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Declining soil fertility is a main constraint toward attaining improved crop yields across the Africa continent. The soils within the area of study are generally poor in fertility, thus the importance of using chemical fertilizer to increase crop productivity. Experimental data from two sorghum cultivars (Gadam and Seredo) grown under four levels of nitrogen (N) and three levels of phosphorus (P) in the year 2014 and 2015 long and short rainfall seasons at Katumani Research Center in Machakos County, Kenya, were used to assess the effect of nitrogen and phosphorus on phenological parameters, harvested biomass and grain yield.
Analysis of variance depicted a statistically insignificant effect of N and P on phenological parameters in majority of the experiments. The two sorghum cultivars were significantly different from each other with Gadam taking approximately twelve (12) and seven days (7) less when compared to Seredo in flowering and maturity. Only Experiment 2 showed statistically significant effect of N and P on harvested biomass. Seredo produced higher average biomass (27.3%) than gadam in all the four experiments. N application significantly increased biomass accumulation in seredo by 10.9 and 25.3% with application of 50 Nkgha-1, 10.9 and 6.5% at 75 Nkgha-1 over the control for Experiments 1 and 4, respectively. Statistically significant response of Seredo among the different rates of fertilizer was present in Experiment 2. Seredo grain yield in Experiments 1, 2 and 4 responded positively to application of nitrogen with yields increase of 28.8%, 13.9% and 14.9%, respectively over the control (N1P1).
The sorghum plants responded more to N inorganic fertilizer. Existence of elevated amounts of soil P in the soil restricted the efficient utilization of P inorganic fertilizer by the sorghum plants.