Lateral movement of strains of Bradyrhizobium from inoculated seed of Macroptilium atropurpureum and Desmodium intortum sown in the field

Lateral movement of bradyrhizobia introduced into field soils as seed inocula was assessed by recording nodule positions, on a two-dimensional grid, of root samples of Desmodium intortum, Neonotonia wighlii and Macroptilium atropurpureum. and from concentric band samples taken around individually spaced plants of D. intortum and M. atropurpureum. Nodules due to inoculum strains were identified by serological typing. Spread of the inoculum varied over sites with soil type and between species, and was influenced by the presence of existing soil strains. However, even when soil strains accounted for very few nodules the inoculum strain had rarely moved laterally more than 15 cm nor downward beyond 10 cm during 63 weeks. For singly spaced plants a 30cm dia × 15 cm core-sample (placed centrally over the tap root) satisfactorily recovered the majority of nodules formed by an inoculum strain up to 16 weeks from sowing. This proportion markedly decreased by 63 weeks when the majority of nodules were > 15cm from the tap root. The data suggest caution in sampling of field experiments in trials designed to evaluate movement and nodulation success of inoculum strains.