effect of nitrogen and phosphorus fertilizer on growth and yield of ironweed (vernonia galamesis)

Abstract

Irooweed (Vernonia galamensis) is a promising new crop for industrial oil but information on its response to fertiliser is scanty. A field experiment was conducted at the University of Nairobi Field Station farm during two seasons (January to May 1998, season 1 and March to August 1998, season 2) to determine the effects of nitrogen (N) and phosphorus (P) fertilizer rates on growth, photosyntheti«ally active radiation (PAR) interception and seed yield of two Vernonia gaiamensis varieties ("ethiopica" and "gibbosa"). N was applied at 0, 75 and 150 kg N/ha, and P at 0, 45 and 90 kg P2Os/ha. The experiment was therefore a 2 x 3 x 3 factorial laid out in randomized complete block design with three replications. N and P application significantly increased total dry matter (TDM, g/plant), photosynthetically active radiation (PAR) interception and leaf area index (LAI) of both varieties in late vegetative and reproductive stages. "Gibbosa" had consistently higher TDM, LAI, PAR and was taller compared to "ethiopica" throughout the growing season. Average seed yield of "gibbosa" was 2.3 times higher than "ethiopica" in both experiments. The highest TDM, LAI and seed yields were obtained at the highest N and P levels. "Gibbosa" had significantly higher number of capsules/plant and but a lower harvest index (HI) compared to "ethiopica".