Results and Discussion
Rhizobial population : With rhizobial population at the initial day, the result showed that rhizobial population of the managed subplot were significantly greater than the control subplots (added with N at 70 ppm only). However, the amounts of rhizobial population among the subplots being added with some certain amounts of native rhizobia taken from soil where yard long bean crop plants had been grown continuously possessed some similar amounts of rhizobial population whether with or without the growth of yard long bean plants. The results revealed that the average values of rhizobial population were within the
range of log MPN 2.53 to 2.57 for the three managed subplot (Table 1). The results indicated that native rhizobial population could always be available in soils where yard long bean plants had been previously grow hence , the results could presumably be applied to some other host crops such as cowpeas, soybeans and others. Therefore, legume hosts by nature could manifest their significant effects on rhizobial population in soils. Nambiar et al.(1984) stated that most of agricultural soils being used to grow peanut crop contained rhizobial population of approximately 10 cells/g dry weight of soil and they further added that Rhizobium of peanut could be grouped as those found with cowpea crop. Similarly, subba Rao (1995) claied that rhizobial species of yard long bean crop plants could be grouped
as those of cowpeas. At day 30 after planting of both yard long bean cultivars , the results showed that amounts of rhizobial population of those managed subplots were significantly greater than the control subplots. The differences were large and statistically significant
(Table 2 ). Rhizobial population of the non-managed subplots was similar to that of the managed subplot (with the growth of white seeded yard long bean plants) with the mean values of log MPN of 2.94 and 2.75 , respectively. Whilst that of the managed subplot (with the growth of KKU 25 yard long bean cultivar), the results revealed that rhizobial population was greater than the rest. The results suggested that KKU 25 yard long bean plants had the highest potential in promoting the growth of rhizobial