Improving Root Growth and Nodule Formation of Two Soybean Cultivars Using Combinations of Local <i>Bradyrhizobium japonicum</i> Inoculants with Cobalt in Gypsum Soil

Abstract A factorial experiment was conducted following a Randomized Complete Block Design (RCBD) with a split-split plot arrangement and three replications during the summer agricultural season on 10/07/2024 at the Soil Science and Water Resources Research Station, Faculty of Agriculture, Tikrit University, Salahuddin Governorate. The study intends to investigate the effects of locally made inoculum derived from the bacterial isolate Bradyrhizobium japonicum . And it will focus on cobalt effects on both growth and yield of the two soybean ( Glycine max L.) varieties in gypsum soil. The experiment consisted of three factors: main factor being soybean varieties (Shaima and Yamasoya); the sub factor was cobalt applied at three different levels (0, 0.5, and 1 kg ha⁻); and the sub-sub factor was inoculation at four different levels (no inoculation, local inoculum A3, inoculum W and commercial inoculum T). the results indicated that the Shaima variety significantly outperformed Yamasoya in root nodule number, fresh and dry weights, and plant height by 16.62%, 16.12%, 20%, and 7.66%, respectively. Inoculated treatments surpassed non-inoculated ones by 321.15%, 276.92%, 220%, and 25.40% in the same traits. Cobalt fertilization (1 kg Co ha −1 ) enhanced all parameters compared to non-fertilized treatments by 28.11%, 31.03%, 30%, and 6.69%. The Shaima-A3 inoculum interaction excelled over non-inoculated Yamasoya by 541.04%, 537.5%, 325%, and 38.06%. Shaima with 1 kg Co ha −1 outperformed non-fertilized Yamasoya by 52.54%, 56%, 44.44%, and 15.42%. The A3 inoculum with 1 kg Co ha −1 surpassed non-treated controls by 758.67%, 671%, 500%, and 36.32%. The triple interaction among factors significantly improved all traits, yielding the highest values.