14th International Cocoa Research Conference

COCOA / COCONUT INTERCROPPING TRIAL IN GHANA: EFFECT OF THE CROPPING SYSTEMS ON SOIL NUTRIENT DYNAMICS

K. Ofori-Frimpong, A. A., Afrifa, K., Osei Bonsu and M. R. Appiah

Cocoa Research Institute of Ghana (C.R.I.G.) P. O. Box 8, New Tafo, Akyem, Ghana

SUMMARY

In search for an evergreen permanent shade for cocoa in Ghana, the effects of some cropping systems on soil properties at three soil depthsof tropical ferric lixisol were investigated at the Cocoa Research Institute of Ghana. The cropping systems were; cocoa spaced at 3.0 m2

with Gliricidia sepium spaced at 12.0 m2; cocoa planted at 2.5 m triangular, with coconut at 8.8 m triangular; cocoa spaced at 2.5 mtriangular with coconut at 9.8 m triangular; cocoa spaced at 3.0 m2 with coconut at 9.1 m triangular; cocoa spaced at 3.0 m2 with coconut at12.2 m x 10.7 m. The cropping systems were compared with an uncultivated bush in an adjacent plot. Land clearing and the cultivation ofthe crops / crop combinations generally influenced the soil chemical properties determined at the end of fifteen years of cropping. Soil pHand organic matter content increased with cultivation. Irrespective of the intercrop system, total N, available P and exchangeable Kcontents of the soils declined. The cocoa / Gliricidia sepium plot recorded the highest level of nitrogen among the intercrop systems butcultivation significantly decreased the nitrogen content of the soils more especially in the cocoa spaced at 3.0 m2 and coconut spaced at12.2 m x 10.7 m plot. Cocoa spaced at 3.0 m2 with Gliricidia sepium or with coconut spaced at 12.2 m x 10.7 m treatments significantlyhad the lowest available P contents. The exchangeable K contents were however similar among the intercrop systems. The results arediscussed in relation to the nutrient requirements of mature cocoa as well as cocoa fertilization programme in Ghana.

ENSAIO DE CULTURA INTERCALAR DE CACAU/COCO EM GANA: EFEITO DE SISTEMAS DE CULTIVO SOBRE AS DINÂMICAS DOS NUTRIENTES DO SOLO.

RESUMO

Foram investigados os efeitos de alguns sistemas de corte nas propriedades do solo a três profundidades diferentes do solo com lixisolférrico tropical no Instituto de Pesquisa do Cacau de Gana, como parte das tentativas para buscar uma sombra perene permanente sempreverde para o cacau em Gana. Os sistemas de corte foram plantados em triângulo a 2.5m com coco em triângulo a 8.8m; cacau plantado emespaçamento de 3m x 3m com Glyricidia sepium plantada a 12.1m x 12.1m; cacau plantado em triângulo a 2.5m com coco em triângulo a9.8m; cacau plantado em espaçamento de 3.0m x 3.0m com coco plantado em triângulo a 9.1m; cacau plantado em espaçamento de 3.0m x3.0m com coco plantado em espaçamento de 12.2m x 11.0m. O método experimental foi de blocos aleatórios completos com quatroréplicas. Realizaram-se análises aos solos dos sistemas de corte e um arbusto não cultivado numa parcela vizinha foi utilizado comotestemunha. A limpeza da terra e o cultivo das culturas / combinações de culturas no geral influenciaram as propriedades químicas do solocalculadas ao fim de 15 anos de cultivo. O pH do solo bem como o conteúdo da matéria orgânica aumentou com o cultivo.Independentemente do sistema de cultura intercalar, os conteúdos de N total, P disponível e K permutável dos solos declinaram.

A parcela cacau / G. sepium registou os níveis mais elevados de nitrogênio entre todos os sistemas de cultura intercalar, mas o cultivodiminuiu de modo significativo o conteúdo de nitrogênio dos solos, especialmente nas parcelas de cacau em espaçamento de 3.0 x 3m e dococo em espaçamento de 12.2m x 11.0m. No cacau espaçado a 3 x 3.0m com G. sepium ou com coco em m espaçamento de 12.2m x11.0m os tratamentos apresentaram índices de P disponível significativamente (Pd”0.05) inferiores. No entanto, os conteúdos permutáveisde K foram semelhantes nos sistemas de cultura intercalar. Os resultados são discutidos em rel espaçamento de 12.2m x 11.0m ostratamentos apresentaram índices de P disponível significativamente (Pd.05) inferiores. No entanto, os conteúdos permutáveis de K foramsemelhantes nos sistemas de cultura intercalar. Os resultados são discutidos em rela ação às necessidades do cacau adulto bem como doprograma de fertilização do cacau em Gana.

