Molecular Soil Biology 2015, Vol.6, No.4, 1-12
7
nitrogen to a cropping system, they must have
appropriate rhizobia partners. The capacity of nitrogen
fixation in differing rhizobia strains
varies (Albareda
et al
.,
2008). Differences between
Bradyrhizobium
strains regarding their effectiveness with different
soybean genotypes have been reported (Tien et al.,
2002; Mahna et al., 2006). Similarly, Annapuma and
Krishnan (2003) revealed that compatibility of
Sinorhizobium fredii
with soybean is strain and
cultivar dependent. Hence, matching rhizobial strains to
host legumes is the most important factor in maximizing
N fixation and productivity of grain legume.
Researches have demonstrated enormous potential for
developing effective N-fixing inoculants to enhance N
fertility in grain legume production. Inoculating
legumes with adaptable and effective species of
specific rhizobia increase the success of their
establishment, nodulation, biomass and N yields
(Adamu et al.
,
2001; Habtegebrail and Singh, 2006).
Variations in nodulation and N
2
-fixation efficiency
frequently occur in a bacteria strain-legume cultivar
specific manner (Sanginga et al
.,
2000). Ampomah et
al. (2008) evaluated the ability of five isolates from
cowpea to nodulate and effectively fix nitrogen on
groundnut (
Arachis hypogeae
), mungbean (
Vigna
radiata
), and soybean (
Glycine max
). Except for
cowpea where all the isolates were symbiotically
effective, there was variation in the symbiotic
effectiveness of the isolates among the different
host-legume varieties. Genotype of both the host and
the competing rhizobia strains have been shown to
influence the outcome. Genetic variation for N
2
-fixation
ability has been reported involving both the legume
and rhizobium components of the symbiotic
association (Sanginga et al
.,
2000).
Inoculation response also varies with degree of
promiscuity of legume species. The International
Institute of Tropical Agriculture (IITA) developed
promiscuous soybean varieties, which are capable of
establishing symbiotic relationship with indigenous
brandyrhizobia, as a practical alternative to inoculation
by African farmers (Dashiell et al
.,
1983). The
resulting soybean varieties form significantly more
nodules in un-inoculated native soils than traditional
varieties (Kasasa et al.
,
1998). Contrarily, Sanginga et
al.
(2000) found that promiscuous soybean is
incapable of nodulating effectively with indigenous
rhizobia in all locations in the moist savanna zone of
Nigeria. As the promiscuous varieties have shown
inconsistence in nodulation, it may be safer to rely on
effective inoculant strains rather than breed for the
ability to nodulate with indigenous rhizobial strains of
unknown potential.
Muhammad (2010) observed the response to inoculation
by promiscuous soybean varieties. Solomon et al.
(2012), Argaw (2014) found significant interactions
between soybean variety and inoculation status for all
traits scored regardless of promiscuity. Understanding
the rhizobia-legume interaction is important for the
development of rhizobial strains and legume cultivars
with high N
2
-fixation potential. Simultaneous
selection for the optimal combination of the rhizobium
and the host usually results in more effective
symbiosis and better growth of the host plant.
Breeding programs can help to develop crop varieties
that nodulate under targeted environment. Molecular
soil microbiology together with screening genotypes
and classical breeding techniques will increase
productivity of symbioses and eventually result
in maximum economic yields of crop plants (Rengel,
2002). Efforts to develop rhizobial inoculants also
need to be accompanied by research that facilitates
their efficient use that is relevant to resource-poor
farmers.
Nitrogen and Phosphorus Uptake
Nutrients are essential components required by living
plants for normal growth and development. Plants
require 17 essential elements of which carbon,
hydrogen, and oxygen are derived from t he
atmosphere and soil water (Uchida, 2000) whereas the
rest are obtained from the soil mineral, soil organic
matter and organic or inorganic fertilizers (Roy et al
.,
2006; Ndakidemi et al.
,
2011). Each nutrient is needed
in different amounts by the plant, and varies in how
mobile it is within the plant. Nutrient uptake by plants
depends mainly on the quantity, concentration and
activities in the root zone soil as well as the capacity
of soil to replenish the concerned nutrients in the soil
solution (Makoi et al
.,
2013).
Nitrogen (N) and phosphorus (P) are chief elements
that are richly available in the atmosphere and soil
respectively, but inaccessible forms to plants (Acharya
et al
.,
2012). These elements are often limiting factors
