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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

SUMMARYThe 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 (S0), medium shade (S1) and heavy shade (S2) 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 S0 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 S0. Cocoa farms in Ghana cannot maintain high yields (above 1000 kg dry cocoa ha−1) beyond 15 years of intensive cropping.

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Total organic phosphorus, inositol phosphates and phosphatase activity of some selected Canadian and Ghanaian soils were compared. The lower organic phosphorus content of Ghanaian soils compared to the Canadian soils may be the result of either the faster rate of mineralization of organic phosphorus in the tropical Ghanaian soils or differences in the nature of organic residues returned to the soils. Wide variations in the amounts of inositol phosphate were observed in both groups of soils. The low activity of phosphatases in the Ghanaian soils may be due to the lower content of organic matter, a lower microbial biomass and consequently a lower phosphatase production. The effect of rotation and fertilizer application on the total organic P, inositol P and phosphatase activity indicated that alfalfa grown in rotation with corn and oats contributed significantly to the total soil organic phosphorus content with no apparent increase in inositol phosphate content. No consistent trends were observed in either the total amount of inositol phosphates or the proportion of the total phosphorus that existed as inositol phosphates for either the fertilized or unfertilized soils. The activity of phosphatase was generally higher in the corn-oat-alfalfa rotation than in the systems of corn-oat and continuous corn. Fertilization decreased the phosphatase activity in all cropping systems.