EXPERIMENTACIÓN DE INTERCULTIVO CACAO/COCO EN GANA: EFECTO DE LOS SISTEMAS DE CULTIVO ENLA DINÁMICA DE LOS NUTRIENTES DEL SUELO

RESUMEN

Como parte de los esfuerzos de búsqueda de sombra permanente perenne para el cacao en Gana, fueron investigados los efectos de ciertossistemas de cultivo en las propiedades del suelo, a tres profundidades del suelo de lixisol férrico tropical, en el Cocoa Research Institute deGana. Los sistemas de cultivo fueron: cacao plantado a 2,5 m triangular con coco a 8,8 m triangular; cacao plantado a 3m x 3m conGlyricidia sepium plantada a 12,1 x 12,1m; cacao plantado a 2,5 m triangular con coco a 9,8 m triangular; cacao plantado a 3,0 x 3,0 mcon coco plantado a 9,1 m triangular; cacao plantado a 3,0 x 3,0 m con coco plantado a 12,2 x 11,0 m. El diseño experimental fue unbloque al azar con cuatro repeticiones. El muestreo de suelos fue tomado de los sistemas de cultivo y de un mato inculto en un loteadyacente usado como estándar. La limpieza de la tierra y el cultivo / combinaciones de cultivos generalmente influenciaron laspropiedades químicas del suelo determinadas al final de los 15 años de cultivo. El pH del suelo y el contenido de materia orgánica seincrementaron con el cultivo. Sin considerar el sistema de intercultivo los contenidos de los suelos, N total, P disponible y Kintercambiable declinaron.

El lote cacao / G. sepium registró el nivel más elevado de nitrógeno entre los sistemas de intercultivo pero el cultivo hizo decrecersignificativamente el contenido de nitrógeno de los suelos, especialmente en el cacao espaciado a 3,0 x 3,0 m y la parcela de cocoespaciado a 12,2 x 11,0 m. Los tratamientos con cacao espaciado a 3 x 3,0 m con G. sepium o con coco espaciado a 12,2 x 11,0 mtuvieron un nivel de contenidos P disponibles significativamente más bajos (Pd”0.05). Los contenidos K intercambiables fueron, sinembargo, similares entre los sistemas de intercultivo. Son discutidos los resultados con relación a los requerimientos de nutrientes de cacaomaduro así como el programa de fertilización del cacao en Gana.

INTRODUCTION

In Ghana selected forest tree species are used as shade trees forcocoa cultivation. In most cases these forest trees are felled foreconomic uses rendering the farms unshaded. As an alternative tothe forest trees, CRIG has recommended the use of Gliricidiasepium as permanent shade tree in cocoa establishment (Anon,1987). G. sepium sheds its leaves in the dry season (November toMarch) and thus exposes the cocoa trees to the dry weathernormally experienced at that period of the year. It is the practice ofmany farmers to use food crops as temporary shade during theearly stage of the establishment of cocoa, which also provide someeconomic returns. As the cocoa canopy closes up the temporaryshade of food crops are replaced by permanent trees, which do notadd extra income to the farmer. The search for an evergreenpermanent shade for cocoa in Ghana is still on going. Croppingsystems of coconut with cocoa are common in several countries(Daswir et al, 1988; Abbas and Dja’far, 1989, Pursglove, 1976).In Ghana, cocoa is not grown under the shade of coconut. OseiBonsu et al (2002) have proposed that the practice of intercroppingcocoa with coconuts as shade trees is both economically andbiologically attractive than intercropping with G. sepium in Ghana.But Appiah (1992) observed high N and organic matter in soilsunder G. sepium.Liyanage (1985) reported that coconut / cocoa system increasedactivity of useful microbeds such as phosphate solubilizers andnitrogen fixers in soils. According to Pursglove (1976), coconutyields of 7500 nuts / ha removes from the soil 67 kg N, 22 kg P2 O5

and 90 kg K2O or more than thrice the quantities of the nutrientsremoved by a cocoa crop of 1000 kg dry beans / ha (Omotoso,1975). Intercropping cocoa with coconuts may be profit

able but at the same time could result in rapid soil nutrientdepletion. This paper reports on the effects of cropping systemsof coconut with cocoa on soil nutrient dynamics in Ghana.

