Should Soil Health Test Results Be Used When Determining Fertilizer Needs In Minnesota?

 

 

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.

 

 

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.

 

 

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.

 

 

 

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/fertilizer-management/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.

 

 

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 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.