Twenty Years' Results from a Shade and Fertilizer Trial on Amazon Cocoa (Theobroma cacao) in Ghana

Abstract

The effects of three shade densities, NPK fertilizers and fertilizer-pest/disease relationship on Amazon cocoa (Theobroma cacao L.) over a 20-year period are reported. Yields equivalent to the mean yield from the unfertilized plots were achieved in about 3, 4 and 6 years in fertilized plots under the no shade (S o ), medium shade (S,) and heavy shade (S 2 ) treatments, respectively. There were three growth phases, each characterized by a different yield trend. No appreciable changes in the exchangeable Ca and Mg were observed during the experiments, but K dropped to 68% of its initial value while the available P of the unfertilized and the S o plots fell by 38%. Fertilizer recommendations for P and K should take account of overhead shade. The effect of N application was often negative. No direct relationship was observed between fertilizer treatment and the incidence of pest/blackpod/swollen shoot virus (SSV) disease of cocoa. SSV infection was greatest under S o . Cocoa farms in Ghana cannot maintain high yields (above 1000 kg dry cocoa ha"') beyond 15 years of intensive cropping. Y. Ahenkorah, B. J. Halm, M. R. Appiah, G. S. Akrofi y J. E. K. Yirenkyi: Resultados de veinte anos de una prueba de sombra y fertilizante en el cacao amazonico (Theobroma cacaoj en Ghana. RESUMENSe informa sobre los efectos de tres densidades de sombra, fertilizantes NPK y la relacion fertilizante-plaga/enfermedad en el cacao amazonico (Theobroma cacao L.) durante un pen'odo de 20 anos. Rendimientos equivalentes al rendimiento medio de parcelas no fertilizadas fueron logradas en unos 3, 4 y 6 anos en las parcelas fertilizadas bajo los tratamientos sin sombra (S o ), sombra mediana (SJ, y sombra intensa (S 2 ) respectivamente. Se dieron tres fases de crecimiento, cada una caracterizada por una tendencia de rendimiento diferente. No se observaron cambios apreciables en el Ca y Mg intercambiable durante los experimentos, pero el K bajo al 68% de su valor inicial cuando el P disponible de las parcelas no fertilizadas y S o bajo un 38%. Las recomendaciones de fertilizante para P y K debenan contemplar la sombra que viene de arriba. El efecto de la aplicacion de N fue con frecuencia negativo. No se observo relacion directa entre el tratamiento de fertilizante y la incidencia de plaga/podredumbre negra de las bellotas/virus de la enfermedad de retono hinchado (SSV) del cacao. La infeccion SSV fue maxima bajo S o . Las plantaciones de cacao en Ghana no pueden mantener altos rendimientos (mayores de 1000 kg cacao seco hectarea" 1 ) mas alia de los 15 anos de cultivo intensive INTRODUCTION

Ecophysiology of the cacao tree

“…This is essential for the survival and the establishment of cacao seedlings in dry and seasonally humid environments (Beer, 1987) since they are highly susceptible to dehydration (Leite et al, 1980). When nutrient availability is not a growth limiting factor, there is a positive correlation between cacao yields and light, as shown by Bonaparte (1975) and Ahenkorah et al (1987). Vernon (1967) concluded that the relationship between cacao yield and available light was approximately linear from 30 to 60% full sunlight.…”

“…It was suggested earlier, with regard to the fundamental points of cultural practices, that the control of both growth and density of cacao trees, as well as the increase in aeration and light penetration are necessary to promote increments in seed production (Vernon and Sunderam, 1972). Nowadays the comprehensive work of Ahenkorah et al (1987) sheds light on the importance of appropriate fertilization in unshaded cacao plantations.…”

“…The higher production of non-shaded cacao implies in a smaller productive lifespan, a larger demand for fertilizers (Owusu, 1978), a decrease in the incidence of cherelle wilt (Asomaning et al, 1971) and larger investments (Ahenkorah et al, 1987). The subsequent decline in productivity (Bonaparte, 1973) is attributed to high losses of exchangeable bases in the soil, attack of insects and diseases, excessive leaf transpiration and increase of soil evaporation (Ahenkorah et al, 1974).…”

“…In Ghana and Ivory Coast for example, 50% of the total cacao farm area is under mild shade whilst an average of 10% in Ghana and 35% in Ivory Coast is managed under no shade (Padi and Owusu, 1998). In a shade and fertilizer trial conducted with Amazon cacao over a 20-year period in Ghana, yield of heavily shaded plots were about half those under the non-shade treatment (Ahenkorah et al, 1987). Despite that, the authors inferred that the economic life of an unshaded Amelonado cacao farm in Ghana may not last for more than 15 years of intensive cropping.…”

Trade-offs between crop intensification and ecosystem services: the role of agroforestry in cocoa cultivation

“…In West Africa, only two published studies were found documenting the beneficial effect of removing shade for achieving higher yield. In Ghana, Ahenkorah et al (1987) reported that the yield of cocoa grown under a moderate level of shade (with an initial density of 67 trees ha -1 reduced to 34 trees ha -1 after 12 years) and fertilizers was only 78 % of that of the full-sun system with the same fertilization, while under a heavy level of shade (with an initial density of 132 trees ha -1 reduced to 68 trees ha -1 after 12 years) the yield was only 50 % of that of the full-sun system. However, this study was conducted under the shade of only one tree species, Terminalia ivorensis, a fast-growing pioneer species of West Africa.…”