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

Improvement of Grain Legume Production in Semi-Arid Kenya 175
colorimetric method described by Anderson and Ingram (1993) was used for soil
organic C. Total N was measured colorimetrically following Kjeldahl digestion
(Bremner and Mulvaney, 1982). Soil P was determined following the method of
Foster (1995). Analyses of soil N and P indicate a deficiency of both nutrients
(0.7 mg N/g soil and 3.0 mg P/kg soil in 0–60 cm soil depth, respectively). These
values are below the threshold values of 2.0 mg N/kg soil and 4.5 mg P/kg soil
for optimum crop growth in tropical soils, respectively (Kapkiyai et al., 1999).
The C/N ratio and CEC are 11.7 and 7.8 mmol/kg, respectively.
2.5. Greenhouse experiments
Glass growth tubes were used as the growth containers while the growth medium
was nutrient free vermiculite. Pre-germinated seeds (Somarsegaran and Hoben, 1985)
were transferred into the glass growth tubes, one seedling per tube. Uninoculated
control plants were used as material control to detect contamination over the growth
period in the greenhouse experiment. The Rhizobia were routinely grown on yeast
extract mannitol (YEMA). For inoculation purposes, the rhizobia were grown in
yeast extract mannitol broth (YEMB). Isolation, presumptive and authentication tests
were conducted according to the methods described by Somasegaran and Hoben
(1985). The tests carried out were Gram staining, growth of isolates on YEMA,
growth on YEMA plus Bromothymol blue (BTB) and growth of isolates YEMA
plus Congo red media. The plates were incubated in darkness at 20 °C for 3–5
days. The commercial Rhizobia strains used, i.e. R446 and R3254 were obtained
from Microbiological Resource Centre (MIRCEN), University of Nairobi, while
strains R578 and R579 were obtained from the Radizin Institut, Iserlohn, Germany.
A native strain RTB was isolated from Kiboko soils where tepary beans had been
grown.
For assessment of effectiveness of the various Rhizobia strains in nitrogen
fixation, a total of seven treatments were used. The experimental design was a
7 × 7 Latin square with 7 treatments. This design was chosen because it eliminates
row and column effects from the experimental error thus increasing the power
of ANOVA (Steel and Torrie, 1981). Each treatment constituted 7 replications.
The treatments were as follows:
(1) Nitrogen (N) tepary bean grown in N rich medium
(2) Control no inoculation of tepary bean and no N
(3) R578 tepary bean inoculated with rhizobium strain R578
(4) R446 tepary bean inoculated with rhizobium strain R446
(5) R579 tepary bean inoculated with rhizobium strain R579
(6) R3254 tepary bean inoculated with rhizobium strain R3254
(7) RTB tepary bean inoculated with rhizobium strain RTB
Nodule assessment was carried out in the greenhouse as described by Gitonga et
al. (1999). The number of Rhizobia in the soil was determined using the MPN
plant infection technique as described by Beck et al. (1993) (equation 3)