  		Table 2. Corn hybrids, planting dates, seeding rates, and starter mixtures and rates used for seven trials.

 
Soil samples were collected immediately before planting corn (after tillage or broadcast fertilization) using a systematic grid-point sampling approach (Wollenhaupt et al., 1994). The spacing between grid lines across the future corn rows coincided with the width of the replications (60–90 m) and was 24 to 36 m along crop rows. Soil samples (10–12 cores, 15-cm depth) were collected from an 80- to 100-m2 area located at the center of each cell. Samples were analyzed for P (Bray-P1 test), K (ammonium acetate test), organic matter (OM) (Walkey–Black test), and pH (1:1 soil/water ratio) following procedures recommended for the North Central Region (Brown, 1998). Iowa State University interpretation classes for corn (Sawyer et al., 2002) were used to classify STP and soil test K (STK) values.

The aboveground portion of corn plants was sampled in Fields 1, 2, 3, 4, and 6 at the V5 to V6 growth stage (Ritchie et al., 1986). Ten plants were collected from the center of cells defined by the treatment strip width and the distance between soil sampling grid lines along the crop rows. Plant samples were dried in a forced-air oven at 65°C, weighed, and ground to pass a 2-mm screen. Total N, P, and K in the tissue were measured by digesting samples with H2SO4 and H2O2 (Digesdahl Analysis System, Hach Inc., Boulder, CO). Total N in the digests was determined with a steam distillation procedure (Bremner, 1960) in 1998 and with a colorimetric procedure (Hach et al., 1985) in 1999. Phosphorus in the digests was determined colorimetrically (Murphy and Riley, 1962) and K by flame photometry. Plant nutrient content is expressed on an oven-dried basis.

Grain yields were measured using farm combines equipped with commercial, impact flow-rate yield monitors and real-time DGPS receivers. The yield monitors were calibrated in adjacent areas of each field following manufacturer's recommended procedures. Differential corrections were obtained through the U.S. Coast Guard AM signal. Yield data were unaffected by field borders because the experimental areas were at least 40 m from any fence or border rows. Each combine trip (a 4.5-m swath) was identified with a unique number that was recorded with the georeferenced yield data. Only yield averages for each treatment strip could be recovered from the electronic card of the yield monitor used in Field 1. At other fields, yield data were recorded at 1-s intervals. The yield data were imported into ArcView GIS (Environ. Syst. Res. Inst. Inc., Redlands, CA) for GIS management and the SAS statistical package (SAS Inst., 2000) for statistical analyses. The raw data were analyzed for common yield monitor problems such as effects of grass waterways or combine stops, and affected data were deleted. ArcView GIS was used to calculate the mean yield for each treatment strip and for portions corresponding to field areas with different soil series or soil test values. The maps in Fig. 1 show examples of the strip trial methodology used and maps generated using ArcView.



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  		Fig. 1. Example of field and geographical information system (GIS) methods used: treatment strips replicated four times (randomized complete block, split-plot design), soil series, and grid soil-sampling cells with values for various soil tests.

 
Treatment effects on grain yield for the entire experimental area of each field were assessed with analysis of variance (ANOVA) for a randomized complete block, split-plot design combined with nearest-neighbor analysis (NNA) to account for spatial correlation of yield. The NNA could not be used for Field 1 because only strip means were recovered from the yield monitor card. This procedure was described by Hinz (1987) and Hinz and Lagus (1991) for small-plot trials, adapted to strip trials harvested with yield monitors by Mallarino et al. (1999b), and used by Bermudez and Mallarino (2002) and Wittry and Mallarino (2003). Yield input data were means of yield monitor points for areas delineated by the width of the combine head (4.5 m) and the length of the soil sampling cell along crop rows (24–36 m across fields). The individual data points are not directly considered because of known diminished accuracy of yield monitors over distances shorter than 30 to 40 m (Lark et al., 1997). A covariate value corresponding to each yield observation was calculated using NNA from the mean yield value of four neighbors of each yield observation (one from each north, south, east, and west coordinate).

