J. HOMCHAN¹, R.J. ROUGHLEY² and R.A. DATE³

¹Department of Soil Science, University of Khon Kaen, Khon Kaen, Thailand.
²New South Wales Department of Agriculture, Horticultural Research & Advisory Station, Gosford, NSW 2250, Australia.
³CSIRO Division of Tropical Crops and Pastures, St Lucia, 4067, Queensland, Australia.

Abstract

The responses to inoculation with highly effective strains of root-nodule bacteria by Stylosanthes humilis and S. hamata were investigated on Korat, Namphong and Yasothon soils, the main soil series of N.E. Thailand. Inoculation with peat-based inoculants containing > 10⁹ bradyrhizobia/g carrier significantly increased plant growth but the response was variable between soils and sites within soils. Based on experiments at 2 sites, failure to respond to inoculation could be attributed to the failure of the inoculant to form a significant proportion of nodules in competition with resident strains even when more than one million bradyrhizobia/seed were applied. Application of the inoculant using gum arabic or polyvinyl pyrrolidone as adhesives under a coating of CaCO₃ failed to increase the number of bradyrhizobia in the rhizosphere of 14-day seedlings or the proportion of nodules due to the inoculant strain. The resident bradyrhizobia were only partially. effective, failing to produce as much dry matter as obtained by applying 100 kg N/ha as urea. Therefore, there is a need to select a more competitive and effective strain for use with Stylosanthes.