CONVERTING ROOT NUMBERS OBSERVED AT MINIRHIZOTRONS TO EQUIVALENT ROOT LENGTH DENSITY

Root length density (RLD) is the measure of plant root growth having greatest general utility. The use of clear material observation tubes, known as minirhizotrons (MR), in field soil has proved to be an effective method for measurement of root growth dynamics. To convert numbers of roots observed at MRs to equivalent bulk soil RLD, workers have used empirical and time-costly calibrations with soil sample-derived RLD. A modified version of an earlier theory for converting MR observations to soil RLD is presented. The theory is based on calculation of the mean root length that would occur inside a MR if it were not present and if all possible root growth orientations at a root interception by the MR are considered. The theory indicates that a dimensionless factor ranging in value from 3.0 to 3.8 will convert MR interceptions per incremental observation area to equivalent soil RLD for MR tubes ranging in diameter from 9.6 to 2.5 cm. An experimental test with cotton (Gossypium hirsutum L.), presented here, and one from the literature using wheat (Triticum aestivum L.), both give numerical support to the theory. Another test from the literature using cotton appeared to indicate that the theory numerically overpredicted RLD. In contrast, a test in corn (Zea mays L.) presented here showed the theory underpredicting RLD. Methodological factors appeared to weaken the significance of the latter test. Inaccuracies in RLD determination from soil sampling may affect the accuracy of empirical calibrations of MR tubes to an extent comparable to uncertainties about the predictive accuracy of the theory.