RHIZOBIAL NODULATION OF ACACIA TREE SPECIES IN SUDAN: SOIL INOCULUM POTENTIAL AND EFFECTS OF PEAT

Abstract

Soil cores were removed in 20 cm fractions to 1 m depth from beneath five apparently unnodulated mature Acacia mellifera trees growing in the clay plains of east Sudan. Seedlings of Acacia mellifera grown in this soil in pots at Khartoum and in a tropicalised glasshouse near Edinburgh, Scotland, produced root nodules regardless of the depth or tree from which the soil had been taken. Supplementary nutrition and inoculation with compatible Rhizobia had no significant effect on nodulation, although nutrition increased seedling growth. It was concluded that the inoculum potential of the soil in Sudan was high, but that nodulation in the field was inhibited by lack of water. Seedlings of A. mellifera, A. Senegal and A. seyal grown in a tree nursery in Sudan produced substantial numbers of nodules when peat was added to the Nile silt/sand medium . Improved aeration seemed the most likely reason for the stimulation of nodule production.