LOW NODULATION AND N2 FIXATION LIMITS YIELD OF PIGEONPEA ON ALKALINE VERTISOLS OF NORTHERN NSW - EFFECT OF IRON, RHIZOBIA AND PLANT GENOTYPE

Pigeonpea grown on the alkaline black earths (vertisols) of northern New South Wales often fails to fix detectable amounts of nitrogen (N), resulting in N deficient crops and depressed seed yields. We aimed to solve this problem through selection of plant genotypes that were adapted to these soil types. Selection criteria included the agronomic characteristics of maturity (days to flowering), determinacy and seed size, and symbiotic characteristics of nodulation and N2 fixation. In addition, the interactive effects of fertilizer Fe and N, plant genotype and strain of rhizobia on those characteristics were examined. The experiments reported were sown between November 1988 and December 1990 on alkaline vertisols (pH range 7.9-8.2) at Tamworth, Currabubula and Breeza, N.S.W. In Experiment 1, 108 genotypes were inoculated with Bradyrhizobium strain CB756 and sown at two sites. Nitrogen fixation could not be detected for 86 of the 100 genotypes sown at the Tamworth Agricultural Research Centre and for 51 of the 62 genotypes sown at Currabubula. Six of the genotypes had Pfix (proportion of plant N derived from N2 fixation) values of 10-19% compared with values of zero for five of the six combinations of three check cultivars (Quest, Quantum and Campea)x2 sites. A further five genotypes produced seed yields up to 0.20 t/ha (14%) higher than the average of the three check cultivars. Effects of fertilizer Fe (applied at sowing as FeEDDHA at rates of 0, 0.25, 1.0, 4.0 and 16.0 kg Fe/ha) and N (as nitram at 0 and 200 kg N/ha) and strain of Bradyrhizobium sp. (CB756 and CB1024) on symbiotic and yield parameters of pigeonpea cv. Quest were examined in Experiment 2. Nodulation at flowering was increased by as much as 300% by Fe; by early pod-fill, effects of Fe has almost disappeared. Pfix values were unaffected by Fe. Throughout growth, shoot dry matters increased with increasing rates of Fe, particularly when N was also applied. In the presence of fertilizer N, seed yields increased with increasing rates of Fe (1.67 t/ha at Fe0 to 2.43 t/ha at Fe16.0). Responses to Fe in the absence of N were reduced. Plants inoculated with rhizobial strain CB1024 outyielded plants inoculated with strain CB756 by 1.0 t/ha (Fe0) and 0.66 t/ha (Fe16.0). In Experiment 3, we examined the effects of fertilizer Fe on symbiotic and yield parameters of 14 genotypes of pigeonpea. There were significant differences between plant genotypes in nodulation, N2 fixation and yields of shoots and seed when fertilizer Fe was not applied and in responsiveness to applied Fe. Seed yields were increased by an average of 0.70 t/ha (108%) for highly responsive genotypes E33, M96, L133 and Campea, but only by an average of 0.32 t/ha (25%) for the low response group (E76, M48, M144 and Quest).