PLANT-SOIL INTERACTIONS ASSOCIATED WITH ACID, WEATHERED SOILS

Plant-soil interactions in weathered soils are so complex that unqualified statements about a suitable pH for plants are risky. Conventional experimental designs and statistical methods may not be appropriate for investigating such complexities. Lime experiments using continuous function designs and observation of plant response to indigenous variability in soil pH permit detailed observations of plant-soil interactions that are frequently not detected. A graphical boundary-line approach to interpreting data can make good sense out of apparent confusion. Increasing the pH of variable-charge soils by adding lime or by indigenous means increased CEC and retarded cation leaching, but Ca solubility changed very little over the range pH 5 to 6. N fixation and yield was closely related to soil pH, soil Mn and Mn uptake by soybean. This result was clearly demonstrated regardless of numerous other limiting factors. Plant yield response curves resolved into distinct segments that corresponded with associated soil properties. Excess Al compounded by Ca deficiency is suspect in the pH range < 5. Excess Mn, and Ca deficiency probably limited yields in the pH range 5.0 to 5.7. Yields were stable. and Ca and P were constant in the pH interval 5.7 to 6.0. Yields abruptly increased in the pH interval 6.0 to 6.3. This was associated with elevated Ca concentrations in soil solutions.