Relationship between Haney Soil Health Test’s (HSHT) H3A-4 and Mehlich-3 Extractable Nutrients in West TN Soils
Research was funded by Southern SARE
Research was funded by the Tennessee Department of Agriculture (Land and Water Stewardship Programs)
Nutifafa Adotey, Assistant Professor & Soil and Nutr. Magt Specialist Xinhua (Frank) Yin, Professor & Cropping System Scientist, University of Tennessee, and Ryan Blair, Extension Area Specialist III and County Standardized Trials Specialist
Introduction
Soil testing is the best available technology for predicting crop nutrient needs and prescribing the appropriate fertilizer recommendation prior to the growing season. A soil test extractant extracts easily exchangeable nutrients to estimate the soil’s nutrient-supplying capacity. There are several soil extractants used by soil testing laboratories in the US including Mehlich-1 and 3, Bray I, DTPA, Lancaster, etc. (Zhang et al., 2021). However, Mehlich-1 and 3 are commonly used by most university and private soil testing laboratories in the Southeast US (Southern Cooperative Series, 2014).
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 (Haney et al., 2017). Some commercial soil testing laboratories offer HSHT and provide corresponding fertilizer recommendations. Some growers have asked how the soil test values from H3A-4 relates to Mehlich-3 extraction and if there is possible conversion between these two tests. Publicly available Information relating H3A-4 to Mehlich-3 extractable is limited to nonexistent. 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 North Central US Laboratory recommendation with a yield goal of 200 bu/a.
Research Methodology
Field trials were established at multiple sites in Tennessee during the 2021 and 2023 corn growing seasons to assess the adequacy of mineralizable nitrogen (N) to predict crop response to N. These trials were leveraged to evaluate the relationship between HSHT’s H3A-4 and Mehlich-3 extractable nutrients. Specialists and scientists at the University of Tennessee recommend phosphorus(P) and potassium (K) fertilizer applications for only low- and medium-test soils, so only the sites with low or medium test P and K soils were included in the trials. Plots were four rows wide x 30 feet long and each treatment combinations were replicated four times in a randomized complete block design. The N fertilizer treatments included: 0, 60, 120, 180, 210, and 240 lb N ac-1. General site description and selected soil properties for each site are presented in Table 1 and 2, respectively. Soil samples were collected multiple depths, however only the 0-6-inches depth is discussed in this publication for the purpose of fertilizer recommendations. Soils were air dried, ground to pass through a 2-mm sieve, and thoroughly mixed and analyzed for soil organic matter, soil pH, Mehlich-3 extractable nutrients, and HSHT’s H3A-4 extractable nutrients (Haney et al., 2017). The relationship between Mehlich-3 and HSHT’s H3A-4 extractable nutrients of interest with regards to row crop production in Tennessee (P, K, sulfur (S), zinc (Zn), and boron (B)) were evaluated using linear model with Sigma Plot 15.0. Fertilizer recommendations for Mehlich-3 were made from UT Fertilizer recommendations; H3A-4 fertilizer recommendations were provided by a North Central US Laboratory.
Findings
The relationship between HSHT’s H3A-4 and Mehlich-3 extractable P, K, S, Zn, and B is presented in Figures 1-5. Mehlich-3 and H3A-4 extractable nutrients were significantly correlated and showed a positive linear relationship between the two tests across years except S. Nonetheless, there was a significant and strong positive relationship between the two tests for S in 2023. Phosphorus and boron highly correlated well (R2 = 0.87 and 0.89, respectively) while potassium and zinc were moderately correlated (R2 = 0.39 and 0.55, respectively) as shown in Table 3. On average, Mehlich-3 extracted approximately 60% more P, 25% more K, 30% more S, 50% more Zn, and 40% more B than H3A-4. 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, K, S, Zn, and B 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. 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. Mehlich-3 had chelating agents added to extract more micronutrients on soil exchange sites. Mehlich-3 also had fluoride added to extract P that may be retained on iron and aluminum complexes, which Mehlich-1 could not extract. H3A-4 is designed to be used in soil health and more closely mimic organic acids exuded 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.
Conclusion
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 trial to develop calibration curves.