MATERIALS AND METHODS

The studies were undertaken on an already established cocoa-coconut intercropping trial at the Cocoa Research Institute ofGhana, Tafo. The soils at the site are deep gravely – clay loamclassified as Ferric lixisols (FAO/UNESCO, 1990). Theexperimental treatments on which the studies were superimposedwere;

T1 - Cocoa spaced at 3 m x 3 m and intercropped withGliricidia sepium spaced at 12.0 m x 12.0 m giving plantdensities of 1074 and 67 for cocoa and Gliricidia sepium/ ha respectively.

T2 - Cocoa spaced at 2.5 m triangular with coconut at 8.8 mtriangular spacing giving plant densities of 1829 and 134for cocoa and coconut / ha respectively.

T3 - Cocoa spaced at 3 m x 3 m with coconut at 9.1 mtriangular giving plant densities of 1074 and 149 forcocoa and coconut / ha respectively.

T4 - Cocoa spaced at 2.5 m triangular with coconut at 9.8 mtriangular giving plant densities of 1739 and 105 forcocoa and coconut / ha respectively.

T5 - Cocoa spaced at 3 m x 3 m with coconut at 12.2 m x 10.7m giving plant densities of 1074 and 90 for cocoa andcoconut / ha.

T6 - Adjacent uncultivated bush.

The details of the design and layout of the original experimentthat was set up in 1987 have been described by Osei-Bonsu et al.(2002).

In March 2002, soil samples were taken from three soil depths at 0– 15 m, 15 – 30 cm and 30 – 45 cm in each of the treatments, airdried, ground and passed through 2 mm sieve for analysis. Soil pHwas determined in water using a glass electrode. Organic carbonwas determined by the wet combustion method of Walkley andBlack (1934). Total nitrogen was by the Kjeldahl method ofBremner (1965), available phosphorus was obtained with theextractant 0.2 N H2SO4 using ascorbic acid as reductant andexchangeable K from 1 M ammonium acetate leachate on AtomicAbsorption spectrophotometer.

RESULTS

The effects of the cropping systems and adjacent fallow on somesoil nutrient changes at the end of fifteen years of cropping for the0 – 15 cm, 15 – 30 cm and 30 – 45 cm depths are presented inTables 1a, 1b and 1c respectively. Land clearing and thecultivation of crops / crop combinations significantly influencedsome of the soil chemical properties. Soil pH increased withcultivation but the crops / crop combinations did not significantlyaffect the soil pH at the three soil depths. Soil pH however, washigher at the 0 – 15 cm depth than at the other two soil depths. Theorganic carbon contents of the cropping systems also increasedwith cultivation. The surface soil (0 – 15 cm depth) had the highestorganic carbon content. Relative to the fallow plot (T6), the meanvalues of organic carbon in the cultivated plots represent anincrease of 21% in the 0 – 15 cm, 43% in the 15 – 30 cm and 74%in the

30 – 45 cm soil depth. Thus the effect of soil organic carbonincrement with cultivation was more marked in the deeper soillayers than in the topsoil layer. In all the three soil depths, thehighest organic carbon concentration was recorded in plot T1.Cultivation significantly decreased (p = 0.001) the total N contentof the soil in all depths. Nitrogen concentration in the soil alsodecreased with depth. The mean values of total N in thecultivated plots represent 73% decline in the 0 – 15 cm, 78% inthe 15 – 30 cm and 53% in the 30 – 45 cm soil depths relative toT6. The differences in the total N concentrations of the closelyspaced coconut plots (T2, T3, T4) were not significant but thelowest concentration of total N was recorded in plot T5 whichwas significantly lower (p = 0.001) than in the other cultivatedplots in all the soil depths. The T1 plot recorded the highest levelof total N among the intercrops. The soil available P levelsshowed that cultivation significantly decreased (p = 0.001) theircontents in the soil. The soil available P contents also decreasedwith depth in all the treatments. Compared with T6, the meanvalues of soil available P in the cultivated plots represent 35%,49% and 44% decline respectively in the 0 -–15 cm, 15 – 30 cmand 30 – 45 cm soil depths. The available P content of the T1plot was significantly lower (p = 0.001) than in the otherintercrops. Among the cocoa-coconut intercrop systems, T5recorded the lowest level of available P. Exchangeable K levels inthe soils also declined with cultivation but the differences in thetreatments were not significant. Relative to T6, cultivationresulted in a mean drop of 60% for the exchangeable K with theT2 recording the highest decrease of about 50% in the 0 – 15 cmdepth.

