Nodulation and nitrogen fixation in promiscuous and non promiscous soybean varieties grown in Kenya
Nitrogen deficiency is a major factor limiting soybean production. Th is problem can be alleviated by the use of nitrogen fertilizers which on the other hand adversely affect the environment, are expensive and unaffordable to most peasant farmers. Alternatively, attention is being paid to improving nitrogen through use of environmental friendly biological nitrogen fixation of soybeans in an attempt to develop sustainable cropping systems. There is however inadequate knowledge on estimates of nitrogen fixation by soybean varieties in Kenya. In light of this, growth experiments were designed to investigate biological nitrogen fixation in promiscuous and non promiscuous soybeans. Non promiscuous and specific Clark was used in greenhouse to confirm presence of Bradyrhizobium japonicum. Results from the field experiment showed that the two promiscuous soybean varieties (TGx 1869 and TGx 1893) nodulated better than the non promiscuous Gazelle. Uninoculated Gazelle did not produced any nodule while uninoculated TGx 1869 and TGx 1893 nodulated (with indigenous soil Rhizobia). Inoculation had effect on nodulation since there was a significant difference (P<0.05) in nodule number and nodule dry weight of inoculated soybeans as compared to uninoculated and N treated soybeans. Nitrogen treated soybeans did not produce any nodule. At early stage of growth, non promiscuous Gazelle was competing effectively with the TGx varieties. Results at podding and final harvest indicated that as growth progressed, the promiscuous soybean varieties fared better than Gazelle in root and shoot dry weight, pod number, pod and stover dry weight. Except in nodule number and nodule dry weight, N treated soybeans had significantly highervalues in all parameters than inoculated and uninoculated soybeans. Inoculated soybeans had higher pod and seed dry weight than uninoculated soybeans. Nitrogen treated soybeans had significantly higher average seed dry weight (22.98g) than inoculated and uninoculated soybeans which had 12.39g and 10.44g respectively. Although there was no significant difference in seed dry weight of inoculated and uninoculated soybeans, inoculated soybeans had higher seed dry weight. The two TGx varieties had higher seed dry weight than Gazelle. Results from greenhouse studies showed that the two TGx varieties nodulated with 85.7% of isolates, Gazelle with 47.6% and Clark 23.8%. Clark was the poorest in nodulation and only nodulated with isolates obtained from soybeans nodulated by Bradyrhizobium japonicum USDA 110. There is need to adopt growing of promiscuous TGx varieties and to exploit biological nitrogen fixation with the view of increasing soybean yields and decreasing overdependence on nitrogen fertilizers for sustainable agriculture.