More recently, with emphasis on sustainable crop production, there are a number of tests that assess soil health. Some of the soil health tests such as the Haney Soil Health Tool (HSHT), which is adopted by USDA-NRCS, assess soil quality parameters as well as nutrient availability. The HSHT uses the H3A-4 extractant to estimate ‘plant available’ nutrients. The H3A-4 extractant simulates plant root environment by using organic acid plant exudates and has been touted as more representative of phosphorus availability. Some commercial soil testing laboratories offer HSHT and provide corresponding fertilizer recommendations. How does the test values and recommendations from H3A-4 relate to Mehlich-3 extraction? Publicly available information relating H3A-4 to Mehlich-3 extractable nutrients is limited.
To address this concern a study was conducted to (i) investigate the relationship between HSHT’s H3A-4 and Mehlich-3 extractable nutrient elements and (ii) compare the P2O5 and K2O fertilizer recommendations by University of Tennessee and Haney Soil Health Test from a commercial Laboratory recommendation with a yield goal of 200 bu/a. Field trials were established at multiple sites in Tennessee during the 2021 and 2023 corn growing seasons to evaluate the relationship between these two tests. Fertilizer recommendations for Mehlich-3 were made from UT Fertilizer recommendations while H3A-4 fertilizer recommendations were provided by a commercial laboratory.
Relationship between HSHT’s H3A-4 and Mehlich-3 extractable P and K
Extractable P and K showed a positive linear relationship between the two tests. Phosphorus highly correlated well (R2 = 0.87) while potassium was moderately correlated (R2 = 0.39). On average, Mehlich-3 extracted approximately 60% more P and 25% more K than H3A-4, respectively. The lower concentration of H3A-4 extractable nutrients is because of the composition of the H3A-4 extracting solution. The solution consists of a dilute mixture of four weak acids with a weakly buffered pH of approximately 3.75, which is higher than the buffered 2.5 pH of Mehlich-3. Although the relationships between some nutrients appear highly correlated, the magnitude of RMSE observed makes it inappropriate to estimate Mehlich-3 extractable P and K from H3A-4 extractions for fertilizer recommendations. Hence, conversion equations generated for these two soil tests can be misleading and would result in inappropriate fertilizer recommendation. An example would be if H3A-4 was 20 ppm P and translated to Mehlich-3 it would be 40 ppm, but the translation error could be +/- 10 ppm. This large error may be driven by two data points that are far from the predicted line. Further research into why some samples fall farther from the predicted line may help explain extracting differences between the two extracts. Maybe the fluoride in the Mehlich-3 is extracting a P fraction not exchangeable with H3A-4 on those soils.
Comparison of fertilizer recommendations
Differences in fertilizer recommendations for 200-bushel corn based on the different tests were observed. This observation indicates that more work should be conducted to explain the difference. Calibration is needed since the Haney test uses a different extracting acid than Mehlich-3. The reliable means to generate appropriate fertilizer recommendations for Tennesseans is by conducting response trials to develop calibration curves.
In summary, data shows that H3A-4 may not translate to Mehlich-3 well, and that is okay. They were both designed with different goals in mind The H3A-4 is designed to be used in soil health and more closely mimic organic acids exudated by plant roots while also allowing for use in measuring nitrogen in the soil. Mehlich-3 cannot be used to measure nitrogen fractions in the soil. These differences are why it is important to have crop yield correlation and calibration trials for each extract so producers can have both options to use in their nutrient management program.