Table 1a. Effect of treatment on soil properties (0 – 15 cm depth).

Mario Cervantes

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DISCUSSION

The higher soil pH and organic carbon values recorded in thecultivated plots compared with those in the uncultivated plotindicate that cultivation had a positive impact on pH and organicmatter levels in the soil. In the cultivated plots, higher return ofcocoa litter to the plantation floor protected the soil surface fromerosion and that could minimise the loss of bases from the soilresulting in a decrease in acidity. Reduction in soil organic carbonwith cultivation has been reported by Jenkinson (1981). Howeveraccording to Halm (1977), the rate of cocoa leaf litter fall is highunder Ghanaian conditions. The higher organic carbon contents ofthe cultivated plots than the uncultivated bush could be due to theaddition of litter from cocoa. Gliricidia sepium also sheds itsleaves in the dry season (November to March) and together withthe cocoa litter could result in increased quantity of organic carbonresidue in the T1 plot hence its higher soil organic carbon content.

The significantly higher total N and available P contents of the T6plot than in the cultivated plots was possibly due to the removal ofthe plant nutrients from the soil by the intercrops. This evidence issupported by the observation that the soil organic carbon was lowerin the T6 plot and could not have contributed much to the nitrogenand phosphorus pools. High total N content of the T1 plot may beattributed to inputs from nitrogen fixation by the G. sepium.Contrary to the expectation that competition for plant nutrientsfrom soil is greatest in high density plantings, among the cocoa –coconut intercrop systems, widely spaced cocoa and coconut (T5)recorded significantly lower total N and available P levels. Thiscould be due to the higher weed cover in the plot. The cumulativeyield of coconut was high in T5 (Osei Bonsu et al., 2002). It maytherefore be speculated that the removal of nitrogen andphosphorus by the coconut for higher yields was one of the factorsfor the low concentrations of the nutrients in this cropping system.Nodules are strong sinks for P (Graham and Rosas, 1979) andCobbina et al. (1992) observed that soil factors that affectnodulation and biological nitrogen fixation in legumes in tropicalsoils include P availability. The significantly lower concentrationof P in T1 could be the result of the utilization of the soil availableP for nodulation by G. sepium. The absence of significantdifferences in exchangeable K contents of soils for the treatmentsconfirms the earlier contention that cocoa growing soils of Ghanaare well buffered against K depletion (Ahenkorah, 1970).However, the highest percentage decrease in exchangeable Kcontent of closely spaced cocoa-coconut (T2) could be explainedby the excessive absorption of the nutrient by the intercrops.

Based on the estimates for the removal of the major plant nutrientsfrom the soil, the quantitative sequential order of importance of themajor nutrients for bearing cocoa and coconut is potassium >nitrogen > phosphorus. Root activity of both crops are greatestwithin the top 15 cm of the soil surface and 60 cm from the base ofthe crops (Ahenkorah, 1975; Pursglove, 1976) which imply thatboth crops actively

feed within the same soil layer. Competition for nutrients,particularly from applied fertilizers by the crops is likely to be

very high in intercrop systems with closer spacings.

CONCLUSION

Intercropping cocoa with G. sepium significantly decreased thesoil available P but increased the soil total N. Among the cocoa– coconut intercrop systems, widely spaced cocoa and coconutcropping systems surprisingly had the lowest concentrations oftotal N and available P in the soil. However, considering thelikely high competition for plant nutrients between cocoa andcoconut, the adoption of widely spaced cocoa and coconut systemby farmers under fertilization may be more promising.

ACKNOWLEDGEMENTS

The authors wish to acknowledge the technical support by thetechnical staff of the Agronomy/Soil Science Division.

We are also thankful to Mr. Ben Anamon for typing themanuscript. This paper is published by the permission of theExecutive Director, Cocoa Research Institute of Ghana.

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