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Corn

Soil Test Calibration for Predicting Corn Response to Phosphorus in the Northeast USA

^a Dep. of Plant Biology and Pathology, 59 Dudley Road-Foran Hall, Cook College, New Brunswick, NJ 08901-8520
^b Plant and Soil Science Dep., 215 Hills Agric. Science, Univ. of Vermont, Burlington, VT 05405
^c Dep. of Plant Science, U-67, Univ. of Connecticut, Storrs, CT 06269-4067
^d Dep. of Crop and Soil Sciences, 410 Agricultural Science and Industries Building, Penn State Univ., University Park, PA 16802
^e Agriculture and Natural Resources Dep., 163 Townsend Hall, Univ. of Delaware, Newark, DE 19716
^f Dep. of Natural Resource Sciences and Landscape Architecture, 214 H.J. Patterson Hall, Univ. of Maryland, College Park, MD 20742
^g Dep. of Plant and Soil Sciences, Bowditch Hall 207, Univ. of Massachusetts, Amherst, MA 01003-0910
^h USDA-ARS, NEPSWL, Room 23, Univ. of Maine, Orono, ME 04469
ⁱ Plant, Soil and Environmental Sciences, Room 409A, 5722 Deering Hall, Univ. of Maine, Orono, ME 04469-5722
^j Univ. of Maine CE, 495 College Ave., Orono, ME 04473-1294
^k CELS-PLS, 214 Woodward Hall, Univ. of Rhode Island, Kingston, RI 02881
^l Division of Plant and Soil Sciences, West Virginia Univ., P.O. Box 6108, Morgantown, WV 26506-6108
^m Plant Biology Dep., Spaulding Hall, Univ. of New Hampshire, Durham, NH 03824
ⁿ Crop and Soil Sciences, 1004 Bradfield Hall, Cornell Univ., Ithaca, NY 14853

^* Corresponding author (heckman{at}aesop.rutgers.edu)

Received for publication April 26, 2005.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
The consensus of soil fertility specialists working in the northeast USA was that soil testing and recommendation systems for P needed to be reexamined because of recent changes in soil testing methodology in the laboratory and corn (Zea mays L.) production technology in the field. Soil tests (M-COL, MM-COL, B-ICP, M1-ICP, and M3-ICP) were performed by either colorimetry or inductively coupled plasma (ICP) emission spectroscopy on samples from soil test calibration studies conducted during 1998 to 1999 at 51 experimental sites chosen to represent a range of soils, including Ultisols, Spodosols, and Alfisols, in northeastern states (Connecticut, Delaware, Massachusetts, Maryland, Maine, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, Vermont, and West Virginia). The mean P measured by M-COL, MM-COL, B-ICP, M1-ICP, and M3-ICP was 8.3, 6.6, 148, 66, and 121 mg P kg^–1, respectively. Production practices followed local state extension recommendations at each site and included P fertilizer treatments: none, 15 kg P ha^–1 banded, or 60 kg P ha^–1 broadcast. Combined analysis of variance over sites showed that plant height at 35 d after planting, silk emergence, grain yield, and grain dry down were enhanced by the broadcast P treatment. There were yield increases (P < 0.10) to the band treatment at only four sites and to the broadcast treatment at nine sites. Cate-Nelson statistical analysis of relative yield in relation to soil test P failed to identify soil test P critical levels for any of the soil test methods. The percentage of experimental sites that had soil test P levels below the currently used critical levels in the region ranged from 14 to 65% of the sites. Results showed that 17 to 47% of those sites testing below the critical level exhibited a yield increase (P < 0.10) to broadcast P. Some of the yield responsive sites had soil test P above currently used critical levels. The calibration data obtained from the present study and the relationships examined between soil test P and relative yield do not necessarily validate the currently used soil test P critical levels nor does the data enable much refinement. This study shows that the current critical levels frequently permit both types of errors in soil test prediction; indicating a need for P fertilization when it may not be needed and not indicating a need for P fertilization when it may be needed. The second type of error is usually avoided by recommendations for crop removal rates of P.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
WHEN soil testing programs began around the middle of the 20th century, collection of soil test field calibration data was a major research activity (Hanna and Flannery, 1960; Hanway, 1963). A considerable amount of data was collected and became the working and living knowledge of practicing agronomists of the era. Much of the data that was collected was kept in personal files and seldom published in peer reviewed journals. As a consequence, much of this body of soil test information was not effectively transferred to the next generation of agronomists.