The treatment effects on yield for field areas having soil test values within different interpretation classes or soil series were assessed using a procedure described by Oyarzabal et al. (1997) and recently used by others (Mallarino et al., 2001; Bermudez and Mallarino, 2002). This analysis was not conducted for Field 1 because only strip yield means were recovered. ArcView GIS software was used to produce the input data for these analyses. The soil test data were the initial values from grid-sampling cells defined by the width of each replication and the distance between grid lines along crop rows. Yield treatment means (four) were calculated for the same areas. Soil test values were classified into the five Iowa interpretation classes for STP and STK (Sawyer et al., 2002). Arbitrary classes were used for pH (<5.5, 5.5–6.2, 6.3–7.0, and >7.0) and OM (<30, 30–40, and >40 g kg–1) because Iowa State University does not provide interpretation classes for pH and OM. For analysis of responses by soil series, each yield value was matched by the corresponding soil series from digitized (scale 1:12 000) soil survey maps (ICSS, 2003). The analysis was performed for the two predominant soils of each field because areas for other soils were small. Thus, each line of the data set for each field consisted of a soil-sampling cell code, soil test values, a yield value for each treatment (four), and a set of codes representing the interpretation classes for the soil tests and the soil series. The F test from a one-way ANOVA was used to estimate the consistency of treatment effects for each soil test interpretation class (for STP, STK, pH, and OM) and for each soil series. The numerator mean square (between groups) represented variation introduced by the treatments, and the denominator mean square represented variation within groups (cells with a similar classification). Results from these analyses were not used when there were less than three cells for any soil test class or soil series within a field. Treatment effects on corn early growth (plant DW) and N, P, and K uptake were analyzed in the same manner as described for yield. However, for these measurements, data were derived from one sampling point from each small cell defined by the width of each treatment strip and the separation distance of grid sampling lines along crop rows.

The within-field variability of crop measurements for the tillage and starter fertilization treatments was compared using Bartlett's test (Steel and Torrie, 1980). The geostatistical software used was S-Plus version 6.0 (Insightful Corp., Seattle, WA). Practical geostatistical methods for agriculture, terminology, and semivariance models were summarized by Marx and Thompson (1987), and more comprehensive information is provided by Journel and Huijbregts (1978). One unidirectional semivariogram (along crop rows) was calculated for all strips of each treatment. Minimum lag distances were 15 m for yield (for this analysis, yield monitor points were averaged over a 15-m distance) and 45 m for early growth and nutrient uptake measurements (the sampling distance along corn rows). Maximum lag distances were 60% of the strip length (120–320 m depending on the field). Although the software fits various models to the sample semivariograms, the spherical model was the best-fitting model in most instances and is the only one presented. This model estimates nugget, sill, and range parameters. The nugget and sill semivariances were used for this study. The nugget represents random variation and structured variability at a scale smaller than the minimum distance between sampling points. As the distance between sample points increases up to a certain value (range), the semivariance increases from the nugget value (or zero) toward a maximum value (the sill).


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Soil Test Values of the Fields in Relation to Potential Crop Response
Most fields (except Field 3) had mean STP (Table 3) at or above the Iowa Optimum interpretation class for corn production (16–20 mg P kg–1). All STP values within Fields 1 and 7 were Optimum or higher, whereas at other fields, STP ranged from Very Low or Low to Very High. All fields had mean STK at or above optimum levels for corn (131–170 mg K kg–1). All STK values within Fields 3 and 6 were optimum or higher; at other fields, STP spanned the Very Low or Low classes to the Very High class. The median STP and STK values for the fields (Table 3) were only slightly lower than mean values and in most fields were classified into the same interpretation classes as the mean values. Corn response is likely (>25% probability) in the low-testing classes, less likely (<25%) in the Optimum class, and unlikely (<5%) in high-testing soils (Sawyer et al., 2002). Therefore, except for Fields 1, 4, and 7 for P and Fields 3 and 6 for K, the fields had areas with STP or STK values for which large corn response to fertilization would be expected. The pH data indicated that most fields had acidic areas, but lime would be recommended for corn only at Field 4 according to Iowa State University interpretations (Sawyer et al., 2002). Mean OM values ranged from 35 to 50 g kg–1 across fields and varied markedly within each field. In this region, the higher OM values usually are associated with moderately to poorly drained soils at lower landscape positions.


