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Phosphorus and Potassium Levels in the Corn Belt

 

Phosphorus and Potassium Levels in the Corn Belt

Crop Insights written by Mark Jeschke, Ph.D.1, Jeff Mathesius2, Kirk Reese3, Brent Myers, Ph.D.4, and Andy Heggenstaller, Ph.D.5

Summary

  • Recent evidence suggests that inadequate soil levels of phosphorus and potassium could be limiting crop yields in several Midwestern states.
  • Phosphorus and potassium removal from the soil through grain harvest has increased in corn and soybean as yields for both crops have increased over the past 30 years.
  • While corn and soybean yields have risen substantially, phosphorus and potassium fertilizer applications have remained relatively flat or declined in some areas; which raises the question of whether deficient soils could be limiting further gains in yield.
  • DuPont Pioneer agronomists and Encirca certified services agents conducted a large survey of soil fertility levels across the Midwestern U.S. in 2016.
  • Results of the soil fertility survey showed that insufficient phosphorus and potassium levels were common, with substantial variation in fertility levels among states.
  • Corn yield data collected from survey locations in fall of 2016 showed that yields trend lower in areas where phosphorous and potassium fall below critical levels.

Introduction

Balanced soil fertility management is critical for achieving crop genetic yield potential and maximizing profitability. In a DuPont Pioneer survey of over 1500 growers, improved soil fertility management was second only to improved hybrid/variety genetics among management practices which growers viewed as most critical to increasing yields.

Recent evidence suggests that inadequate soil levels of phosphorus and potassium could be limiting crop yields. A survey conducted by the International Plant Nutrition Institute (IPNI) in 2015 of soil samples submitted to labs across the U.S. showed that samples testing below the critical levels of phosphorus and potassium for major crops were common in several states (Murrell and Fixen, 2015).

In order to further evaluate current soil fertility levels and potential limiting effects on crop yields, DuPont Pioneer agronomists and Encirca certified services agents conducted a large survey of soil fertility levels across the Midwestern U.S. in 2016. This survey included a total of 22,402 samples collected from 8,925 fields across 12 states.

Corn yield data collected from survey locations in fall 2016 showed that yields trend lower in areas where phosphorous and potassium fall below critical levels.

Evidence of Fertility Deficiencies

A 2015 IPNI Survey of soil test levels in North America summarized median soil test levels and the frequency of samples testing below critical levels for key plant nutrients. Results of this survey showed that the frequency of samples testing below state critical levels ranged from 31-83% for phosphorus and 9-65% for potassium (Table 1).

Table 1. Critical levels for phosphorus (Bray and Kurtz P1 equivalent) and potassium (ammonium acetate equivalent) and percent of samples testing below critical levels for major crops in a 2015 IPNI survey (Murrell and Fixen, 2015).

This table shows critical levels for phosphorus and potassium and percent of samples testing below critical levels for major crops in a 2015 International Plant Nutrition Institute (IPNI) survey.

What is a Critical Level?

The critical soil test level for a given nutrient is defined as the level below which a profitable yield response in the year of application would be expected based on university research (Fixen et al., 2006). Critical levels can vary by geography based on factors such as soil characteristics, climate, major crops grown in the area, and cultural practices.

A fertilizer application plan based on soil test results can differ depending on a grower’s approach to fertility management:

  • The nutrient sufficiency approach involves applying just enough fertilizer to maximize profitability in the current year. For soils testing above the critical level, no additional fertilizer is applied.
  • The build-maintenance approach involves building soil fertility levels up to the critical level and, once this level is reached, applying fertilizer at rates equivalent to the amount of nutrients removed by the crop.

Soil test level categorizations and fertility management recommendations can vary among state universities, and not all universities use the critical level concept as a part of their soil fertility system. The critical levels for phosphorus and potassium used in this DuPont Pioneer study were first published by IPNI in 2006 as a way to bring together the various university soil fertility rating systems into a single system of state-level critical values to allow analysis of broad soil fertility trends across geographies and over time (Fixen et al., 2006).

