Evaluation of Methods for Soil Calcium for Peanut Production in Coastal Plain Soils.

Sufficient Ca in the upper 7 to 10 cm of soil (e.g., pegging zone) is essential for peanut production.  Soil testing can determine whether Ca is sufficient for production using a variety of methods; however, the best method for pegging zone Ca is usually determined by what is used routinely in soil testing laboratories.  Comparison of methods among routine soil tests on pegging zone samples and their ability to supply Ca to peanut is needed.  The objective of this study is to evaluate relationships of four major Ca tests including Mehlich 1 (M1), Mehlich 3 (M3), 1 N neutral ammonium acetate (AA), and 0.01 M sodium nitrate (SN) for Coastal Plain soils.  Results showed that soil test Ca using the M1 and M3 methods were similar and significantly greater than the AA and SN methods.  Application of gypsum shortly before soil sampling had minimal to no effect on correlations among soil tests; however, application of lime led to weaker correlations, indicating conversion among soil test Ca in soils recently treated with lime was less accurate.  In untreated and gypsum-treated soils, slopes of regression lines among soil test Ca were not significantly different, suggesting that the relationships among soil test Ca did not change when gypsum was applied.  However, in lime-treated soils, M1 test extracted significantly larger amount of Ca than M3, AA, and SN, possibly due to inclusion of undissolved lime in soils.  This result indicated that M1 may overestimate soil available Ca in soils recently treated with lime.  The AA test for Ca had the best relationships to peanut yield, grade and seed Ca, while the SN test was the weakest.  In order to improve efficiency of soil testing and accuracy of supplemental Ca recommendation for peanut, soil Ca can be extracted by M1 routinely and then converted to soil Ca by AA using conversion equations to better predict effect on crop; however, recently limed soils may not provide accurate results with M1 method.