Crack fertilization is a soybean cultivation technique for nodulation control in which midterm subsoiling is used to supply fertilizing materials to deep soil just before the flowering stage. This study examined the effects of fertilizing materials and the continuous application of nodulation control, on soybean yield enhancement in two field experiments. The survival of nodule bacteria in deep soil was also evaluated by a bioassay of nodule bacteria in a root box. When the nodule bacteria on biochar were continuously applied without any other chemical fertilizers for three successive years, seed weight was significantly heavier being up to 1.34 times that of the control. The application of nodulation control in the previous year but not in the experimental year did not have residual effects on seed weight. The enhancement of seed weight in a farm field converted from a paddy was much lower. This may be partly attributed to the midterm tillage practice, which destroys the crack structure after the nodulation control, together with soil water status and cultivar differences. Nodule growth and nitrogen fixation activities significantly increased in the soybean plants grown on the soil collected from the subsoil to which nodule bacteria on biochar had been applied the previous year. This suggests that nodule bacteria in the subsoil survived in the biochar habitat for at least a year after application. These results indicate that nodulation control by the crack fertilization technique leads to yield enhancement when nodule bacteria on biochar are continuously applied.

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