Water Soluble Phosphorus (WSP) Correlation to Mehlich-3 P (M3P) and M3 Phosphorus Saturation Ratio (M3PSR).

Phosphorus (P) loss from agricultural fields to freshwater ecosystems in the Midwestern United States is of great concerns. Water-soluble P may be the most appropriate environmental estimator of P concentrations in runoff. Mehlich-3 P (M3P) is the standard soil test in Indiana and is used to maker fertilizer recommendations and to limit manure application rates. The IN database relating M3P to water soluble P (WSP) is relatively small and limited both geographically and across soil properties. The objective of this study was to determine the relationship among M3P, M3PSR, and WSP in relation to other soil properties. Laboratory analyses were conducted on 565 Indiana soils from different geographic quadrants (NE, NW, SE, SW). Soil test P (STP) was determined by Mehlich-3 (Mehlich, 1984) extractions and WSP extractions (1 g soil, 10 ml deionized water (DDI) with/without 0.01M CaCl_2, shaken for 1 h at 120 epm). Two drops of chloroform were added to inhibit microbial activity. Samples were centrifuged at 2000g for 10 min, filtered, and analyzed on ICP-optical emission spectroscopy. Mehlich-3 extractions showed that theses soils varied greatly in the amount of P, K, Mg, Ca, Fe, Al, and CEC. Preliminary results showed a range of 5-1258, 25-899, 26-902, 129-6777, 57-514, 53-1360, and 2.2-42.1 (mg kg^-1) for P, K, Mg, Ca, Fe, Al, and CEC, respectively. Mehlich-3PSR ranged from 0.007-2.17, whereas water soluble P (DDI) and 0.01M CaCl₂ ranged from 0.63-101.8 and 0.32-45.5 mg kg^-1, respectively. Mehlich-3P data showed a good correlation with M3PSR (R²=.85). Water soluble P (DDI) showed a greater correlation with both PSR and M3P than 0.01M CaCl₂ diluted salt solution. A greater correlation was observed between M3P and both WSP and 0.01M CaCl₂ as opposed to M3PSR. Multiple stepwise regression techniques were used to determine which soil parameters best explained the relationship between M3P, M3PSR, and WSP. The relationship between M3P, M3PSR, and WSP may likely be used as an indicator of the mobility of P concentrations in runoff and thus plays an important role in excessive P accumulation in surface water, especially from manure fertilized soils. The results and implications of this study will be presented in this poster.