Production Practices and Quality Assessment of Food Crops. Vol. 1

Improvement of Grain Legume Production in Semi-Arid Kenya 171
2. IMPROVEMENT OF TEPARY BEAN YIELDS THROUGH
BIOLOGICAL NITROGEN FIXATION IN SEMI-ARID KENYA
2.1. Biological nitrogen fixation: potentials and limitations
There has been tremendous interest in biological nitrogen fixation as an alternative
to mineral fertilisers in agricultural systems (Gitonga et al., 1999; Hornetz et
al., 2000; Maingi et al., 2001; Shisanya, 2002). The Kenyan government has encouraged
complementation of inorganic fertilisers through alternatives such as
biofertilizers (using N 2 – fixing agents) as pointed out in the National Development
Plan (Republic of Kenya, 2002). Biological nitrogen fixation (BNF) contributes to
productivity both directly, where the fixed N 2 is harvested in grains or other food
for human and animal consumption or indirectly through enhancement of soil fertility
(Giller and Cadish, 1995). Biological nitrogen fixation represents a particularly
renewable source of N for many farming systems (Peoples and Jensen, 1999). Global
estimates of BNF to soil nitrogen vary from 135–175 million tonnes annually
(Paul, 1988). The calculated annual amounts fixed range from 3–160 kg N
ha –1 yr –1 shoot N for annual pasture species, 37–128 kg N ha –1 yr –1 for lucern, and
14–160 kg N ha –1 yr –1 for pulses (Peoples et al., 2001). Other sources of nitrogen
accumulation in the soil include decomposition of organic matter, action of lightning
and use of N fertilisers among others.
Legumes of all categories contribute to N fertility in various cropping practices
including fallow, agroforestry and rotational systems. Low input legume-based
agriculture exists in a continuum between subsistence farming and intensive arable
and pastoral systems. Pastoral systems reliant solely on fixed N are capable of
moderately high production with modest N losses (Ledgard, 2001). The principal
factors regulating BNF can be summarised in terms of environmental or management
constraints to legume growth (basic agronomy, nutrition, water supply, diseases
and pests) or result from local practices that impact on percentage N fixation (Peoples
and Jensen, 1999). The low moisture contents in soils of Sahelian Africa can
reduce nodule functioning in symbiotic legumes through drought-induced collapse
of lenticels, decreased nitrogenase activity, and reduced respiratory capacity of
bacteroids and decline in leghaemoglobin content of nodules (Guerin et al., 1990).
In many parts of the world, reduced water supply limits fixation in the field. In
the semi-arid tropics, crops often suffer water stress for various periods during
the growth cycle (Hornetz et al., 2001). Drought can also affect longevity of introduced
rhizobia and a decline occurs with low moisture and soil desiccation (Hornetz
et al., 2000). Consequently, nodulation fails to occur through loss of infection sites
due to changes in the morphology of infectible root hairs (Sprent and Sprent,
1990). In the tropics, legumes have the ability to produce nodules on more acid soils
and soils deficient in phosphorus (P) and calcium (Ca), and other nutrients than
in temperate countries (Shisanya, 2002). In the temperate regions nodulation is
poor in seasons characterised by low light intensity (Ledgard, 2001).
A study in Australia found that N fixation was primarily regulated by biomass
production and that both pasture and crop legumes fixed between 20 and 25 kg shoot
N for every tonne of shoot dry matter (DM) produced (Peoples et al., 2001).