University soil fertility recommendations sometimes vary among different areas within a state based on different soil characteristics and major crops. Always consult your local university soil fertility guide for the most relevant and detailed information on soil fertility management practices in your area.

Crop removal of phosphorus and potassium has increased over the past 30 years.

Phosphorus and potassium removal from the soil through grain harvest has increased in both corn (Table 2) and soybean (Table 3) as yields for both crops have increased over the past 30 years. Increases in yield, and corresponding increases in phosphorus and potassium removal, have been particularly great in the northern and western Corn Belt. Yield increases were not as great on a percentage basis over this time frame in the central and eastern Corn Belt, mostly because initial yields at the start of the 30-year period were greater in these states.

Table 2. Average corn yield and calculated crop removal of phosphorus and potassium for select corn-producing states in 1986 and 2016, and percent increase over the 30-year period.

This table shows average corn yield and calculated crop removal of phosphorus and potassium for select corn-producing states in 1986 and 2016, and percent increase over the 30-year period.

1 USDA-NASS. Average yields for 1986 and 2016 represent trendline values based on a linear regression of average state yields from 1960-2016.
2 Removal rate of 0.35 lbs/bu P2O5 (IPNI, 2014)
3 Removal rate of 0.25 lbs/bu K2O (IPNI, 2014)

Table 3. Average soybean yield and calculated crop removal of phosphorus and potassium for select soybean-producing states in 1986 and 2016, and percent increase over the 30-year period.

This table shows average soybean yield and calculated crop removal of phosphorus and potassium for select soybean-producing states in 1986 and 2016, and percent increase over the 30-year period.

1 USDA-NASS. Average yields for 1986 and 2016 represent trendline values based on a linear regression of average state yields from 1960-2016.
2 Removal rate of 0.73 lbs/bu P2O5 (IPNI, 2014)
3 Removal rate of 1.2 lbs/bu K2O (IPNI, 2014)

Phosphorus and potassium fertilizer application rates have remained relatively flat or declined in some areas.

This table shows trends in phosphorus fertilizer applied per planted acre of corn (1986-2010) and soybean (1990-2015).

1 USDA-NASS. Average quantities for 1986 and 2010 represent trendline values based on a linear regression of phosphorus applied per planted acre of corn from 1986-2010. Years selected based on data availability.
2 USDA-NASS. Average quantities for 1990 and 2015 represent trendline values based on a linear regression of phosphorus applied per planted acre of soybean from 1990-2015. Years selected based on data availability.
3 No data available prior to 1995 for phosphorus application to corn.
4 No data available prior to 2000 for phosphorus application to corn.

This table shows trends in potassium fertilizer applied per planted acre of corn (1986-2010) and soybean (1990-2015).

1 USDA-NASS. Average quantities for 1986 and 2010 represent trendline values based on a linear regression of potassium applied per planted acre of corn from 1986-2010. Years selected based on data availability.
2 USDA-NASS. Average quantities for 1990 and 2015 represent trendline values based on a linear regression of potassium applied per planted acre of soybean from 1990-2015. Years selected based on data availability.
3 No data available prior to 1995 for potassium application to corn.
4 No data available prior to 2000 for potassium application to corn.

While corn and soybean yields have risen substantially over the last 30 years, phosphorus and potassium fertilizer applications have not necessarily kept pace. Fertilizer usage data from USDA-NASS show that average quantities of phosphorus and potassium applied per planted acre of corn have remained relatively flat or declined in some states over the past 30 years (Table 4 and Table 5). In some cases, these trends have been offset by increases in phosphorus and potassium applied to soybean acres. Most states have experienced an increase in phosphorus and potassium fertilizer applied to soybean over a similar time period.

The fact that corn and soybean yields have substantially increased over the past 30 years without substantial increases in phosphorus and potassium applications in some cases can be regarded as favorable, in that gains in productivity have been achieved without a corresponding increase in inputs. However, it raises the question of whether these higher yields have been achieved, in part, by mining soils of phosphorus and potassium and whether deficient soils could be limiting further gains in yield.

