Comparative Assessment of Carrot (Daucus carota L.) Yield and Heavy Metal Accumulation under Poultry Manure and NPK Fertilization in Mining and Non-mining Areas of Ghana

Abstract

One important vegetable crop that is commonly used in human meals is the carrot (Daucus carota L.). Mining operations and continuous use of excessive amounts of fertilizers containing metals are the main causes of heavy metal pollution in the soil. The objective of this research was to promote the sustainable production of healthy carrots in the semi-deciduous forest agroecological zone. The study was implemented concurrently at two locations, employing a Randomized Complete Block Design (RCBD) with three replications. Fertilizer treatments included NPK (15:15:15) at rates of 300, 350, and 400 kg ha⁻¹; poultry manure applied at 5, 15, and 20 t ha⁻¹; and an integrated treatment combining half the recommended rate of NPK (15:15:15) (300 kg ha⁻¹) with the rate of poultry manure (25 t ha⁻¹). Before the tests began, soil analysis was done. Both sites had soil that was below the WHO-recommended concentrations of all tested heavy metals at both sites. Significant treatment-related variations (p < 0.05) in growth and yield characteristics were recorded at both locations.  Heavy metal concentrations in carrot roots were below WHO threshold values, while poultry manure applied at 25 t ha⁻¹ significantly improved carrot growth and yield, recording 23.94 t ha⁻¹ and 21.09 t ha⁻¹   in Kumasi and Obuasi, respectively. In Kumasi, transfer factor values ranged from 0.0155–0.0606 for arsenic, 0.1012–0.2197 for mercury, 0.0155–0.0201 for lead, 0.0336–0.0532 for cadmium, and 0.1747–0.3093 for copper, while in Obuasi they ranged from 0.0141–0.0181 for arsenic, 0.0322–0.0750 for mercury, 0.0071–0.0122 for lead, 0.0082–0.0166 for cadmium, and 0.0582–0.0954 for copper. These are all well below the critical threshold value of unity (TF = 1).