Although it is widely recognized that soil test information and fertilizer recommendations require continual updating and reevaluation (Peck and Soltanpour, 1990), there has been little current national or regional emphasis on soil test calibration research with the possible exception of the presidedress soil nitrate test (PSNT) (Magdoff, 1991; Heckman, 2002). This may in part be because soil test calibration research is perceived academically as lacking originality and as low priority, and grant funds to carry it out are limited. Yet its importance to farmers and the environment cannot be denied (Sharpley et al., 1994).

Federal and state support for soil test calibration research has greatly diminished. A symposium held at the American Society of Agronomy meetings (Voss, 1996) concluded that "if resources became available, it is strongly suggested that the research be done regionally as opposed to each individual state and that current data management systems be used so that at any future date the data and analysis can be reviewed and that research data can be added to the data bank."

There are five extractants that are widely used by university and private soil test laboratories that operate within the northeast region of the USA, and the soil test P critical levels associated with the various extractants vary among individual states (Table 1). Several university soil test laboratories within the region have recently adopted the Mehlich-3 soil test based on correlations with previous soil test methods and with very limited field calibration research conducted on local soils. Another recent development in soil testing is the analysis of the soil test extractions by ICP in place of colorimetric analysis (Mallarino, 2003). Thus, given the developments in crop production technology, increasing corn yield levels, and soil test laboratory methodology, it is imperative that soil test calibration for P be revisited.

In most states P fertilizer recommendations are based solely on a P soil test; but in Vermont, Modified Morgan-extractable Al is used to adjust P fertilizer recommendations (Magdoff et al., 1999). Modified Morgan-extractable Al ("Reactive" Al) was introduced into the Vermont soil testing system in the late 1970s based on research by Lee and Bartlett (1977). More recent work (Magdoff et al., 1999; Jokela et al., 1998) further supported the use of extractable Al as an indicator of the P fixing capacity of the soil. So, in the Vermont system, for a given soil test P below the critical level, the amount of P recommended increases as the amount of reactive Al increases (Jokela et al., 2004). Research in the province of Quebec (Khiari et al., 2000) examined use of extractable Al to improve P recommendations for potato (Solanum tuberosum L.). They found that the P/Al ratio extracted with Mehlich-3 extractant was a better indicator of both crop response and environmental risk than extractable P alone.

The objective of this northeast regional project was to evaluate corn responses to banded and broadcast P fertilizer using current crop production technology and soil test methods, and to determine how well the currently used soil test P critical levels are able to predict when corn responses to P fertilization should or should not be expected.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Field experiments were conducted on P fertilization of corn grown at 64 experimental sites over a 12 state area (Connecticut, Delaware, Maine, Maryland, Massachusetts, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, Vermont, and West Virginia) during 1998 and 1999. Of the 64 sites, 13 sites were deleted from the study for various reasons such as animal damage or weather-related crop failure. Most of the trials were in farmers' fields, but six were conducted on experiment station land. Table 2 lists experimental sites and soils. Crop management practices were those normally used by the farmer (except P fertilization), provided they were in agreement with local extension recommendations and nonlimiting rates of N and K fertilizers were applied. Fields with a recent (>5 yr) history of manure application were excluded from use in this study. Each experimental site included treatments of 0, 15 kg ha^–1 P banded, and 60 kg ha^–1 broadcast as triple superphosphate. The banded P was placed 5 cm beside and 5 cm below the seed. Plots consisted of four rows (row width was 76 cm) with a minimum length of 8 m. The treatments were replicated four times in a randomized complete block design. Corn was planted between 15 April and 6 June. Conventional tillage was used at all experimental sites. Fertilizers were broadcast and incorporated immediately before planting by disk or chisel plow. Soil samples were collected in the spring from the 0- to 15-cm depth by randomly collecting 15 cores (2.25-cm diam.) from each control plot. Soil samples were air-dried and crushed to pass a 2-mm sieve. The University of Delaware Soil Test Lab analyzed all samples for soil pH, organic C, and extractable P and Al by the Mehlich-1, Mehlich-3, and Bray-P1 methods. The same set of samples was also analyzed for extractable P and Al at the University of Maine Soil Test Lab by the Morgan method and the Modified Morgan soil test method. All soil test methods were performed following Recommended Soil Test Procedures for the Northeast (Sims and Wolf, 1995). All extractions were analyzed by ICP, but P extracted using Morgan and Modified Morgon solution was also determined colorimetrically.

