CALIBRATION OF THE SINGLE-BUFFER AND DOUBLE-BUFFER SMP LIME REQUIREMENT METHODS BY ROOT ELONGATION BIOASSAY

Chemical and biological lime requirement (LR) reference values of 154 soils were obtained by six months of incubation of each soil with five levels of calcium carbonate (CaCO3). Levels of CaCO3 addition differe d among soils according to their characteristics. Chemical LR values w ere based on the individual neutralization curves to achieve a desired pH (pH(d)) value of 5.5, 6.0, and 6.5 in water or 5.0, 5.5, and 6.0 i n 0.01M calcium chloride (CaCl2). Biological LR values were estimated to achieve 90% relative root elongation on each soil after a growth pe riod of 48 h using wheat cv Abe. Chemical values of LR suggest that SM P method is valid for a wide range of mineral soils from different geo graphic regions. However, the proportion of soil acidity reacting with the buffer is not constant. Results indicate that values obtained wit h the routine methods need to be calibrated with equations different f rom the originals. The use of curvilinear models to adjust one single pH of the soil-buffer system improved substantially its accuracy, allo wing the single-buffer (SE) results to be comparable to the more time consuming and labourious double-buffer (DB) technique. No advantage wa s noticed with the use of curvilinear equations for DB technique. The adoption of pH(d) in 0.01M CaCl2 leads to an increase of precision of the predicted LR. Regression equations are provided for calculating LR rates to different pH(d) values. Accuracy is high (r(2)=0.887) even f or pH values (5.0 in 0.01M CaCl2) lower than normally considered in me thods based on buffer solutions. An overestimation of biological LR va lues was observed with both SMP methods. Notwithstanding, a calibratio n can be also made with the root bioassay, adjusting the chemical valu es to lime rates based on biological constraints related with aluminum (Al) toxicity. Regression equations are provided. Once more, the use of quadratic model for SE method allows an accuracy (r(2)=0.836) compa rable with the DB technique (r(2)=0.850).