Soybean nodulation patterns in relation with soil properties under three tillage systems.

Soybean (Glycine max (L.) Merrill cv. Prata) nodulation patterns were evaluated under three tillage systems (LC = conventional tillage or moldboard plowing, LV = vertical tillage or chisel plowing and SD = no-till or direct drilling). The study was done in a Typic Hapludoll from the pampean subhumid region. The soil presented a high number of naturalized Bradyrhizobium japonicum. The mean differences in nodulation were detected in the upper 5 cm of the main root (P0-5). The number, the total dry weight (PST) and the individual dry weight of the nodules were higher under SD than under LV and LC (p<0.01). In the two evaluation dates (70 and 119 days after planting), the differences in P0-5 nodulation were positively related with the relative soil water content (0-20 cm) at sowing [Number: R2 (70 dds) = 0.77 and R2 (119 dds) = 0.70 p<0.01 and PST: R2 (70 dds) = 0.70 and R2 (119 dds) = 0.73 p<0.01]. Neither difference due to the tillage systems in the soil NNO3 levels and in the soybean grain yields, nor correlation with the nodulation pattern were observed. We conclude that the better moisture conditions in the soil during the seeding under SD favored the initial infection with Bradyrhizobium japonicum.

Key words: soil water, nitrogen, Bradyrhizobium japonicum, no till, dryland faIming, Pampean subhumid region.