Th e issue of competition for nodulation has received much attention in studies on soybean [Glycine max (L.) Merr.] crops because native soil rhizobia oft en preclude nodulation of inoculated high-quality strains. In this work, soil placement and enhanced motility of Bradyrhizobium japonicum were investigated as strategies to improve the competitiveness of soybean inoculants applied in the presence of large populations of native (or indigenous) soybean-nodulating rhizobia. Th e fi rst strategy might allow for the enhanced distribution of inoculant rhizobia in the surface soil, compared with inoculant that is directly applied to soybean seed, while the second strategy is expected to enhance root colonization. Th e eff ects of soil placement and enhanced bacterial motility on percentage nodule occupation, grain yield, and grain N contents were examined during the 2004-2005 soybean season at three locations in Argentina, comprising soils classifi ed as entic hapludol, argillic pelludert, and typical argiudol. Th e experimental design was a randomized complete block design, and main eff ects and interactions were also analyzed with a factorial design. Strain LP 3008, previously selected for increased motility, was more eff ective than the commonly inoculated strain E 109. While E 109 occupied, on average, 13.2% of nodules with seed inoculation, LP 3008 occupied 28.2%. In comparison, in-furrow inoculation yielded 24.2 and 37.2% average nodule occupancies for E 109 and LP 3008, respectively. However, these gains in nodulation effi cacy lead to a modest, nonsignifi cant yield increase, and grain N content was unaff ected.
Termos para indexação: aborto de flores, aborto de legumes, estádios de desenvolvimento.
SOYBEAN YIELD POTENTIAL INFLUENCED BY SOIL PHOSPHORUS CONTENT AND ROW SPACINGABSTRACT -The experiment was performed during the 1994/95 growing season at the EEA/UFRGS, in Eldorado do Sul, RS, Brazil, in a dark red Podzolic soil (Rhodic Paleudult). The objective was to evaluate the soybean potential yield at the growth stages R2 (flowering), R5 (beginning of pod filling) and R8 (maturity). The treatments were arranged in subplots in a randomized complete block design, and consisted of two soil P levels (3 and 15 ppm), and two row spacings (20 and 40 cm). The early, determinate cultivar OCEPAR 14 was used. On the average of the treatments, potential yield in R2 was of 18 t/ha and of 10 t/ha in R5, and the final yield, in R8 of 4.6 t/ha. The treatment of 15 ppm of P reached greater potential yield in the three growth stages, due to less flower and pod abortion. The row spacing of 20 cm showed greater potential yield at the growth stages evaluated, but the percentage of yield reduction due to flower and pod abortion were similar.
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