Modulation Promiscuity of Soybean Genotypes.

Abstract

The objective of the study was to obtain information on the existence of Bradyrhizobium japonicum strains in Ghanaian soils, evaluate their effectiveness with the view to improving nodulation, nitrogen fixation and yield potential of soybean. Eight soil series were screened for nodulation capabilities of soybean using six cultivars (four promiscuous and two non-promiscuous). The soils were Adenta, Akuse, Anyinase and Bekwai; the rest were Hatso, Nyigbenya, Nzima and Toje. Four cultivars nodulated in four soils and one in five soils. There was no nodulation in Anyinase, Bekwai and Nzima soils. Bragg, a non-promiscuous genotype, nodulated considerably well contrary to documented reports in the literature that non-promiscuous American soybean genotypes do not normally nodulate in tropical soils. Most Probable Number (MPN) counts carried out established some relationship between nodulation and bradyrhizobia population in the soils used for the studies. Symbiotic effectiveness test carried out on 60 selected isolates from the screening experiment showed that 15% of the isolates were highly effective, 65% ineffective and 20% moderately effective. Inoculation studies were carried out on three soybean cultivars namely Bragg (Non- Promiscuous American genotype), Bengbie (Promiscuous) and TGx (Promiscuous) using five isolates from the screening experiment and two imported isolates from Thailand in the Bekwai soil. Generally inoculation led to improvement in shoot dry matter and total N, although the levels were different among the cultivars and isolates and thus showing that plant genotypes and bradyrhizobia strains significantly influenced inoculation response. Inoculation and N fertilization response carried out on four soybean cultivars, Bragg, Bengbie and TGx and Non-nodulating soybean genotype, in Adenta and Bekwai soils showed better nodulation and percent N-fixed in Adenta than Bekwai. This could be attributed to the higher bradyrhizobia count in Adenta than in Bekwai.Total N fixed was however higher in Bekwai than Adenta. This means that other factors inherent in Bekwai had enhanced plant growth and total N accumulation, and hence total nitrogen fixed. Bekwai soil had higher nitrogen, organic matter and phosphorus and was thus more capable of providing nutrients and plant requirement for better plant growth than Adenta. The higher nodulation, percent and total N fixed at the 10 kg N/ha rate than at 100 kg N/ha application could be attributed to the depressing or inhibitory effects that inorganic nitrogen fertilizers have on nodulation and nitrogen fixation.