Contribution of Urea Nitrogen to Soil Fertility: Production of Indigenous Underutilized Vegetables and Food Security in Southwest Nigeria

One in nine of the world's population suffers from chronic hunger in 2011-13. With increasing demand for high valued food and adverse impacts of climate change in Africa and globally, indigenous underutilized vegetables of southwest Nigeria which are collected wild as weeds, are gradually disappearing.  The vegetables have both nutritional and therapeutic uses. Cultivation of the vegetables for commercial purpose could contribute to climate change adaptation and mitigation in the region. Urea fertilizer is the most fertilizer used for vegetable production worldwide. There is the need for information on the effects of urea nitrogen, fertilizer on soil quality and production of the vegetables. This study examined the effects of nitrogen fertilizer (urea) on soil organic carbon, total nitrogen, pH and exchangeable cations, and yield and nutrient composition of Solanum macrocarpon (Igbagba) and Solanum scabrum (Ogunmo) in Iwaro, Ondo State, Ogbomosho, Oyo State derived savanna and OmiOkun Ile-Ife, Osun State, rainforest agro-ecological zones of southwestern Nigeria. The study was a 4 x 4 arranged into a Completely Randomized and replicated four times. Urea was applied at 0, 40, 80 and 160 kg N ha^-1. Soil properties were determined before and after fertilizer application. Vegetable shoot yield and nutrient contents were determined. Data collected were subjected to statistics analysis. Results showed that soil organic carbon and total nitrogen were significantly higher and more acidic while exchangeable cations were not affected in Omiokun compared with that of Iwaro and Ogbomosho. Fresh shoot yield from Omiokun, Iwaro and Ogbomosho were 9.01, 3.21 and 5.87 kg 6m^-1 for Solanum macrocarpon and 19.58, 9.83 and 8.06 kg 6m^-1 for Solanum scabrum, respectively, which increased consistently with increasing rates of N application. S. marcrocapon leaf nutrient contents in % range from 3.5 to 10.1Ca, 4.03 to 9.6 Mg, 4.6 to 5.8 K and 0.5 to 0.9 Na, and 27.0 to 143.2 mg100g^-1 for Fe while 1.9 to 6.1 Ca, 1.6 to 4.6 Mg, 2.2 to 3.1 K and 0.2 to 0.5 Na, and 51.7 to 71.4 mg100g^-1 for Fe were obtained for S. scabrum, It was concluded that the soils have low fertility.  Application of 80 kg N ha^-1  was recommended for S. scabrum and S. macrocarpon. Through greater production and consumption of indigenous vegetables, malnutrition can be reduced significantly and healthy diets promoted, and income increased. Vegetable production, processing  and value addision could serve as a source of employment for the teeming youth population of Nigeria.