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Extension > Minnesota Crop News > Should soil health test results be used when determining fertilizer needs in Minnesota?

Monday, September 26, 2016

Should soil health test results be used when determining fertilizer needs in Minnesota?

by Lizabeth Stahl, Extension Educator – Crops; Fabián G. Fernández  and Daniel E. Kaiser, Extension Nutrient Management Specialists

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soil test
Photo 1. Soil sampling
Soil health and how to improve and maintain it has been a hot topic in agriculture recently. Soil tests, including the Haney Test, have been developed to help measure indicators of soil health such as microbial activity, the amount of carbon in the soil, and nutrient availability. As more farmers use soil health tests, the question arises if results from these tests can or should be used in determining fertilizer needs for crops like corn in Minnesota.

The importance of correlation and calibration


To help address this question, it is critical to recognize the importance of correlation and calibration in developing fertilizer recommendations. When soil is sent to a laboratory for analysis, specific procedures and extractants are used to determine the estimated availability of nutrients. Different extracts and procedures typically result in different estimates of nutrient availability. The process of correlation helps determine the relationship between plant nutrient uptake or yield and the amount of nutrient extracted from the soil. A soil test is considered "correlated" when lower yield and plant growth can be predicted at lower soil test values, and higher yield and plant growth can be predicted at higher soil test values.

It is then important to determine how much fertilizer is needed to meet crop nutrient needs at different soil test levels. This is called calibration. The University of Minnesota has conducted extensive correlation and calibration research across Minnesota in developing fertilizer guidelines, using standard soil testing procedures and extractants, and this research continues.

Little to no correlation and calibration research has been done with the procedures and extractants utilized by nonstandard tests like the Haney Test in Minnesota.

It is also important to note that soil test results from a lab may include fertilizer recommendations that are not based on correlation and calibration research conducted in Minnesota and that recommendations listed as a "University" recommendation may not be from the University of Minnesota. This is important as soil type, soil pH, precipitation, temperature, soil organic matter, cropping rotation, and soil parent material influence soil nutrient availability and ultimately fertilizer application needs.

Nitrogen comparisons


To help compare results from the Haney Test to standard soil testing procedures, trials were conducted at the Southwest Research and Outreach Center in Lamberton in 2015, supported by the Minnesota Soybean Research and Promotion Council and NCR-SARE. When samples were collected in the spring, the Haney Test indicated much lower available nitrogen than standard soil testing procedures (37 to 97 lbs/acre less, Table 1). These lower amounts with the Haney Test would trigger a higher nitrogen application rate than when using standard testing procedures and U of MN Fertility Guidelines. Differences were minimal at the fall sampling date (1 to 17 lbs/acre greater available nitrogen with the Haney Test).

Table 1. Comparison of results for estimated available nitrogen from soil samples analyzed with the Haney test and standard soil testing procedures. As per protocol, samples were collected from a 0-6 inch depth for the Haney test and 0-6 and 6-24 inch depths for the standard test. The larger number in each comparison is bolded.
Sample date 6/1/15 Sample date 10/14/15
Haney Test Standard soil test procedures Haney Test Standard soil test procedures
Tillage Total N available NO3-N
(0-6")
NO3-N
(6-24")
NO3-N
(0-24")
Total N available NO3-N
(0-6")
NO3-N
(6-24")
NO3-N
(0-24")
pounds N per acre pounds N per acre
MP 68 46 119 165 36 8 27 35
RIP 62 32 82 114 41 12 19 31
ST 78 29 86 115 41 10 14 24

Potassium and phosphorus comparisons


In 2016, twelve long-term, on-farm sites were established for comparisons between the Haney test and standard soil testing procedures, with funding from the Minnesota Agricultural Fertilizer Research and Education Council. At each site this spring, around the V4 stage of corn, 0-6 inch depth soil samples were collected, mixed and divided for analysis via the Haney test and standard soil test procedures. To illustrate the differences between tests, results were divided into response categories and recommended broadcast fertilizer rates were generated following guidelines described in the U of MN Extension publication “Fertilizing Corn in Minnesota” (http://www.extension.umn.edu/agriculture/nutrient-management/nutrient-lime-guidelines/fertilizing-corn-in-minnesota/).

Large differences were found in estimated available potassium between the two methods (Table 2). In every comparison, less potassium was extracted with the Haney test than with the standard procedure, and the resulting fertilizer recommendations were 90 to 175 lbs/acre greater with the Haney Test compared to the standard procedure.

