PREDICTING LIMING NEEDS OF SOYBEAN USING SOIL-PH, ALUMINUM, AND MANGANESE SOIL TESTS

Whether a tropical-soil should be limed or not for a particular crop i s strongly dependent on the levels of soil aluminum (Al) which can be determined with soil tests. Soil pH is used to predict whether lime is needed in less-weathered soils, although some evidence indicates a so il Al test would be more accurate. The objectives of this study were t o determine and to compare the accuracies of four soil tests to:separa te soils requiring lime from those that do not, and to determine the c ause of acid-soil injury to soybean [Glycine max (L.) Merr.]. Soybean was grown in the greenhouse on four surface soils representing the maj or land resource areas of Louisiana and were amended with eight rates of lime, yields determined, and soils analyzed for soil pH, extractabl e Al, CaCl2-extractable Al, CaCl2-extractable manganese (Mn), and Al s aturation. Acid-soil injury in soybean grown on the Litro clay and Sto ugh fsl was probably caused by soil-Al effects while low soil calcium (Ca) and high soil Mn was likely responsible for lower yields from the Mahan fsl. Leaf Ca from the limed Mahan-soil treatment was 5-fold gre ater and leaf-Mn 7-fold less than control levels. Regression analyses' R(2) values were similar for all soil tests except for CaCl2-extracta ble Mn, which was lower. Soil tests were compared across soil type by selecting treatments that had the same 85% relative yield. Using this data subset, there was no difference in the soil pH among the four soi ls, while there were significant differences among soils for all other soil test measurements indicating the superiority of soil pH for iden tifying acid-soil injury. Critical rest values were 5.1 soil pH, 30 mg kg(-1) extractable Al, 7% Al saturation, 0.7 mg . kg(-1) CaCl2-extrac table Al, and 9 mg . kg(-1) CaCl2-extractable Mn.