Effect of nitrogen application and plant age on edible leaf yield and quality of Black Nightshade (Solanum nigrum l.) plants.
Two experiments were carried out to investigate the effects of nitrogen application rates and plant age on the edible leaf yield and nutritive quality of black nightshade (Solanum nigrum L.) plants. The first experiment was between February and June; and the second between July and November 1995, at Egerton University Njoro, Kenya. Nitrogen rates were 0, 26, 52, 78 and 104 Kg N / ha; applied either as one, two or three split applications. The experiment had 15 treatments arranged in a split plot design, replicated thrice. Nitrogen rates constituted the main plots, while the application methods constituted the sub - plots. Seedlings were transplanted when 8 weeks old. Edible shoots were harvested fortnightly starting from 8 weeks after transplanting, and continued up to 20 weeks after transplanting. Edible leaf yield increased with increasing nitrogen application rates. Increase in yield was not significant in season 1, but was significant in season 2. Nitrogen at a rate of 104 kg N / ha. gave the highest yield (15 t/ha.) in season 2, but it was not significantly different from application rate of 52 kg N / ha. (14.8 t/ha.) and 78 kg N / ha. (14.8 t/ha.). Leaf yield increased significantly with increasing plant age. Yield increased up to 14 weeks (2.4 t/ha.) and 18 weeks (2.7 t/ha.) after transplanting in season 1 and season 2 respectively; thereafter yields declined with successive harvests. Harvested leaves were dried, and analyzed for oxalate and phenolic contents. Plant content decreased with increasing plant age from 611.0 mg/lOOg 8 weeks after transplanting to 480.7 mg/lOOg 20 weeks after transplanting, in season l. In season 2 the lowest oxalate content of 652.7 mg/l00mg was recorded during the first harvest, i.e. 8 weeks after transplanting. At the last harvest (20 weeks after transplanting) the oxalate content was 707.1 mg/lOOg. The highest oxalate content during season 2 was 938.5 mg/lOOg, recorded 12 weeks after transplanting. Plant age had no significant effect on phenolic content in season 1; but its effect in season 2 was significant. The lowest phenolic content in season 2 (1153 mg/100g) was observed 16 weeks after transplanting; while the highest content (1507 mg/iOOg) was recorded at 20 weeks after transplanting. Tthis study shows that nitrogen application at 52 kg Nlha. applied in a single dose is the most economical. It is also evident that nitrogen fertilizer does not increase oxalate and phenolic contents in Solanum nigrum to harmful levels.