As farmers increasingly use soil health tests, many wonder if these test results can or should determine fertilizer needs for corn and other Minnesota crops.
These tests, such as the Haney test, help measure soil health indicators such as microbial activity, the amount of carbon in the soil and nutrient availability. While these tests can be useful for contrasting management practices, they shouldn’t be used to determine fertilizer application rates.
Unlike University of Minnesota fertilizer guidelines, soil health tests often don’t follow the standard procedures used to develop recommended rates. This could cause farmers to use too much or too little fertilizer.
Understanding fertilizer guidelines
When determining your crop’s fertilizer needs, it’s recommended to stick with standard soil testing procedures and fertilizer guidelines that have been correlated and calibrated through many years of research in your own state.
The standard process
When soil is sent for analysis, the laboratory uses specific procedures and extractants to determine a nutrient’s estimated availability. 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 how much of a nutrient is 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.
Then, it’s important to determine how much fertilizer is needed to meet crop nutrient needs at different soil test levels. This is called calibration.
Who uses the standard process (and who doesn’t)
The University of Minnesota has been conducting extensive correlation and calibration research across the state to develop fertilizer guidelines, using standard soil testing procedures and extractants.
However, little to no correlation and calibration research in Minnesota has been done with the procedures and extractants used by non-standard tests like the Haney test.
The importance of local guidelines
Also be aware that soil test results from a lab may include fertilizer recommendations that aren’t 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 because soil type, soil pH, precipitation, temperature, soil organic matter, cropping rotation and soil parent material influence soil nutrient availability—and ultimately fertilizer application needs.
Following guidelines from other states can also cause crop producers to over- or under-apply fertilizer—just as non-standard tests like the Haney test can do.
Soil health tests, like the Haney test, can give you a snapshot of soil characteristics not typically measured by standard testing procedures. You can use them to help demonstrate contrasts in management practices.
However, using non-standard soil tests to determine fertilizer application rates can result in:
Higher input costs.
Potentially negative environmental impacts from over-applying nutrients.
Reduced yield and profits if you apply fewer nutrients than a crop needs.
Comparison: Haney test vs. standard soil-testing procedures
To better understand whether soil health tests could help determine fertilizer needs, University of Minnesota researchers conducted comparison trials. They used the Haney test (a non-standard test) and standard soil testing procedures to estimate:
Available nitrogen, potassium and phosphorus per acre.
Corresponding fertilizer recommendations.
In 2015, the Southwest Research and Outreach Center in Lamberton conducted trials to compare results from the Haney test to standard soil testing procedures.
When samples were collected in the spring, the Haney test indicated much lower available nitrogen than standard soil testing procedures (37 to 97 pounds per acre less), as shown in Tables 1 and 2.
These lower amounts with the Haney test would trigger a higher nitrogen application rate than when using standard testing procedures and University of Minnesota fertilizer guidelines. Differences were minimal at the fall sampling date (1 to 17 pounds per acre greater available nitrogen with the Haney test).
In table 1, soil samples were analyzed on June 1, 2015 using the Haney test to determine total N available and standard soil testing procedures for the other values. As per protocol, samples were collected from a 0- to 6-inch depth for the Haney test and 0- to 6-inch and 6- to 24-inch depths for the standard test. The larger number in each comparison is bolded.
Table 1: Estimated available nitrogen per acre in June
Tillage | Total N available (Haney) | NO3-N at 0-6 in. (standard) | NO3-N at 6-24 in. (standard) | NO3-N at 0-24 in. (standard) |
---|---|---|---|---|
Moldboard plow | 68 lbs. | 46 lbs. | 119 lbs. | 165 lbs. |
V-rip | 62 lbs. | 32 lbs. | 82 lbs. | 114 lbs. |
Strip tillage | 78 lbs. | 29 lbs. | 86 lbs. | 115 lbs. |
In table 2, soil samples were analyzed on Oct. 14, 2015 using the Haney test to determine total N available and standard soil testing procedures for the other values. As per protocol, samples were collected from a 0- to 6-inch depth for the Haney test and 0- to 6-inch and 6- to 24-inch depths for the standard test. The larger number in each comparison is bolded.
Table 2: Estimated available nitrogen per acre in October
Tillage | Total N available (Haney) | NO3-N at 0-6 in. (standard) | NO3-N at 6-24 in. (standard) | NO3-N at 0-24 in. (standard) |
---|---|---|---|---|
Moldboard plow | 36 lbs. | 8 lbs. | 27 lbs. | 35 lbs. |
V-rip | 41 lbs. | 12 lbs. | 19 lbs. | 31 lbs. |
Strip tillage | 41 lbs. | 10 lbs. | 14 lbs. | 24 lbs. |
In 2016, 12 long-term, on-farm sites were established in southwestern and west-central Minnesota for comparisons between the Haney test and standard soil testing procedures.
In spring 2016, around the V4 stage of corn, researchers collected, mixed and divided soil samples 0 to 6 inches in depth. Then, they analyzed them via the Haney test and standard soil test procedures.
To illustrate the differences between tests, researchers divided results into response categories (from very low to very high, shown in Table 3) and generated recommended broadcast fertilizer rates into guidelines for fertilizing corn in Minnesota.
Potassium
Researchers found large differences in estimated available potassium between the two methods (Table 3). In every comparison, the Haney test:
Extracted less potassium than the standard procedure.
Led to fertilizer recommendations that were 90 to 175 pounds per acre greater compared to the standard procedure.
In table 3, researchers analyzed samples with the Haney test and standard soil testing procedures. Then, they generated fertilizer recommendations using University of Minnesota Fertilizer Guidelines (table 10) for a corn yield goal of more than 200 bushels per acre. The larger number in each comparison is bolded.
