Effect of shade tree planting and soil management on rehabilitation success of a 22-year-old degraded cocoa (Theobroma cacao L.) plantation

Since productivity of current cocoa plantations is stagnating or declining, increasing global demand for cocoa products puts pressure on tropical, primary forests to be cut for new plantations. To address this problem, sustainable increase of cocoa yield on current plantations is needed. On a 22-year-old, 61 ha cocoa plantation in peninsular Malaysia, shade tree planting and soil management treatments, including targeted mineral fertilization, intense organic fertilization and combinations of these with compost application and cover crop (Centrosema pubescens) planting, were assessed as a rehabilitation strategy of degraded cocoa plantations with sub-optimum yield. Treatment effects on cocoa yield and on proxy indicators soil fauna abundance and diversity, and soil nutrient status, was tested 3.5 years after implementation in four harvesting rounds between 28 November 2014 and 12 March 2015. Across all treatments, large differences were found in the yield of the two clones tested (PBC130 and PBC123). The effect of shade trees (mainly Musa sp., with some permanent shade trees) on yield interacted with the factor 'clone'. Mean wet bean yield of PBC123 trees in full-sun cultivation was 0.74 kg as compared with 1.08 kg for trees in the agroforestry system (46% increase). Mean wet bean yield of PBC130 trees, however, was 20% lower in the same agroforestry system (0.08 kg) in comparison with mean wet bean yield of trees in full-sun cultivation (0.10 kg). Soil management treatments did not result in clear yield improvement as compared with common-practice, control plots. However, cover crop treatment had a positive effect on proxy indicators soil nutrient status and endogeic soil fauna diversity. It is concluded that in the rehabilitation of degraded cocoa plantations, possible yield impact of applying a straightforward agroforestry system, as the one in our experiments, is high. Furthermore, it was shown that in a degraded plantation with a long history of intensive fertilization, improved soil management has little effect. Positive effects of shade trees on cocoa yield are therefore probably linked with the creation of an environment that improves cocoa crop physiology and reduces pressure of pests and diseases, rather than with the improvement of soil quality as a consequence of intercropping with trees.