G. Milev, P. Yankov

Abstract. A field experiment was carried out in the trial field of Dobrudzha Agricultural Institute – General Toshevo for a period of three successive years with the aim to study the degree of compaction of soil on nodulation of common bean (Phaseolus vulgaris L.). The common bean cultivar Dobrudzhansky 2, which is well adapted to the local environment, was used for this investigation. The soil in the trial field was slightly leached chernozem with very good distribution of natural strains of bean nodule bacteria Rhizobium leguminosarum biovar. phaseoli. The degrees of soil compaction were achieved through an increasing number of tilths with heavy rollers for the respective variants of the trial. The effect of the different bulk density of soil was traced on the nodulation indices: number and dry weight of nodules per plant. The correlations between soil moisture, bulk density, nodulation parameters and seed yield were analyzed. As a result from these analyses it was found that the higher bulk density had stronger effect on the index dry weight of nodules, while the effect of this variable on the number of nodules per plant was lower. These effects were equally well expressed under conditions of higher soil moisture and very dry soil. The increased bulkdensity of soil significantly reduced yield under drought while under favorable soil moisture this harmful effect was much lower and statistically insignificant.