Establishing a Database of Calcareous Soils in Wisconsin.

Wisconsin requires Bray P1 as the routine soil test phosphorus method for fertilizer recommendations and phosphorus loss risk predictions.  Highly calcareous soils (>5 g inorganic carbon/kg) have been shown to have the potential to neutralize the Bray P1 extracting solution causing erroneous fertilizer recommendations.  Previous research has also shown that the carbonate dolomite (CaMgCO₃) does not affect the Bray P1 test whereas the much more soluble calcite (CaCO₃) does.  Wisconsin has a region of calcareous subsoils located in the northeast part of the state where the carbonate has been identified as dolomitic, but there may be other calcareous regions in the state containing enough calcite to make Bray P1 an inappropriate choice for soil test phosphorus.  The objective of this research is to establish a database of calcareous soils in Wisconsin and identify them as dolomitic or calcitic.  A subsample of high pH (>7.0) soils (n=133) from Wisconsin was obtained from the UW Madison Soil Testing Lab.  Samples were analyzed for pH, soil test P (Bray P1 and Mehlich 3), Bray P1 filtrate pH, total inorganic carbon (IC), and calcite and dolomite content.  In summary, no soils with high levels of calcite were found in these 133 samples.  Specifically, the soil pH ranged from 6.59-7.63; Mehlich 3 ranged from 10-180 mg P/kg soil; and Bray P1 ranged from 65-222 mg P/kg soil.  Bray P1 filtrate pH ranged from 3.1-4.8, indicating that the Bray P1 extracting solution was not neutralized in any sample.  The results of the calcite/dolomite volumetric calcimeter method corroborated these findings, with carbonate content ranging from 3.8-22.6 g calcite/kg soil and 0-88.3 g dolomite/kg soil.  Total IC ranged between 0.1 – 11.0 g IC/kg soil.  These findings give no evidence that Bray P1 is inappropriate for Wisconsin soils, but these 133 soils only begin the search for areas in Wisconsin that may contain high levels of calcite.