All data were analyzed using the Statistical Analysis System (SAS Inst., Cary, NC). Fisher's Least Significant Difference (LSD) test was used for mean separation within each individual site. An analysis of variance was performed on the combined data using the PROC MIXED procedure. Sites, years, and blocks were considered random variables in the model because we were interested in making inferences for our fixed effects (treatments) for any year and site and not only for the sites and years represented by our data. Differences between means were tested using the Tukey-Kramer test of significance for the combined analysis.

Statistical significance was assessed at the 0.05 level except for corn grain and silage yield, which was assessed at 0.10. The Cate-Nelson method (Cate and Nelson, 1971) was calculated using the GLM procedure in the SAS statistical software (Goodnight et al., 1990) while selecting a horizontal critical level of 93% relative yield.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Across the 34 sites where the corn was harvested for grain, the yields ranged from 2.5 to 17.7 Mg ha^–1, and across the 17 sites harvested as silage, the yields ranged from 7.94 to 23.50 Mg ha^–1 (Table 3). Variation in rainfall and irrigation practice among sites were factors influencing crop yield. From a total of 51 experimental sites harvested as grain or silage, only nine sites exhibited yield increases (P < 0.10) from the 60 kg ha^–1 P broadcast fertilization and only four sites from the 15 kg ha^–1 P band treatment.

Early season corn plant growth increases to the P treatments were often visually apparent in the field, and these early growth differences were sometimes associated with crop yield increases. Thus, early season growth increases of corn to P fertilization may sometimes translate into increased crop yield, but this relationship is not strong, with an r value of 0.45. Previous research (Mallarino et al., 1999) has shown that early corn growth enhancements from banded P often do not translate into increased crop yield at maturity.

Tissue analysis of the earleaf revealed that, of the 37 sites that were sampled, the P concentration ranged from 1.93 to 3.45 g kg^–1. Only one site had an earleaf P concentration below the normal expected range of 2 to 4 g kg^–1 (Jones et al., 1990). Just two sites for the broadcast treatment and one site for the band treatment exhibited increases in tissue P concentration. The combined analysis of variance over sites for earleaf P concentration on the P treatments was not significant (Table 4).

Grain moisture content measured at harvest averaged 243 g kg^–1 for the controls, 243 for the band treatment, and 240 g kg^–1 for the broadcast treatment. Only two experimental sites for the broadcast treatment and one site for the band treatment exhibited reductions in grain moisture content due to the treatments but the combined analysis of variance over sites for grain moisture content was significant (Table 4).

When averaged across sites, corn yield, early growth, and other crop response indicators were generally more responsive to 60 kg ha^–1 broadcast than the 15 kg ha^–1 band treatment. For this reason, the relationships between crop response and soil test P were examined from the perspective of the high rate of broadcast P (Fig. 1 and 2) . Others studies (Mengel et al., 1988; Randall and Hoeft, 1988; Rehm et al., 1988) have similarly shown that benefits of banded P diminish in comparison to high rates of broadcast P.

View larger version (25K): [in this window] [in a new window]   Fig. 1. Relationships between soil P extracted by five soil test extractants and relative yield as calculated based on the broadcast treatment. Triangle symbols are statistically significant at the P > F 0.10 level; circle symbols are not significant. Vertical lines indicate current soil test P critical levels for individual states.

View larger version (25K): [in this window] [in a new window]   Fig. 2. Relationships between soil P extracted by five soil test extractants and relative plant height at 35 d after planting as calculated based on the broadcast treatment. Triangle symbols are statistically significant at the P > F 0.05 level; circle symbols are not significant. Vertical lines indicate current soil test P critical levels for individual states.

