Tillage depth effects on nodulation, nitrogen fixation and yield of three soybean varieties in the Northern Savanna zone of Ghana

Most of the farmers in the Northern Savanna zone of Ghana cannot afford the required quantities of synthetic fertilizers to apply to their crops to increase their yields. The inclusion of legumes such as soybean in the cropping systems of the zone has been found to be one of the alternatives. In order to assess the yield and Nitrogen (N) fixed by three varieties of soybean as affected by four tillage depths, a field experiment was conducted at Nyankpala in 2007 and 2008 cropping seasons. The experiment was laid in a split-plot design replicated 4 times with the main factor being tillage depths of 0, 10, 20, and 30 cm while the sub-plot treatments were soyabean varieties (Jenguma, Quarshie and Salintuya-2). The results showed that, conventional tillage significantly (p < 0.05) reduced the bulk density of the soil and significantly (p < 0.05) increased the porosity in the 0 to 15 cm layer of the soil. There were significant influences of tillage on all the plant variables measured. Plant height, grain yield, root length density, nodules per plant and the N fixed by soybean were all highest on the deepest tillage (T30) with the lowest values of these variables on the no-till plots (T0). Among the varieties, Salintuya-2 produced significantly higher values in all the variables except plant height than Quarshie and Jenguma. However, there were no significant differences in values of these variables obtained between Quarshie and Jenguma in the experiment. Correlation analysis also showed that, grain yield was positively correlated to percent of N fixed and root length density but the relationship with plant height was not significant. Also, N content fixed was not significantly correlated to plant height. Tillage under semi-arid conditions such as found in northern Ghana is therefore necessary for high N fixation and grain yield of soybean and among the soybean varieties, Salintuya-2 is found to be the best variety in both the grain yield and N fixed in this study.   Key words: Tillage depth, soybean varieties, N fixation, cropping systems, fertilizers.