Evaluation of Mehlich-3 as a Multi-Element Extractant of Micronutrients and Sulfur in a Soil–Ryegrass System Amended with Varying Biochar Rates from Two Feedstocks

Mehlich-3 (M-3) is used as a universal nutrient extractant due to its ability to simultaneously extract multiple elements. This study aimed to assess M-3 for the simultaneous determination of plant-available zinc (Zn), copper (Cu), iron (Fe), manganese (Mn), boron (B), and sulfate (SO4-S) in a soil amended with switchgrass- (SGB) and poultry litter-derived biochars (PLB), which were used to vary soil pH values (5.7–7.6) and organic carbon (OC) content (2.0–5.5%) in the short-term. Soil and ryegrass (Lolium perenne) were sampled from a growth chamber experiment and analyzed for plant-available and tissue phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), micronutrients (B, Cu, Fe, Mn, and Zn), and sulfur contents. The commonly accepted extractants diethylenetriaminepentaacetic acid (DTPA), for micronutrients, and 0.008 M monocalcium phosphate (MCP), for SO4-S, were used for the evaluation. Relationships between M-3 and DTPA were not reliable for micronutrient availability, although highly significant relationships for Zn and Cu were found. However, M-3-extractable S was highly correlated with S contents in ryegrass tissues regardless of the treatments and provided a 1:1 relationship between MCP and M-3. This offers the potential to eliminate MCP by simply adding S determination after extraction with M-3. Although this research evaluated the d-index for an easier linear relationship between the traditional and proposed methods, more research using several soil samples is needed to establish models and find conversion equations for micronutrients and SO4-S between DTPA-sorbitol, MCP, and M-3.