Comparison of results from the 2015 IPNI survey to those of a similar survey conducted in 2001 provides an indication of soil fertility trends over the past 15 years. These surveys show that the frequency of samples testing below the state critical level for phosphorus increased in Wisconsin, Illinois, Michigan, Ohio, and Indiana (Table 6).

Table 6. Phosphorus median test levels and percent of samples below critical levels in IPNI surveys conducted in 2001 and 2015.

This table shows phosphorus median test levels and percent of samples below critical levels in International Plant Nutrition Institute (IPNI) surveys conducted in 2001 and 2015.

* Median Bray and Kurtz P1 equivalent levels.

Conversely, survey results suggest an improvement in phosphorus fertility levels in Minnesota and South Dakota during this period. Changes were not as great for potassium, where only one state (Kansas) had an increase in samples below the critical level (Table 7). Illinois, Iowa, and Wisconsin all had a decrease in samples below the critical level. Several other states remained relatively unchanged.

These results are not indicative of an across-the-board drawing down of soil phosphorus and potassium levels in recent years. Fertility levels appear to have declined in some areas, improved in others, and stayed relatively constant in many cases. While it does not appear that the overall soil fertility picture has gotten significantly worse, these results do indicate a continuing opportunity to improve crop yields by remedying deficiencies where they exist, particularly as crop nutrient demands continue to go up with higher yielding hybrids and varieties.

Table 7. Potassium median test levels and percent of samples below critical levels in IPNI surveys conducted in 2001 and 2015.

This table shows potassium median test levels and percent of samples below critical levels in International Plant Nutrition Institute (IPNI) surveys conducted in 2001 and 2015.

* Median ammonium acetate equivalent soil K levels.

DuPont Pioneer Soil Fertility Survey

In order to further evaluate current soil fertility levels and potential limiting effects on crop yields DuPont Pioneer Agronomists and Encirca certified services agents conducted a large survey of soil fertility levels across the Midwestern U.S. in 2016. Fields included in the survey were either enrolled in the Encirca® Yield services or planted with an on-farm agronomy research trial in 2016 (Figure 1).

Survey Methods

Samples were collected in fall of 2015 and spring of 2016, with the majority of sampling taking place between March and June of 2016; therefore, the samples are generally reflective of soil phosphorus and potassium available for the 2016 crop. All sample fields were planted to corn in 2016. Samples consisted of six soil cores collected from the upper six inches of the soil profile using an 8-inch soil probe. In order to maintain consistency among samples, all soil samples were submitted to Waypoint Analytical labs in Atlantic, IA and Champaign, IL for analysis. Samples were analyzed for pH, phosphorus, potassium, sulfur, iron, manganese, zinc, copper, and boron; although this summary focuses specifically on phosphorus and potassium. Phosphorus and potassium test results were compared to IPNI critical levels for corn production for each state to determine the percent of samples in each state that fell below recommended levels.

This map shows locations of fields sampled as a part of the DuPont Pioneer soil fertility survey.

Figure 1. Locations of fields sampled as a part of the DuPont Pioneer soil fertility survey. A total of 22,402 samples were collected from 8,925 fields across 12 states.

Survey Results

Results of the DuPont Pioneer soil fertility survey showed that insufficient phosphorus and potassium levels were common throughout the Midwestern U.S., with very few instances in which the frequency of samples testing below critical levels within a state was less than 25%. Results for both phosphorus and potassium varied widely among states (Figure 2 and Figure 3).

Map showing percent of soil samples that were below critical levels for phosphorus in 2016 DuPont Pioneer sampling.

Figure 2. Percent of soil samples that were below critical levels for phosphorus in 2016 DuPont Pioneer sampling.

