THE NUMBER OF BRADYRHIZOBIUM SP (LUPINUS) APPLIED TO SEED AND ITS EFFECT ON RHIZOSPHERE COLONIZATION, NODULATION AND YIELD OF LUPIN

In field experiments we investigated the roles of inoculum potential o f Bradyrhizobium sp. (Lupinus) and quantity of peat used to inoculate seed on the nodulation and yield of lupins. Within the range 0.125-3 t imes the Australian recommended rate of peat application (2.5 g peat k g-1 lupin seed), the amount of peat had no effect on nodulation or gra in yield. In the first experiment, seven inoculum potentials were appl ied within the range log10 0.32-6.28 bradyrhizobia per seed in 7, 10-f old increments which spanned the recommended rate of log10 5.55 per se ed. Inoculum potentials of log10 6.27 and 5.27 improved the colonizati on of lupin rhizospheres and increased early nodulation, nodule number and nodule mass. Nodule mass was increased from 65 to 393 mg plant-1 at 43 days by increasing the inoculum from log10 4.27 to 6.27 bradyrhi zobia seed-1. Grain yield and % N in the grain were not significantly different (P > 0.05) between potentials of log10 4.27 and 6.27. In the second experiment, higher potentials of 6.80 and 7.28 further improve d rhizosphere colonization and increased nodule mass. Studies of the s urvival of the inoculum, during inoculation, sowing and in the soil, i dentified a large mortality factor; 95% of bacteria died between inocu lation and sowing and of those surviving, 83% died after 22.5 h in the soil. These observations have important implications for setting new standards for commercial inoculants and for emphasizing the care neede d in handling inoculated seed to reduce the death of bradyrhizobia in the period between inoculation and sowing.