Poor nodulation of lupins and tropical legumes in northern New South Wales

Abstract

Nodulation problems with tropical legumes and lupins in northern New South Wales appeared due to the inoculant's failure to establish itself around the seedling root, and were more severe with direct drilling. The cause of this was investigated in a number of laboratory, glasshouse and field experiments. Krasnozem soils were found to be more inhibitory than three other types, but this did not appear related to pH or manganese content. Raising the pH of krasnozem soil with lime did not improve survival of rhizobia unless the soil was autoclaved. Soil extracts suppressed multiplication in sterile nutrient sand except when micro-filtered, but a greater increase occurred with no extract. Extracts of both carpet grass and kikuyu depressed nodulation of glycine (Neonotonia wightii) but only the carpet grass extract adversely affected Siratro (Macroptilium atropurpureum). Root washings of the two grasses restricted multiplication of Rhizobium lupini, kikuyu having the greater effect. Lupin seedlings growing in the presence of kikuyu roots had fewer rhizobia in the root zone and were less well nodulated except when inoculated at a high rate. The susceptibility of WU425 and the clover strain TA1 appears not to differ and the host plant (lupin or clover) did not influence rhizobium numbers in the rhizosphere. In the field both the number of rhizobia in the root zone of lupins and the percentage of plants nodulated, increased with increasing rate of inoculation. The results indicate that suppression is due to both micro-organisms and a heat labile chemical factor.