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  		Table 3. Descriptive statistics for soil test P, K, pH, and organic matter.{dagger}

 
Field Average Crop Responses
Grain Yield and Moisture
Tillage increased (P ≤ 0.05) grain yield in four fields, and the response ranged from 251 to 498 kg ha–1 (Table 4). Neither the strip tillage in Field 6 nor the tillage done by disking in Fields 1 and 4 increased yield compared with no-till. The response to strip tillage in Field 3 was comparatively similar to tillage done by disking in other responsive fields. Vetsch and Randall (2002) also reported inconsistent yield differences for corn managed with no-till or strip tillage although instances in which strip tillage increased yield compared with no-till were more numerous. The methods used in this study do not allow for supported explanations of tillage effects because important factors such as residue cover, soil temperature, and other soil physical properties were not measured.


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  		Table 4. Corn grain yield response to tillage and starter fertilization for seven trials.

 
Starter fertilization increased (P ≤ 0.05) yield from 93 to 522 kg ha–1 in three fields and decreased yield slightly (97 kg ha–1) in one field (Table 4). The yield reduction is not reasonable and was attributed to Type I random error (resulting from rejecting the null hypothesis when it was true) because the small rate of NP starter fertilizer applied in the furrow did not decrease plant population (not shown) and increased early corn growth and nutrient uptake (discussed below). A tillage x starter interaction was not observed (P ≤ 0.05) at any field, which indicates that the starter effect was proportionally similar for both tillage treatments. Across the seven fields, tillage increased yield 2.5% (P ≤ 0.05), and starter fertilization increased yield 1.1% (P ≤ 0.06).

Grain moisture was not influenced (P ≤ 0.05) by tillage or starter fertilization in any field (data not shown). However, across all fields, analyses showed that tillage reduced grain moisture by 3 g kg–1, and starter fertilizer reduced it by 5 g kg–1. Studies in more northern regions (Minnesota and Wisconsin) have shown that starter fertilization can have a larger effect on grain moisture (Vetsch and Randall, 2002; Wolkowski, 2000).

Early Plant Growth and Early Nutrient Uptake
Tillage increased (P ≤ 0.05) early plant DW in five fields, and starter fertilization increased yield in all fields (Table 5). The interaction between tillage and starter treatments was not significant in any field. Several studies (Al-Darby and Lowery, 1987; Imholte and Carter, 1987; Vetsch and Randall, 2002) have shown that early corn growth in conventional tillage is greater than with no-till. The largest plant DW responses to tillage were observed in Fields 1 and 5 (approximately 50%).


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  		Table 5. Early plant dry weight (DW) (V5 to V6 growth stage) response to tillage and starter fertilization for seven strip trials.

 
The treatments seldom influenced early plant N, P, and K concentrations (data not shown). Tillage decreased (P ≤ 0.05) plant N concentration in Fields 5 and 7, decreased P concentration in Field 3 and increased it in Field 5, and did not influence K concentrations. Starter fertilization did not influence plant N concentration, decreased P concentration in Fields 3 and 7, and did not influence K concentration. These inconsistent results were not surprising because fertilization effects on nutrient concentrations in plant tissue often are masked or diluted by relatively larger effects on plant growth (Mallarino et al., 1999a).

