MODULATION PROMISCUITY OF SOYBEAN GENOTYPES

ABSTRACT

The objective o f the study was to obtain information on the existence o f Bradyrhizobium

japonicum strains in Ghanaian soils, evaluate their effectiveness with the view to improving

nodulation, nitrogen fixation and yield potential o f 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 o f 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.