SOIL QUALITY ASSESSMENT OF URBAN FARMING IN THE FEDERAL CAPITAL TERRITORY, ABUJA, NIGERIA

Abstract

This study assessed the seasonal and spatial variations in soil properties and heavy metal concentrations in irrigated urban vegetable farms in the Federal Capital Territory (FCT), Abuja. The study objectives include the seasonal variation of soil nutrients on sites where vegetable farming is practiced as well as the suitability of the vegetables for human consumption among others. The study employed a quasi-experimental design. Sampling was conducted in four major sites where vegetables are farmed using a systematic-random approach. Each site included three cultivated plots and one control plot, from which thirty-two composite soil samples (four per site) at 0–15 cm depth and sixteen plant samples were collected during wet and dry seasons. Statistical evaluations, including ANOVA, t-tests and descriptive statistics were employed, and the test of significance is at 0.05 level. Soils were predominantly sandy loam to loam in texture, with slightly acidic to near-neutral pH (5.53– 6.94), moderate Organic Carbon (0.2567–0.3333%) and Nitrogen (0.3567–0.5270%), and high Available Phosphorus (24.87–29.91 mg/kg). Exchangeable bases and Effective Cation Exchange Capacity (7.15–8.15 cmol/kg) indicated generally favourable fertility conditions. Heavy metals in soils followed the trend Fe > Zn > Cu > Pb. While Fe and Zn concentrations were within acceptable limits, Cu levels exceeded acceptable values at some locations particularly (A3W and A3B) with corresponding Geoaccumulation index (Igeo) values indicating moderate contamination (up to 2.635). Pb concentrations remained negligible across all sites and seasons (Igeo < 0). Pollution Load Index (PLI) values exceeded 1.0 in some wet season plots, indicating localized moderate contamination, but were lower during the dry season. Plant tissue analysis of four vegetables (Amaranthus retroflexus, Corchorus olitorius, Ocimum tenuiflorum, and Telfairia occidentalis) revealed species- and season-dependent variations in heavy metal uptake. Zn and Cu were the most prevalent heavy metals in the edible parts of the vegetables, while Pb was undetected or below toxic thresholds. Seasonal differences were significant for some metals and species, including a decline in Cu levels in Telfairia occidentalis stems during the dry season. Health risk assessments showed that Estimated Daily Intake (EDI) and Health Risk Index (HRI) values for all heavy metals were well below safety limits, indicating no significant non-carcinogenic health risks from vegetable consumption. In conclusion, the soils of these urban farms are moderately fertile and currently safe for vegetable cultivation. However, localized Cu accumulation and seasonal variations underscore the need for regular monitoring, improved soil management, and sustainable irrigation practices by farmers, policy makers and other relevant stakeholders, to ensure long term food safety and environmental health in urban agriculture