Tillage increased N uptake in five fields, and starter fertilization increased N uptake in most fields (Table 6). The responses to strip tillage in Fields 3 and 6 were large (48 and 41 mg plant–1, respectively). These responses were comparable to responses in other fields where the tillage was done by disking. Tillage effects on N uptake could have resulted from increased soil N mineralization, increased N uptake resulting from increased plant growth, or a combination of both processes. On average, tillage and starter fertilization increased (P ≤ 0.05) N uptake 30 and 21%, respectively.


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  		Table 6. Early plant N uptake (V5 to V6 growth stage) response to tillage and starter fertilization for seven strip trials.

 
Tillage increased P uptake significantly in Fields 1, 5, and 7 (Table 7). Tillage also increased early plant growth in two of these fields (Fields 5 and 7) and increased P concentration in Field 5. Previous research (Mackay et al., 1987) showed that tillage can increase P availability and consequently early P uptake when compared with untilled soils. However, the tillage effect on P uptake could also be an indirect result of more favorable conditions for early plant growth. Starter fertilization increased P uptake in all fields. Relative responses were higher in Fields 3 and 6 (5.4 and 4.2 mg plant–1, respectively) where median STP was Low and the tillage treatment was strip tillage. On average, starter fertilization and spring tillage increased (P ≤ 0.05) mean P uptake 30 and 20%, respectively.


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  		Table 7. Early plant P uptake (V5 to V6 growth stage) response to tillage and starter fertilization for seven strip trials.

 
Tillage increased (P ≤ 0.05) early plant K uptake in four fields, and starter fertilization increased K uptake in all fields (Table 8). These responses were attributed to tillage-induced increases in early plant growth. Only the starter fertilizer used in Fields 1, 2, and 7 actually contributed K, and the rates applied were small.


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  		Table 8. Early plant K uptake (V5 to V6 growth stage) response to tillage and starter fertilization for seven strip trials.

 
Crop Response Variability
Tillage and starter fertilization influences on corn yield variability were inconsistent (Table 9). The yield variability was influenced (P ≤ 0.05) by the treatments only in Fields 4 and 5. In Field 4, tillage increased yield variability, and starter did not influence it. In Field 5, the tillage–starter treatment combination increased yield variability compared with the other treatments. Nugget semivariances were very small compared with sill semivariances in most fields, which indicates large spatially structured variation, but neither parameter was consistently affected by the treatments across fields. No theoretical reason clearly indicates an increase or decrease of yield or growth variability when tillage or starter fertilization is used, but some Corn Belt producers believe these practices would reduce yield variability across a field. Our results did not support this expectation.


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  		Table 9. Corn grain yield variability and spatial structure for the tillage and starter fertilization treatments.

 
The within-field variation of corn early plant DW and nutrient uptake was affected by tillage and starter treatments in five fields. Because of the numerous data involved (four treatments, three plant measurements, and seven sites), only data for plant DW are shown in Table 10. The within-treatment variation was largest (P ≤ 0.05) for the tillage–starter combination. Except for Field 7, the smallest variation was for the no-till without starter fertilization treatment. Early plant DW and nutrient uptake variability was not consistently influenced by the treatments and usually had less spatial structure than grain yield (as indicated by larger nugget semivariance compared with sill semivariance).


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  		Table 10. Early plant dry weight (DW) (V5 to V6 growth stage) variability and spatial structure for the tillage and starter fertilization treatments.

