Catherine Mathenge1, 2, Moses Thuita2, Joseph P Gweyi-Onyango1 and Cargele Masso2*
1Department of Agricultural Science and Technology, Kenyatta University, P.O BOX 43844-00100, Nairobi, Kenya
2International Institute of Tropical Agriculture, c/o ICIPE, P.O. Box 30772-00100, Nairobi, Kenya
*corresponding author: C.Masso@cgiar.org
Inoculation of soybean is an efficient way of increasing effective rhizobia population in the rhizosphere of the crop, but their performance is limited by soil nitrogen (N). The objective of the study was to determine the critical level of N below and above which, response to inoculation is hindered. Two greenhouse trials were set up and a field trial. The first greenhouse trial included unamended sixty soils (i.e. N: 0.03-0.21 %; organic carbon ≤ 2.10%), with and without inoculation. The 2nd trial consisted of two soils selected based on the results of the first trial (i.e. 0.06% and 0.08% N) and amended with five rates of vermicompost (phymyx) to various N levels (i.e. N: 0.06/0.08-0.21%), with and without inoculation. The results of the 2nd greenhouse trial are being validated in field conditions using similar treatments. For the sixty soils, the correlation between soil N and the growth parameters was low because of variability in other soil properties (r: 0.29-0.55); however, shoot biomass was higher in soils with high N. In the two amended soils, the highest shoot dry weight and nodules fresh weight were recorded at soil N level of 0.17%, beyond which nodulation was suppressed and shoot dry weight reduced. There was a significant interaction effect of soil and inoculation for N uptake (p<0.001) as result of the differences in soil properties, while the starter N from vermicompost did not hinder the performance of the rhizobia inoculant.