CHAPTER ONE INTRODUCTION
Soybean (Gfycine m‹zr (L.) Merril) is a leguminous crop, which has assumed world- wide importance through its industrial and nutritional uses. Its potential for cultivation in Ghana has attracted interest over the years.
The crop has the ability to fix atmospheric nitrogen and become less dependent on soil nitrogen. Fixation of atmospheric nitrogen in the root nodules contributes up to 70% of the total nitrogen uptake by the plant (Weber, 1966).
Soybean production in the tropics is constrained by poor nodulation due to absence of appropriate Bradyrhizobium spp_ and deficiency of phosphorous, potassium, molybdenum and sulphur in the soil (Singh and Rachie, 1987). Phosphorous appears to be the most limiting nutrient for the growth of legumes in tropical and subtropical regions (Ae et al., 1991). In Ghana the problem of phosphorus deficiency and its attendant effect on low crop yields have long been recognized (Nye, 1952).
Several researchers have reported that inoculation of soybean with B. japonicum increased yield (Dadson and Acquaah, 1984; Mahmoud et al.,. 1991; Badawy et al., 1992 and Mahmoud and El-far, 1991).
Although bradyrhizobial inoculation technology is simple and extremely cost effective for soybean production legume inoculation is not practised by most African farmers.
A practical alternative to inoculation has been the development of soybean varieties capable of forming effective symbiosis with indigenous cowpea bradyrhizobia (Kueneman et al., 1984).
In Ghana, information on the inoculation of promiscuous soybean genotypes and their response is lacking. For effective comparison between the nodulation of promiscuous soybeans cultivars in our soils, it is important to compare with the non- promiscuous cultivar.
For legumes, P enhances both nodulation and N2 fixation (Israel, 1987). In this regard there is the need to study the responses of the different cultivars to varying levels of phosphorus.
The objective of this study therefore was to evaluate the effect of Bradyrhizobium inoculation and phosphorus application and their combined effect on nodulation, growth and yield of both promiscuous and non-promiscuous soybean cultivars.
- The effect of bradyrhizobial inoculation on nodulation and nitrogen fixation in soybean
- Variability in Bradyrhizobium strains
Poor nodulation of soybean by indigenous bradyrhizobia is one of the major constraints to the successful production of soybean in Africa (Singh and Rachie, 1987). Where no soybean crop has been grown before, it is usually necessary to inoculate with an efficient Bradyrhizobium strain to maximize yield when fertilizer nitrogen is not applied (Dadson and Acquaah, 1984). Significant responses to bradyrhizobial inoculation are observed when the crop is grown in areas where it has not been previously cultivated (Abel and Erdman, 1964; Kang, 1975) and yield increases as high as six-fold have been obtained (Bromfield and Ayanaba, 1980). In some soils where soybean has been grown previously, continued inoculation may still be necessary (Rao et al., 1985) apparently because of poor survival of introduced rhizobia.
A practical alternative to the use of inoculants in developing countries may be the development of cultivars capable of forming an effective symbiosis with the indigenous rhizobia. Through breeding, a number of cultivars which nodulate freely have been released in recent times (Kueneman et al.,1984).
On the other hand, the development of “promiscuous” soybean varieties with unknown nitrogen fixing abilities and rhizobial association may not be a very attractive alternative compared to the conventional inoculation methods when the benefits of the legume/rhizobial symbiosis is to be maximized (Woomer ef al.,1992). Consequently, if the available high yielding soybean cultivars are to be grown under tropical conditions of low soil N (Pulver el al.,1992), inoculation will be essential.
Mandimba and Mandibaye (1996) found that inoculation with B. japonicum strains TAL 377 and TAL 379 improved nodule number per plant up to 355 and 380% and nodule mass per plant by up to 366 and 182%, respectively, compared to the uninoculated control. Furthermore, inoculation increased shoot N, plant biomass and yield. Inoculation was equivalent to the application of 75 and 65 kg N/ha for TAL 377 and TAL 379 strains respectively.