 
Summary Discussion for Whole-Field Crop Responses
Tillage or starter fertilization increased yield in three of seven fields, and a tillage x starter interaction was not observed. These results indicate that tillage did not reduce the magnitude or frequency of crop response to starter fertilization. Larger and more frequent no-till corn responses to starter fertilization were expected because soil tilth and partial removal of crop residues have been reported to increase early plant growth. In Fields 2 and 5, the tillage treatment was applied immediately before planting, and this practice could explain a lack of tillage x starter interaction. However, there was no significant interaction for the other fields where the tillage treatment was applied 2 to 4 wk before planting. Across all fields, tillage increased yield by 2.5%, early plant DW by 27%, P uptake by 20%, and N uptake by 21%. Starter fertilization increased early plant DW and nutrient uptake in all fields. Across all fields, starter increased yield by 1.1% (only at P ≤ 0.06) and early plant growth and N or P uptake by approximately 30%. These results indicate that starter fertilization and spring tillage increase corn early growth and nutrient uptake. However, these early crop responses may not be reflected in a grain yield response. The results also showed that within-field yield variability seldom was affected by tillage or starter fertilization, but early plant DW and nutrient uptake variability often were higher for the tillage–starter treatment combination.

The yield response to starter fertilization could not be explained solely on the basis of mean or median soil test values. The observed yield response to starter fertilization in Fields 2 and 4 was reasonable because either STP and (or) STK of large areas were within a responsive soil test interpretation class. However, the yield response in Field 7 cannot be explained by STP and STK because values were mostly high. Although unidentified factors may have determined the response to starter in this field, we suggest that the starter N was responsible for the response because the N rate was the largest among all fields (27.2 kg N ha–1), and the uniform N rate (157 kg N ha–1) was applied 5 mo before planting. Previous research showed that responses to NPK starter often were explained by N in the mixture (Randall and Hoeft, 1988; Scharf, 1999).

Early plant DW responses to starter fertilization (which were observed in all fields) were not related to mean or median soil test values of each field. The two largest early plant DW responses (1.14 and 1.22 g plant–1) were observed in fields with Low or Very High median STP and Very High median STK (Fields 3 and 7). The two smallest responses (0.24 and 0.47 g plant–1) were observed in fields with Optimum or higher-than-Optimum STP (Fields 2 and 4) or borderline between Low and Optimum STK (Field 4). Other studies found that early growth response to starter fertilization often does not directly relate to soil test values (Mengel et al., 1992; Randall and Hoeft, 1988; Bermudez and Mallarino, 2002).

Analysis of Crop Responses for Different Areas within Each Field
Crop Response to Starter Fertilization for Field Areas with Different Soil Test Values
Study of yield response to starter fertilization for field areas with STP or STK testing within different interpretation classes revealed different within-field yield response in Fields 2 and 3 (data not shown). In these fields, starter fertilization increased (P ≤ 0.05) yield only in areas testing Very Low or Low in STP. This result indicates that the whole-field response observed for Field 2 (Table 4) probably was the result of yield responses only in low-testing field areas. The significant response to starter in low-testing areas of Field 3 is in contrast to a lack of significant whole-field response (Table 4) and indicates that this response was not large enough to offset a lack of response for other field areas. The yield response to starter in low-testing field areas of these fields and the whole-field response in the predominantly low-testing Field 4 confirm expected corn response to starter or P fertilizer in low-testing soils shown in previous studies of within-field responses (Bermudez and Mallarino, 2002; Wittry and Mallarino, 2003). However, the whole-field response to starter in Field 7 and a lack of differential response across areas of this field and other fields (not shown) indicate that the yield response to starter cannot be accurately predicted for high-testing soils.

Early plant DW and early P uptake responses to starter fertilizer analyses for areas grouped according to the STP interpretation classes showed that statistically significant responses were observed across all classes (data not shown). These results indicate that starter fertilization increased early plant DW independently from the STP level, which agrees with the results for whole-field analyses and previous work (Welch et al., 1966; Randall and Hoeft, 1988; Rehm et al., 1988; Bermudez and Mallarino, 2002).