Preplant soil test P levels ranged from very low to very high across the 51 experimental sites (Table 2). The number of sites that had soil test P levels below current critical levels varied depending on the soil test extractant and a state's particular soil test critical level in use (Table 1). For example, the percentage of experimental sites that had initial soil test P levels below the current critical level ranged from 14 to 65% of the sites. By definition, experimental sites that test below the soil test critical level are likely to respond positively to the application of P fertilizer (Mallarino and Blackmer, 1992). Results show that 17 to 43% of those sites testing below the critical level exhibited a yield increase to the application of P fertilizer (Table 1) and 25 to 50% of the sites testing below the critical level exhibited an early growth increase to P fertilizer. Also, some sites that tested below the particular soil test critical levels in use did not respond to P fertilizer, and a few sites (depending on the state designated critical level) that would be predicted to not respond did exhibit responses to P fertilizer (Fig. 1 and 2). Depending on the soil test critical level employed, two to six of the nine yield responsive sites had soil test P levels above the critical level. All of these sites, however, occurred in the soil test range (Table 1) where maintenance (crop removal or partial crop removal) rates of P fertilization (Heckman et al., 2003), ranging from 20 to 80 kg ha ^–1 P depending on the grain or silage yield goal, would be recommended. Thus, current soil test recommendations for maintenance applications of P would have likely protected against a potential loss of corn yield at most of the responsive experimental sites that had soil test P levels above the critical level. Some experimental sites (7-MA and 35-MA in Tables 2 and 3) were problematic in that they exhibited yield increases at very high soil test P levels. Site 35-MA, for example, had a soil test P level of 319 mg kg^–1 M3-ICP with a large and significant yield increase (relative yield = 75%).

The M3-ICP P/Al ratio was shown by Khiari et al. (2000) to be a reliable method for making both agronomic and environmental P recommendations for potato production in Quebec, and this approach has been adopted by the province for other field crops as well (CRAAQ, 2003). Consequently, we included yield and plant height response data for the P/Al ratio for comparison with that for M3-ICP P (Fig. 1 and 2) but this approach also did not exhibit a clear relationship.

Soil testing for Morgan and Modified-Morgan P are based on colorometric, not ICP, analysis of the extracts. States that use these extractants perform colorometric analysis (except Maine, which performs ICP analysis but adjusts results to colorometric based on correlation) and their threshold levels are based on colorometric P analysis. While results from ICP and colorometric analysis are well correlated (R² of 0.99 and 0.98), those from colorometric are 2.0 and 1.5 mg kg^–1 lower for Morgan and Modified-Morgan, respectively (Fig. 3 ). These represent substantial differences given critical levels in the range of 4 to 7 mg kg^–1.

View larger version (12K): [in this window] [in a new window]   Fig. 3. Morgan and Modified Morgan P analyzed by ICP and colorometric methods.

The difficulties with soil test P calibration research are highlighted by the contrasting recent success in establishing clearly defined presidedress soil nitrate test (Magdoff, 1991) critical levels. Most of the states in the region have independently conducted research and have come to remarkably close agreement that the PSNT critical level for corn is between 20 and 25 mg kg^–1 (Heckman, 2002). Even on other annual crops, the PSNT critical level is in the same range, and the PSNT is consistently about 85% accurate for correctly predicting whether N is needed. The situations for N and P are different for many reasons, but first, crops grown on soils of this region are typically much more responsive to N than to P. With the abundance of high P fertility soils in the region (Fixen, 2002), it is becoming increasingly difficult to find good field sites for soil test P calibration research (Beegle and Oravec, 1990). Second, the chemistry and transport mechanisms for nitrate uptake and for P uptake in the soil–plant system are fundamentally different, with nitrate being very soluble and moving to the root primarily by mass flow, and P being strongly adsorbed and moving to the root primarily by diffusion. While proposals (Barber, 1995) to improve soil testing for P using a mechanistic approach may hold promise, it has not yet been realized, and the development of reliable predictors for crop responses to P fertilizer continues to be a research challenge brought on with increasing urgency as soil test P levels are now being used for regulatory purposes.

	ACKNOWLEDGMENTS

 
The research reported in this publication was supported by the Northeast Region Sustainable Agriculture Research and Education Program.

	REFERENCES

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (5) Citing Articles via Google Scholar Google Scholar Articles by Heckman, J. R. Articles by Reid, W. S. Search for Related Content PubMed Articles by Heckman, J. R. Articles by Reid, W. S. Agricola Articles by Heckman, J. R. Articles by Reid, W. S. Related Collections Phosphorus Nutrient Management Soil Fertility and Productivity Maize Management