Table 3: Estimated available potassium (K) and recommended fertilizer application rates
Site | Extractable/ estimated available K: Standard | Extractable/ estimated available K: Haney | Soil test range: Standard | Soil test range: Haney | K2O to broadcast: Standard | K2O to broadcast: Haney | Difference in fertilizer rates (Haney to standard) |
---|---|---|---|---|---|---|---|
1 | 162 ppm | 56 ppm | Very high | Low | 0 lbs. per acre | 160 lbs. per acre | +160 lbs. per acre |
2 | 94 ppm | 31 ppm | Medium | Very low | 90 lbs. per acre | 205 lbs. per acre | +115 lbs. per acre |
3 | 158 ppm | 32 ppm | High | Very low | 30 lbs. per acre | 205 lbs. per acre | +175 lbs. per acre |
4 | 134 ppm | 56 ppm | High | Low | 30 lbs. per acre | 160 lbs. per acre | +130 lbs. per acre |
5 | 130 ppm | 43 ppm | High | Low | 30 lbs. per acre | 160 lbs. per acre | +130 lbs. per acre |
6 | 154 ppm | 43 ppm | High | Low | 30 lbs. per acre | 160 lbs. per acre | +130 lbs. per acre |
7 | 283 ppm | 115 ppm | Very high | Medium | 0 lbs. per acre | 90 lbs. per acre | +90 lbs. per acre |
8 | 156 ppm | 44 ppm | High | Low | 30 lbs. per acre | 160 lbs. per acre | +130 lbs. per acre |
9 | 273 ppm | 68 ppm | Very high | Low | 0 lbs. per acre | 160 lbs. per acre | +160 lbs. per acre |
10 | 339 ppm | 95 ppm | Very high | Medium | 0 lbs. per acre | 90 lbs. per acre | +90 lbs. per acre |
11 | 134 ppm | 31 ppm | High | Very low | 30 lbs. per acre | 205 lbs. per acre | +175 lbs. per acre |
12 | 154 ppm | 26 ppm | High | Very low | 30 lbs. per acre | 205 lbs. per acre | +175 lbs. per acre |
Phosphorus
Researchers also found differences for phosphorus. Using the Haney test led to phosphorus fertilizer recommendations that would vary from 85 pounds per acre lower to 35 pounds per acre higher than the standard procedure (Table 4).
Note that the standard procedure for phosphorus uses the Olsen test 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.
In table 4, researchers analyzed samples with the Haney test and standard soil testing procedures. Fertilizer recommendations were generated using University of Minnesota fertilizer guidelines (table 9) for a corn yield goal of more than 200 bushels per acre. The larger number in each comparison is bolded.
Table 4: Estimated available phosphorus (P) and recommended fertilizer application rates
Site | Soil pH (standard test) | Extractable P: Standard | Extractable P: Haney | Soil test range: Standard | Soil test range: Haney | P2 O5 to broadcast: Standard | P2 O5 to broadcast: Haney | Difference in fertilizer rates (Haney to standard) |
---|---|---|---|---|---|---|---|---|
1 | 7.9 | 6 ppm | 11 ppm | Low | Medium | 85 lbs. per acre | 50 lbs. per acre | -35 lbs. per acre |
2 | 7.5 | 14 ppm | 36 ppm | High | Very high | 15 lbs. per acre | 0 lbs. per acre | -15 lbs. per acre |
3 | 5.6 | 36 ppm | 19 ppm | Very high | High | 0 lbs. per acre | 15 lbs. per acre | 15 lbs. per acre |
4 | 7.4 | 26 ppm | 66 ppm | Very high | Very high | 0 lbs. per acre | 0 lbs. per acre | 0 lbs. per acre |
5 | 7.9 | 4 ppm | 8 ppm | Low | Medium | 85 lbs. per acre | 50 lbs. per acre | -35 lbs. per acre |
6 | 7.2 | 7 ppm | 19 ppm | Low | Very high | 85 lbs. per acre | 0 lbs. per acre | -85 lbs. per acre |
7 | 7.4 | 28 ppm | 50 ppm | Very high | Very high | 0 lbs. per acre | 0 lbs. per acre | 0 lbs. per acre |
8 | 7.8 | 5 ppm | 25 ppm | Low | Very high | 85 lbs. per acre | 0 lbs. per acre | -85 lbs. per acre |
9 | 7.7 | 16 ppm | 17 ppm | Very high | Very high | 0 lbs. per acre | 0 lbs. per acre | 0 lbs. per acre |
10 | 7.9 | 21 ppm | 26 ppm | Very high | Very high | 0 lbs. per acre | 0 lbs. per acre | 0 lbs. per acre |
11 | 6.5 | 16 ppm | 11 ppm | High | Medium | 15 lbs. per acre | 50 lbs. per acre | 35 lbs. per acre |
12 | 7.1 | 28 ppm | 14 ppm | Very high | High | 0 lbs. per acre | 15 lbs. per acre | 15 lbs. per acre |
Corn nitrogen rate calculator.
Hochmuth, G., Mylavarapu, R., & Hanlon, E. (2014). Developing a soil test extractant: The correlation and calibration processes (Bulletin SL409). University of Florida Institute of Food and Agricultural Sciences Extension.
Kaiser, D.E., Fernández, F., & Coulter, J.C. (2018). Fertilizing corn in Minnesota.
Stahl, L.A.B. (2015). Long-term tillage systems – What are we seeing from an agronomic perspective?
Stahl, L.A.B. (2016). The impact of long-term tillage systems on measures of soil health.
Reviewed in 2018