A discussion on Mehlich-3 phosphorus extraction from the perspective of governing chemical reactions and phases: Impact of soil pH

Authors: Penn, Chad; RUTTER, EDMOND - Oklahoma State University; ARNALL, D - Oklahoma State University; CAMBERATO, J - Purdue University; Williams, Mark; WATKINS, P - Oklahoma State University

Submitted to: Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/29/2018
Publication Date: 7/2/2018
Citation: Penn, C.J., Rutter, E.B., Arnall, D.B., Camberato, J., Williams, M.R., Watkins, P. 2018. A discussion on Mehlich-3 phosphorus extraction from the perspective of governing chemical reactions and phases: Impact of soil pH. Agriculture. 8(7):106. https://doi.org/10.3390/agriculture8070106.
DOI: https://doi.org/10.3390/agriculture8070106

Interpretive Summary: Proper use of agronomic soil test extractants such as Mehlich-3 (M3) is critical to make proper fertilizer phosphorus (P) recommendations. Precise P recommendations improve agronomic and economic efficiency as well as help to prevent over-applications that cause water quality problems. However, certain soil properties can affect the ability of the the soil test extractant to perform properly. The objectives of this study were to investigate the impact of soil pH on P extractability by M3 and water in different soils containing equal total P, and to ascertain information about mechanisms of M3-P extraction. We found that soil pH had an impact on M3-P that contradicted that found by water soluble P (WSP). M3-P gave the incorrect notion that P is more plant available at acid pH levels; this would mislead agricultural producers in thinking that their soil P levels were adequate when they were not. This means that M3-P would over-estimate plant available P and risk of dissolved P transport in low pH soils in such cases. The results suggest that M3-P is best utilized in combination with WSP as an indicator of soil P fertility and risk of causing P pollution. These results directly impact the soil testing industry, agricultural producers, and agencies interested in water quality.

Technical Abstract: Mehlich-3 (M3) is one of the most common agronomic and environmental phosphorus (P) extractants for determining P fertilizer requirements and potential for non-point source pollution. Understanding how soil properties impact M3 extractability can improve our ability to properly use this soil test. Objectives of this study were to investigate impact of soil pH on P extractability by M3 and water in different soils containing equal total P, and to ascertain information about mechanisms of M3-P extraction. Soil pH at four field sites were previously adjusted to a range of approximately 4.5-7.5. Soils (Grant, Dale, Teller, Easpur) were characterized and extracted for P with M3 and water. Extraction of Mehlich-3 P decreased 40 to 55% with increasing pH which was potentially due to changing P forms, partial neutralization of extractant pH, and consumption of extractant fluoride (F-) by non P-containing calcium (Ca) minerals. Water-soluble P (WSP) increased with increasing pH up to pH 6-7. Mehlich-3 P and WSP were not positively correlated except for one soil. Mehlich-3 P is best utilized with WSP as indicators of quantity and intensity, respectively. Use of M3-P alone at pH <5.5 may overestimate solubility. Further research should examine suitability of M3-P at pH >7.