Survey results for phosphorus largely aligned with results from the 2015 IPNI survey. In both surveys, North Dakota and South Dakota had that highest frequency of sub-optimal soil phosphorus levels (above 60% for both states in both surveys), and Indiana had the lowest frequency of sub-optimal levels. Michigan had a higher frequency of samples below the critical level for phosphorus in the DuPont Pioneer survey (57%) than in the IPNI survey (36%), which could be a result of different sampling distribution.

Map showing percent of soil samples that were below critical levels for potassium in 2016 DuPont Pioneer sampling.

Figure 3. Percent of soil samples that were below critical levels for potassium in 2016 DuPont Pioneer sampling.

Survey results for potassium showed Indiana, Nebraska, and Kansas with the lowest frequency of sub-optimal potassium levels (below 25%) and North Dakota, Minnesota, Iowa, Wisconsin, and Michigan all above 50%. For several states, results were very similar to those of the 2015 IPNI survey; however, North Dakota, South Dakota, Iowa, Minnesota, and Michigan all stood out as having much higher frequency of sub-optimal potassium levels in the DuPont Pioneer Survey than the IPNI survey. These discrepancies may be partly attributable to differences in sampling distributions within the states.

Yield Analysis

The soil samples collected in this study were intersected with 2016 corn yield maps by selecting a 50-ft radius of yield data around the soil sampling point and then calculating an average yield value for that area. A quadratic plateau function was fit to these data to look for trends in the response of corn yield to phosphorus and potassium soil test levels. The relationship between potassium and phosphorus and yield respectively shows similar yield response thresholds as those presented by Iowa State Extension (Mallarino et al., 2013) with no evidence of yield response beyond optimum phosphorus and potassium soil test levels (Figure 4 and Figure 5).

Five Tips for Managing Soil Fertility

  1. Know your soil test levels.
  2. Don’t reduce nutrient application rates in low-testing soils, even if the fields are rented.
  3. Don’t apply buildup rates within two years that are higher than needed to optimize yield goals.
  4. Don’t fertilize in high-testing soils if budgets are tight.
  5. Avoid practices that inhibit root development and nutrient uptake.

This chart shows corn yield relationship to soil test phosphorus in the 2016 DuPont Pioneer soil fertility survey compared to Iowa State soil fertility ranges.

Figure 4. Corn yield relationship to soil test phosphorus in the 2016 DuPont Pioneer soil fertility survey compared to Iowa State soil fertility ranges (Mallarino et al., 2013).

This chart shows corn yield relationship to soil test potassium in the 2016 DuPont Pioneer soil fertility survey compared to Iowa State soil fertility ranges.

Figure 5. Corn yield relationship to soil test potassium in the 2016 DuPont Pioneer soil fertility survey compared to Iowa State soil fertility ranges (Mallarino et al., 2013).

References

Fixen, P.E., T.W. Bruulsema, A.M. Johnson, R.L. Mikkelsen, T.S. Murrell, C.S. Snyder, and W.M. Stewart. 2006 The fertility of North American soils. Potash and Phosphate Institute.

IPNI. 2014. IPNI Estimates of Nutrient Uptake and Removal.

Mallarino, A.P., J. Sawyer, and S. Barnhart. 2013. A General Guide for Crop Nutrient and Limestone Recommendations in Iowa. Iowa State University Extension. PM 1688.

Murrell, T.S., and P.E. Fixen. 2015. Soil test levels in North America. International Plant Nutrition Institute. IPNI Publication No. 30-3115.

United States Department of Agriculture National Agriculture Statistics Service. 2017. Accessed April 25, 2017.

1Mark Jeschke, Ph.D., Agronomy Information Manager, DuPont Pioneer
2Jeff Mathesius, Agronomy Research Manager, DuPont Pioneer
3Kirk Reese, Agronomy Research Manager, DuPont Pioneer
4Brent Myers, Ph.D., Crop Scientist Decision Support, DuPont Pioneer
5Andy Heggenstaller, Ph.D., Encirca® services - Commercial Unit Lead, DuPont Pioneer

Thanks to Scott Murrell and Paul Fixen of the International Plant Nutrition Institute for their review and input on this manuscript.

June 2017

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