pod numbers at 42 DAE Fig. 3c. However, a significant difference P 5 0.05 was observed in
pods dry weights Fig. 3a. The N-fertiliser 1 treatment gave larger pod dry weightplant com-
paring to the control. From Fig. 3b, pod dry weightplant was significantly larger P 5 0.05 in
3 than 1. There was no significant difference P 5 0.05 in pod dry weightplant of intercrop
plants at final harvest.
Treatment 3 gave the largest seed dry weight plant followed by 1 Fig. 4a, significantly differ-
ent P 5 0.05 from 2. Significant differences P 5 0.05 were observed in total common bean
seed yieldha between the pure and intercrop treatments Table 1. There were no significant
differences P 5 0.05 in the 100-seed dry weights for all.
The analysis of soil nitrogen before and after the cropping season showed that plots with com-
mon beans as pure crop and intercrop had sta- bilised and partly enriched soil N during the
vegetative periods Fig. 5. Phosphorus content in the soil increased in all of the plots by between 5
and 66, being lowest in plots with common beans – maize intercrop and highest in pure com-
mon bean inoculated plants Fig. 6.
4. Discussion
Most researchers are aware of the benefits of inoculating legume seeds with suitable strains of
nodulating bacteria. A simple trial to determine the need for inoculation requires only three treat-
ments Vincent, 1974: a an uninoculated control to check for the presence or absence of native
rhizobia and their effectiveness; b an inoculated treatment using Rhizobium effective for the
present host; and c an uninoculated treatment plus N.
Plant dry weight was used indirectly to estimate N
2
fixation in the present study. There were no significant differences in the growth parameters
measured in the early development of common beans. This implies that they were deriving their
nutrients from the soil and seed and none had an advantage over the other. At final harvest, inocu-
lated beans and N treated common beans had significantly higher dry weights than the control.
This is an indication that inoculation had a sig- nificant effect on N
2
fixation and therefore the indigenous rhizobia were not so efficient in fixa-
tion Wani et al., 1995. These results are contrary of those found by Pilbeam et al. 1995, who
reported that both N-fertilisation and inoculation did not improve yield of common beans in the
study area. Response to inoculation is expected: a in soils in which the specific rhizobia are
absent or sparse; and b where indigenous rhizo- bia are ineffective or partially effective in N
2
fixation Vincent, 1974; Halliday, 1984; Gitonga et al., 1999. When a legume is inoculated success-
fully, the resulting functional nodules quickly make the legume independent of soil N, the plant
produces a protein-rich seed and the soil is left enriched in N Ayanaba, 1977; Ayanaba and
Bromheld, 1980.
There were no significant differences in most measured parameters at harvest, comparing to
pure stands, except for higher values obtained for pure uninoculated beans without N application.
This is an indication that intercropping sup- pressed the legume growth. Similar observations
have been made by Ayisi and Poswell 1997, and Hornetz 1997 under adequate water supply. This
could have been due to competition between the common beans and maize for limited N and
phosphorus nutrients in the soil Brockwell et al., 1995, sharing of fixed N
2
with the maize and shading which reduced photosynthesis Tanaka
and Fujita, 1979. The mineralisation rate under such climatic conditions has been found to be so
fast within the short growth cycle period Hor- netz,
1997; Eichinger,
1999. According
to Ledgard et al. 1985, Danso et al. 1986, 1988,
Hardarson et al. 1988 and Launauce 1996, among others, for temperate and cold climates, it
seems that very little N is transferred in a short- term period, up to 6 months, because mineralisa-
tion must take place. The effect of intercropping seems to overshadow treatment effects in common
beans such that no significant differences were observed in the intercropped beans in various
treatments. N
2
fixation with common beans is usually not well succeeded either because selected
Rhizobium strains are not selected from indige-
nous Rhizobium, well adapted for the actual envi- ronmental conditions, by the high soil tempera-
tures and by the low soil moisture. Maize as a C4 plant is usually a more competi-
tive crop at the expense of a legume in maize – bean intercropping systems Crookston and Hill,
1979; Gitonga et al., 1999. Analysis of different plant tissues did not reveal any significant differ-
ences in nitrogen and carbon contents between treatments and cropping systems of common
beans data not shown. For maize, however, the yield in intercropped plants was significantly
lower P 5 0.05 than control plants in pure stands Table 1. This, apparently, could be at-
tributed to lower nutrient uptake of the inter- cropped
maize plants
hence weaker
yield performance. Similar observations have been
noted by Gitonga et al. 1999 in green gram – maize intercrops in the study area. Maize grain
yield of the pure crop was in agreement with that obtained by Kimotho et al. 1997 in semi-arid
eastern Kenya.
Soil analysis of nitrogen showed that plots with common beans as pure crop and in intercrop had
partly enriched the soil N during the vegetative cycle Fig. 5. This can be interpreted that indige-
nous as well as commercial rhizobia fix nitrogen with common beans in both pure and intercrop.
Maize plots were characterised by a decrease in N, possibly caused by their high nutrient demand.
Phosphorus content in the soil increased in all the plots, being the highest in inoculated treatment
plots. This shows that amendment of soil with TSP increased soil phosphorus. Improvement of
soil phosphorus in Kiboko soils has been reported by Hornetz 1997 in tepary beans – sorghum in-
tercrop after a cropping season. This could be attributed to increased soil microbial activity es-
pecially mycorrhiza which facilitates the release of large quantities of insoluble nutrients like phos-
phorus for the plants Ames et al., 1983; Menge, 1983.
5. Conclusions