Crop Responses to Starter Fertilization and Tillage for Soil Series within a Field
The yield response to tillage differed for the two dominant soil series within a field only in Field 7 (Table 11) where there was a response in areas with Dinsdale soil but not in areas with Klinger soil. The Dinsdale series is a well-drained loam or silty clay loam soil of upland and moderately sloping topographic positions while the Klinger series is described as a somewhat poorly drained, silty loam soil found in upland positions with small slope (ICSS, 2003). Yield responses to starter fertilization differed between soil series only in Field 4 (Table 11). In this field, the starter fertilization increased yield in areas with Dickinson soil and not in areas with Klinger soil. The Dinsdale series is classified as an excessively well-drained soil found in upland positions with moderate slope (ICSS, 2003). Soil test P or K for these soil series do not explain the differential responses because levels were High or Very High. We expected larger yield response to both starter fertilization and tillage in low-laying areas with fine-textured and poorly drained soils than in well-drained soils of upland positions because these conditions may result in comparatively cooler soil temperatures and lower nutrient availability in early spring.


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  		Table 11. Corn yield response to tillage and starter fertilization for the two predominant soil series for five fields.

 
Early plant DW and nutrient uptake responses to tillage differed between soil series (P ≤ 0.05) in Fields 1, 2, 6, and 7 (Table 12). Because nutrient uptake responses were explained by plant DW differences, only plant DW data are shown. In Fields 1, 2, and 7, tillage increased early plant DW in areas with Maxfield, Donnan, or Dinsdale soils but not in areas with Muscatine, Kenyon, or Klinger soils. The Donnan and Dinsdale soils are well-drained to moderately well-drained loam or silty clay loam and are found in high and moderately to strongly sloping topographic positions. The Maxfield soil is a poorly drained soil found in nearly level landscape positions. Early plant DW and nutrient uptake responses to starter fertilization differed among soils series only in Fields 2 and 6. In Field 2, there was a response in the Donnan soil and no response in the Kenyon soil. The results for this field are difficult to explain with the methods used. Measured STP and OM were similar for these soils, and texture and internal drainage were also similar (ICSS, 2003). In Field 6, responses were observed in the Marshall soil (a well-drained soil found in high landscape positions) but not in the Colo soil (a poorly drained soil found in low landscape positions). The larger response for the Marshall soil is reasonable because it had much lower STP. However, larger early plant DW responses could have also been expected for the Colo soil because it is more poorly drained and could be colder in early spring. We expected larger early growth response to both starter fertilization and tillage in field areas with low-laying and poorly drained soils that may result in cool soil temperatures and low nutrient availability in early spring.


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  		Table 12. Early plant dry weight (DW) (V5 to V6 growth stage) response to tillage and starter fertilization for the two predominant soil series for seven fields.

 

   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Tillage increased grain yield, early DW, and early nutrient uptake of corn in three of seven fields. In responsive fields, the yield response was always proportionally smaller than the response of early growth and nutrient uptake. On average, tillage increased yield 2.5%, early growth 27%, and N or P uptake approximately 20%. Yield responses to starter fertilization were small, infrequent, and unaffected by tillage. Early growth and nutrient uptake responses to starter were much larger than yield responses and occurred even when there was no yield response. On average, starter increased yield 1.1%, early growth 29%, and N or P uptake approximately 30%.

Grain yield and early DW or nutrient uptake responses to starter were poorly related to STP or STK. A large yield response usually was observed when soil test values were below levels considered optimum for corn in Iowa but sometimes was also observed in high-testing soils. Yield responses in high-testing soils were partly attributed to the N or P in the starter mixture.

Tillage and starter fertilization seldom influenced yield variability. However, early growth and nutrient uptake variability was higher for the tillage–starter combination in five of the seven fields. Parameters of modeled semivariograms showed that the spatial structure of the variability of yield or plant measurements was not consistently affected by tillage or starter fertilization.

Overall, the results showed that spring tillage and starter fertilization for corn in Iowa fields that have been under no-till management resulted in infrequent and small grain yield responses and that tillage did not influence the yield response to starter. In contrast, starter fertilization increased early growth and early nutrient uptake in all fields. Large early growth and nutrient uptake responses to starter fertilization do not translate into large or frequent grain yield response.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Project 4062.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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