May 1994                                          SCSB# 380

RESEARCH-BASED SOIL TESTING INFORMATION
AND FERTILIZER RECOMMENDATIONS
FOR PEANUTS ON COASTAL PLAIN SOILS

Chapter 2 Soil Test Methodology C. C. Mitchell and S. C. Hodges Soil test methodology was not a major issue among the three primary southeastern U. S. states producing runner type peanuts. Georgia, Florida, and Alabama all use the Adams-Evans buffer for lime requirement determination and the Mehlich-1 extractant for P, K, Ca, and Mg (Sou. Coop. Ser. Bull. No. 190 1984). Other peanut-producing states of South Carolina and Virginia also use the Mehlich-1 procedure. North Carolina and Oklahoma use the Mehlich-3 procedure while Texas uses an acid, ammonium acetate procedure. This discussion will focus on the states of Alabama, Georgia, and Florida. Representatives from these states actively participated in the subcommittee meetings. Differences existed in the Mehlich-1 extraction procedure. Alabama and the Georgia (Tifton) Lab use a weight: volume ratio of 1:4 for soil and extracting solution whereas Georgia (Athens) Lab and the Florida Lab use a volume: volume ratio of 1:4 with an assumed soil weight per unit volume. These labs ultimately calculate extractable nutrients in terms of parts per million or pounds per acre assuming 2 x 106 pounds of soil per acre furrow slice (parts per 2 million or mg 2kg-1). Differences in actual and assumed soil bulk densities could result in differences in the reported value for an extractable nutrient. However, previous annual SERA-IEG-6 exchange samples and regional reference sample analyses have not identified probable cause for quality control concerns among those labs using the same extractant even though slight differences in methodology may exist. Nevertheless, a sample exchange was conducted to determine if the differences in extraction procedure could be a cause for differences in soil test calibration and interpresentation. A random collection of 10 soils from peanut farms in southeastern Alabama was distributed to the Auburn University Soil Testing Laboratory, the University of Georgia (Athens) Lab, and the Georgia Coastal Plain Experiment Station (Tifton) Lab. Samples had a disturbed density ranging from 0.98 g cm-3 to 1.43 g cm-3 with a mean density of 1.32 g cm-3. There was a strong correlation between laboratories (see table). There were a few outlying errors in results from each of the labs but errors appeared random. These results led to the conclusion that soil test methodology was quite consistent among the three labs, and any differences in fertilizer recommendations that may exist across state lines were due to calibration/interpretation of the data and not to analytical differences.  Linear Correlations Among Test Results from Three Labs Testing Soil Samples from Peanut Fields    Laboratory comparison  Analysis Range in values Auburn vs Tifton  Tifton vs Auburn  Athens vs Auburn  pH  5.3-6.8  0.96 0.94 0.87  P  10-62 lb/acre  0.99 0.97 0.98  K  30-336 lb/acre 0.99 0.94 0.93  Ca  14-354 lb/acre 0.99 0.99 0.99  Org. matter  0.83-7.3%  -- -- 0.99 Each analysis was also rated (low, medium, high, etc.) according to current interpretations used by the respective laboratories. The only differences in ratings occurred when a sample’s analysis fell near a critical level between two ratings. However, with only 10 samples in the survey, a complete range in analyses for each procedure was not possible. Most samples were in the “high” range for Ca according to all laboratories. Document Prepared by: Leigh H. Stribling, lstribli@acesag.auburn.edu Alabama Agricultural Experiment Station Auburn University

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