CHOOSING MODELS FOR SOIL CHRONOFUNCTIONS AND FITTING THEM TO DATA

The effectiveness and utility of soil chronofunctions is examined in t he light of existing pedogenic theory. Statistical treatments applied in chronofunction research are reviewed, including linear transformati ons of raw data, which may improve the utility of the chronofunction. We advocate using a particular statistical model only if it can be jus tified based on our current understanding of the functioning of the pe dologic system. We emphasize the potential difficulties of using linea r chronofunction methods; simple linear and logarithmic functions are not always the best option for chronofunctions. Hyperbolic, polynomial or nonlinear functions might improve not only the fit of the chronofu nction but also advance our understanding of the pedologic system. Whe n the chronofunction explaining the most variance is not best suited t o a process-based understanding of pedogenic theory, other functions w ith slightly smaller r2 values that have been judged suitable by a pri ori reasoning or theory might better reflect the functioning of the pe dologic system. We justify limited extrapolation of some chronofunctio ns to time zero and discuss how this can be useful in the identificati on of pedogenic thresholds and 'step functions'. Interpretation of the Y-intercept of chronofunctions can, at times, aid our understanding o f the soil system near time zero and be useful in the identification o f the existence and timing of thresholds.