CONVERTING ROOT NUMBERS OBSERVED AT MINIRHIZOTRONS TO EQUIVALENT ROOTLENGTH DENSITY

Root length density (RLD) is the measure of plant root growth having g reatest general utility. The use of clear material observation tubes, known as minirhizotrons (MR), in field soil has proved to be an effect ive method for measurement of root growth dynamics. To convert numbers of roots observed at MRs to equivalent bulk soil RLD, workers have us ed empirical and time-costly calibrations with soil sample-derived RLD . A modified version of an earlier theory for converting MR observatio ns 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 fact or ranging in value from 3.0 to 3.8 will convert MR interceptions per incremental observation area to equivalent soil RLD for MR tubes rangi ng in diameter from 9.6 to 2.5 cm. An experimental test with cotton (G ossypium hirsutum L.), presented here, and one from the literature usi ng wheat (Triticum aestivum L.), both give numerical support to the th eory. Another test from the literature using cotton appeared to indica te 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 th e latter test. Inaccuracies in RLD determination from soil sampling ma y affect the accuracy of empirical calibrations of MR tubes to an exte nt comparable to uncertainties about the predictive accuracy of the th eory.