Differences were also found for phosphorus, ranging from -85 lbs/acre to + 35 lbs/acre, when using the Haney test versus the standard procedure (Table 3). Note that for the standard procedure for phosphorus, the Olsen test is used when soil pH is greater than 7.4, otherwise the Bray test is used. The same extractant (H3A extractant) is used regardless of soil pH with the Haney test.

Table 2. Comparison of results for estimated available potassium (K) and recommended fertilizer application rates for samples analyzed with the Haney test and standard soil testing procedures. Fertilizer recommendations were generated using U of MN Fertilizer Guidelines for a 200+ bu/acre corn yield goal. The larger number in each comparison is bolded.

Extractable/ estimated available K (ppm)

Soil test range based on UMN fertilizer guidelines
K2O to broadcast based on UMN fertilizer guidelines (lb/A)

Difference in fertilizer application rates (Haney - Standard)

Site
With Standard test With Haney test With Standard test With Haney test With Standard test With Haney test
1 162 56 V. High Low 0 160 +160
2 94 31 Med V. Low 90 205 +115
3 158 32 High V. Low 30 205 +175
4 134 56 High Low 30 160 +130
5 130 43 High Low 30 160 +130
6 154 43 High Low 30 160 +130
7 283 115 V. High Med 0 90 +90
8 156 44 High Low 30 160 +130
9 273 68 V. High Low 0 160 +160
10 339 95 V. High Med 0 90 +90
11 134 31 High V. Low 30 205 +175
12 154 26 High V. Low 30 205 +175

Table 3. Comparison of results for estimated available phosphorus (P) and recommended fertilizer application rates for samples analyzed with the Haney test and standard soil testing procedures. Fertilizer recommendations were generated using U of MN Fertilizer Guidelines for a 200+ bu/ac corn yield goal. The larger number in each comparison is bolded.


Soil pH (from standard test


Extractable P (ppm)
Soil test range based on UMN fertilizer guidelines P2O5 to broadcast based on UMN fertilizer guidelines (lb/A)

Difference in fertilizer application rates (Haney - Standard)

Site
With Standard test With Haney test With Standard test With Haney test With Standard test With Haney test
1 7.9 6 11 Low Med 85 50 -35
2 7.5 14 36 High V.High 15 0 -15
3 5.6 36 19 V.High High 0 15 15
4 7.4 26 66 V.High V.High 0 0 0
5 7.9 4 8 Low Med 85 50 -35
6 7.2 7 19 Low V.High 85 0 -85
7 7.4 28 50 V.High V.High 0 0 0
8 7.8 5 25 Low V.High 85 0 -85
9 7.7 16 17 V.High V.High 0 0 0
10 7.9 21 26 V.High V.High 0 0 0
11 6.5 16 11 High Med 15 50 35
12 7.1 28 14 V.High High 0 15 15

Avoid the risk or over- or under-application of nutrients


Soil health tests can give a snapshot of soil characteristics not typically measured by standard testing procedures and they can be used to help demonstrate contrasts in management practices. Using non-standard soil tests to determine fertilizer application rates, however, can result in higher input costs, potentially negative environmental impacts from nutrient over-application, or reduced yield and profits if nutrients are applied below crop needs. When determining fertilizer needs for your crop, it is recommended to stick with soil testing procedures and fertilizer guidelines that have been correlated and calibrated through many years of research in your own state.

For more details, check out the “University of Minnesota Extension Fertility Guidelines for Field Crops” publication and other crops-related information on the University of Minnesota Extension Crops website at: z.umn.edu/crops.

References


Corn nitrogen rate calculator. http://cnrc.agron.iastate.edu/

Hochmuth, G., R. Mylavarapu, and E. Hanlon. 2014. Developing a Soil Test Extractant: The Correlation and Calibration Processes. University of Florida Institute of Food and Agricultural Sciences Extension. Bulletin SL409. http://edis.ifas.ufl.edu/ss622.

Kaiser, D.E., F. Fernandez, and J.A. Lamb, J.A. Coulter, B. Barber. 2016. Fertilizing Corn in Minnesota. U of MN Extension. http://www.extension.umn.edu/agriculture/nutrient-management/nutrient-lime-guidelines/fertilizing-corn-in-minnesota/

Stahl, L.A.B. 2015. Long-Term Tillage Systems – What are we seeing from an agronomic perspective? http://www.extension.umn.edu/agriculture/soils/presentations/docs/long-term-tillage-agronomics-stahl.pdf.

Stahl, L.A.B. 2016. The Impact of Long-Term Tillage Systems on Measures of Soil Health. http://www.extension.umn.edu/agriculture/crops-research/south/2015/docs/2015-long-term-tillage-impact-on-soil-health-poster.pdf.

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