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

174 Chris A. Shisanya
on drought-adapted legumes, e.g. TB in semi-arid SE-Kenya (Gitonga et al., 1999;
Maingi et al., 2001). The study by Pilbeam et al. (1995) found significant N increases
in the plant tissues of drought-adapted cowpea (Vigna unguiculata L. Walp) in the
study area. Additionally, N fertilizer application had no significant effect on the
dry matter yield and N content of common bean (Phaseolus vulgaris L. cv. B9).
A limiting factor in the dry lands of southeast Kenya is that the prevalent soils,
i.e. Fluvisols, Luvisols and Ferralsols (Eichinger, 1999) release important available
plant nutrients only after a very short period of cultivation and through leaching
(Hornetz, 1997). Systematic research on the eco-physiological demands, drought
resistance and yield potentials of TB in potential cultivation areas in the ASALs
of southeast and northern Kenya began in the late 1980s and intensified in the 1990s
(Hornetz, 1988, 1990, 1991a, b, 1997; and Shisanya, 1996, 1998). No research
attention has been given to N 2 fixation in TB, which has recently become a prominent
legume crop among resource poor small-scale farmers in SE-Kenya (Shisanya,
1999). The main objective of this study was therefore to investigate N 2 fixation in
TB through inoculation by different host-specific rhizobia strains under the semiarid
environment of SE-Kenya.
2.4. Biological nitrogen fixation experiments with tepary bean in semi-arid
1.2. Kenya
2.4.1. Experimental site
The experiments were carried out at KARI Kiboko sub-centre (latitude 02°12′ S,
longitude 37°43′ E, altitude 975 m a.s.l.), located at about 160 km southeast of
Nairobi, the capital town of Kenya (Figure 1). The soils of the study area are well
drained Fluvisols, Ferralsols and Luvisols (Eichinger, 1999). The soil pH of the
experimental field is 7.9 (measured in 0.01 ML –1 CaCl 2). Rainfall is bimodally
distributed, with median monthly maximum in April (126 mm) and November
(138 mm). The medial annual rainfall is about 582 mm yr –1 . The SR (October–
January) generally has more rainfall and are more reliable than the LR (March–June)
(Musembi and Griffiths, 1986). The lengths of the agrohumid periods for droughtadapted
crops are 50–55 days (LR) and 65–70 days (SR) (Jaetzold and Schmidt,
1983). Average monthly temperatures are highest in February (24.3 °C) and October
(23.4 °C) (KMD, 1984), prior to the onset of the rains in March and November,
respectively.
2.4.2. Soil sampling and analyses
Soil samples were collected to a depth of 60 cm using a soil auger before planting.
Five sub-samples were collected from each plot, bulked, mixed thoroughly in
polythene bags and transported to the laboratory. Sub-sample was air-dried while
the remainder was refrigerated at 4 °C. The air-dried samples were sieved (2 mm)
and a sub-sample was further ground to pass through a 0.25 mm sieve for total C
and N analyses. Carbon and nitrogen contents in the soil were determined. The