Soil carbon stocks and main nutrients under cocoa plantations of different ages

ABSTRACT Soil organic carbon and main nutrient stocks play key roles in maintaining soil fertility and yields of agricultural crops including cocoa (Theobroma cacao). The amount of nutrients removed from the soil by the cocoa trees could be influenced by its physiological growth stage. A study was carried out to examine the changes in some soil physical and chemical properties with main nutrients availabilities under cocoa plantations at the Cocoa Research Institute in the Eastern Region of Ghana. Cocoa plantations of different ages (5, 10, 14, 18 and 23 years) were chosen based on similar land history. Soils at the study sites are classified as Xanthic Ferralsol (WRB series). On each plantation, one acre (0.4ha) plot was demarcated and divided into three blocks for soil sampling. Five core samples were taken at 0–15 cm depth per block, bulked together and subsample taken to the laboratory for analyses. Particle size analyses showed that all soils were of sandy loam texture. All soils were acidic with pH below 5.5, relatively low exchangeable K and Ca. Bulk density values varied with ages at highest in 18 years. Soils under 23 years old plantations had relatively lower bulk density than those of the other ages. Total nitrogen content under the 5, 10, and 18 years of cocoa plantations was below the critical values considered adequate for good cocoa growth. Soil carbon stock and available phosphorus content of the soil generally tend to increase with age of the cocoa plantations. These differential variations in the soil nutrients, namely, total N, available P, exchangeable K, Ca, and Mg among five different cocoa plantation ages suggest that total nitrogen, available P, and exchangeable Mg were higher than the critical levels for good cocoa growth under 23 years of cocoa plantations only, but exchangeable K and Ca were lower the critical levels for all ages of cocoa plantations.