The production of promiscuous soybean by smallholder farmers in Kenya would improve soil fertility through biological nitrogen fixation (BNF), boost food security, and contribute to generation of cash. The present study was conducted to determine the effects of soil amendments on growth and yields of promiscuous soybean cultivars under varying soil carbon levels. Field experiments using early maturing SB 19 and late maturing SB 20 promiscuous soybean cultivars and different levels of phosphorus (P), potassium (K) and sulphur (S) fertilizers were conducted in two sites in the south-eastern slopes of Mt. Kenya, approximately 1500 m above sea level. The soybean cultivars were observed for phenology, plant biomass production, pod fresh weight, 1000 seed weight and haulm weight. Significant differences were observed on most yield components due to field carbon level, soybean cultivar and fertilizer amendments, while the plant height was only affected by fertilizer application and soy bean cultivar. However, the effects due to the interaction of these factors were not significant. Therefore, the benefits of combined use of BNF by soybean and application of PKS fertilizers could be a promising entry point into maximized fertilizer use efficiency by smallholder systems in Kenya.