Development of a composite soil degradation assessment index for cocoa agroecosystems in southwestern Nigeria

Abstract

Abstract. Cocoa agroecosystems are a major land-use type in the tropical rainforest belt of West Africa, reportedly associated with several ecological changes, including soil degradation. This study aims to develop a composite soil degradation assessment index (CSDI) for determining the degradation level of cocoa soils under smallholder agroecosystems of southwestern Nigeria. Plots where natural forests have been converted to cocoa agroecosystems of ages 1-10, 11-40, and 41-80 years, respectively representing young cocoa plantations (YCPs), mature cocoa plantations (MCPs), and senescent cocoa plantations (SCPs), were identified to represent the biological cycle of the cocoa tree. Soil samples were collected at a depth of 0 to 20 cm in each plot and analysed in terms of their physical, chemical, and biological properties. Factor analysis of soil data revealed four major interacting soil degradation processes: decline in soil nutrients, loss of soil organic matter, increase in soil acidity, and the breakdown of soil textural characteristics over time. These processes were represented by eight soil properties (extractable zinc, silt, soil organic matter (SOM), cation exchange capacity (CEC), available phosphorus, total porosity, pH, and clay content). These soil properties were subjected to forward stepwise discriminant analysis (STEPDA), and the result showed that four soil properties (extractable zinc, cation exchange capacity, SOM, and clay content) are the most useful in separating the studied soils into YCP, MCP, and SCP. In this way, we have sufficiently eliminated redundancy in the final selection of soil degradation indicators. Based on these four soil parameters, a CSDI was developed and used to classify selected cocoa soils into three different classes of degradation. The results revealed that 65 % of the selected cocoa farms are moderately degraded, while 18 % have a high degradation status. The numerical value of the CSDI as an objective index of soil degradation under cocoa agroecosystems was statistically validated. The results of this study reveal that soil management should promote activities that help to increase organic matter and reduce Zn deficiency over the cocoa growth cycle. Finally, the newly developed CSDI can provide an early warning of soil degradation processes and help farmers and extension officers to implement rehabilitation practices on degraded cocoa soils.

Long-term dynamics of cocoa agroforestry systems established on lands previously occupied by savannah or forests

“…This improvement can be explained by the increase of litter -and linked litter cycling -in S-cAFS over time when compared to the very low soil organic content (SOC) content and litter cycling of the initial savannahs. This was not the case in F-cAFS established in high clay content soils where a decrease in pH, K and Ca and an increase of the C:N ratio may point to a low but steady decline of soil quality and nutrient availability -results in line with the recent study of Adeniyi et al (2017). Noticeably, soil Cu concentrations seemed to increase with time at least in S-cAFS and could point to a putative soil contamination by pesticides, as farmers use copper oxide or hydroxide to treat against black pod disease (Sonwa et al, 2008;Jagoret et al, 2011).…”

“…It suggests that these attributes, after converging at intermediate time scales, may keep evolving in contrasting directions for S-and F-cAFS. This could be related to the different types of soils studied (Adeniyi et al, 2017) as well as farmers' differential management within a generation and between generations (Jagoret et al, 2018a) -both putatively altering ecosystem's functioning.…”

“…Land-use change from savannah or forest into cAFS probably affects both above-and below-ground ecosystem functioning and related services. For instance, conversion of forest into cAFS was shown to alter litter inputs and soil properties, putatively altering nutrient cycling (Beer et al, 1998;Schroth et al, 2001;Hartemink, 2005;Adeniyi et al, 2017). Similarly, transitioning from savannah to another (agro or eco)system or vice-versa was shown to alter both litter inputs and soil properties (Don et al, 2011;Nouvellon et al, 2012;Sugihara et al, 2014).…”

Integrated Soil Fertility Management for Small Holder Cocoa Farms: Using Combination of Cocoa Pod Husk Based Compost and Mineral Fertilizers

“…This can be attributed to soil acidity, causing interference with the availability and uptake of certain nutrients, such as P [27], and the relatively low use of mineral fertilizers [30]. Our results of the farmers interviewed further confirms communications from earlier studies that have consistently shown that most Nigerian cocoa farmers do not use chemical fertilizers [31,4,32,33], hence inadequate nutrients in the soil.…”