Assessment of Soil Fertility Decline Indicators in Long-Term Farmed Plots in Gombe State, North East Nigeria

This study assesses indicators of soil fertility decline in plots with contrasting agricultural histories in Gombe State, located in Nigeria's North East geopolitical zone. Long-term continuous cropping without adequate nutrient inputs is the dominant management pattern for smallholder farmers in the Sudan savanna zone of Gombe State, and documenting the trajectory of fertility decline is essential for designing restoration interventions. Using a comparative field design, surface soil samples (0 to 20 cm) were collected from three categories of plots: long-term farmed plots with more than 15 years of continuous cultivation, medium-term plots with 5 to 14 years of cultivation, and recent plots with less than five years since forest clearing. Five replicate plots per category were selected in Gombe, Akko, and Funakaye local government areas. Samples were analysed for organic carbon, total nitrogen, available phosphorus, exchangeable potassium, calcium, and magnesium, cation exchange capacity, pH, and bulk density. Findings revealed systematic declines in all measured fertility indicators with increasing cultivation duration. Long-term plots showed 68 percent lower organic carbon, 55 percent lower available phosphorus, and 40 percent lower cation exchange capacity compared to recently cleared land. Bulk density increased progressively with cultivation age. The study establishes a soil fertility decline trajectory for Gombe State savanna soils and recommends immediate introduction of integrated soil fertility management programmes targeting long-term farmed plots as priority rehabilitation areas. Keywords: soil fertility decline, long-term farming, Gombe State, organic carbon, continuous cultivation.