Assessment of Soil Nitrogen Status and Depletion Rates in Intensive Vegetable Gardens in Lagos State

This study assesses soil nitrogen status and seasonal depletion rates in intensive urban and peri-urban vegetable gardens in Lagos State, South West Nigeria. Urban and peri-urban vegetable production in Lagos State is an important source of fresh vegetables for the metropolis, but the intensive cultivation cycles with multiple crops per year create high nitrogen demand and progressive soil nitrogen depletion if external inputs are insufficient. Understanding nitrogen dynamics in these high-intensity production systems is essential for sustainable soil management guidance. This study collects soil samples from 40 vegetable garden plots in Alimosho, Ikorodu, and Badagry local government areas at the beginning and end of three successive cropping cycles. Total nitrogen, ammonium-N, nitrate-N, and soil organic carbon are measured alongside records of crop type, fertiliser input, irrigation source, and yield. Net nitrogen mineralisation rates are computed from inorganic nitrogen accumulation under controlled incubation. Findings reveal that average total nitrogen in sampled vegetable plots is 0.11 percent, well below the critical threshold of 0.15 percent for vegetable crop nutrition. Seasonal depletion of 28 to 42 kilograms of nitrogen per hectare per crop cycle is observed in plots receiving no organic matter inputs. Plots using irrigation water from wastewater sources show significantly higher residual nitrogen due to irrigation-mediated nitrogen loading. The study concludes that nitrogen depletion is a systemic sustainability challenge in Lagos urban vegetable gardens. It recommends integrated nitrogen management programmes combining compost and mineral nitrogen supplementation.

Keywords: soil nitrogen, vegetable gardens, Lagos State, nitrogen depletion, urban agriculture.