The effect of pruning and nutrient management practices on cacao (Theobroma cacao) yields in the Solomon Islands : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science, Massey University, Palmerston North, New Zealand

Abstract

Cacao (Theobroma cacao) is a vital agricultural commodity in the Solomon Islands (SI), yet yields remain low across smallholder farms due to a complex interplay of agronomic, economic and institutional factors. The aim of this study was to investigate and evaluate management practices that have potential to assist cacao growers improve yields. This research involved an initial survey of 60 growers, and assessed the soil and tree nutrient status on selected farms. The effect of a range of different nutrient inputs, including conventional fertilisers and composts, were evaluated using three different field trials. A final grower survey with 14 growers was used to gain a better understanding of the potential influence of cacao tree rejuvenation practices, primarily intensive pruning and grafting, on farm productivity.

The first grower survey in this study aimed to document current farm management practices and assess soil and plant nutrient status across smallholder cacao plantations. The survey showed that cacao yields were low for most farms and identified differences in pruning practices as potential causes of yield variation. Growers who adopted intensive pruning achieved average yields that were 23% higher than those using only standard pruning, and 128% higher than those who did not use any pruning practices. Growers who used both intensive pruning and selected grafting practices appeared to have additional yield benefits. All the growers that used intensive pruning alone or with selected grafting practices had previously received training and tools from support programmes. Overall, most of farms sampled had low soil and leaf nutrient status. Therefore, there is a need to identify cost effective methods to replace nutrient losses that occur over time.

Results from the fertiliser and compost response field trials showed limited and inconsistent results. Only at Site 1, where cacao is grown as a monocrop, was there strong statistical evidence (p=0.059) of a cacao yield response to conventional fertiliser addition. The fertiliser treatment applying 100 kg N, 90 kg P and 150 kg K ha⁻¹ had 189% higher number of harvested cacao pods compared to the Control (no fertiliser) treatment in the first year of the trial. However, no other treatment effects were observed. High tree variability was one of the factors likely influencing the ability to show more significant effects of nutrient inputs on yields. There was also no consistent influence of conventional fertiliser or compost treatments on soil and cacao leaf nutrient status at any of the trial sites.

This study also highlighted that the cost of conventional fertilisers in the SI is major constraint to their use by growers. In the SI, the cost of bagged fertiliser is more than three times that in New Zealand. In some cases, growers would need as much as a 300% increase in yield to achieve a break-even return on the cost of fertiliser. This would require unrealistic yield responses to achieve a return on the fertiliser investment. Therefore, fertiliser use will be cost prohibitive for most cacao growers unless fertiliser prices decrease markedly. In addition, because some growers sell their cacao beans for use in single origin chocolate marketed as being organically grown, this is another reason for growers to be careful about using conventional fertilisers. Therefore, the use of improved recycling of tree residues, including pod husks, and the use of organic certified nutrient inputs will be important to help growers who want to maintain organic status. Overall, there is little evidence that conventional fertiliser use would help smallholder growers be more financial sustainable, especially for those with intercropping farm systems. In addition, cadmium (Cd) is becoming an emerging issue in cacao, with access to markets potentially limited due to the presence of elevated levels in cacao beans from some regions in South America (Thomas et al., 2023). Because fertiliser can be a source of Cd, then it is important to ensure that fertiliser use also doesn’t increase soil and cacao bean Cd levels. Soil Cd concentrations were low (<0.25 mg Cd kg⁻¹) at both field trial sites tested, for both the Control treatment and 90 kg P ha⁻¹ P fertiliser treatment, which is promising as it is associated with lower levels of Cd in dry cacao beans.

The second grower survey investigated the adoption of pruning and grafting techniques among 14 selected growers, evaluated their impact on farm productivity, and explored the potential main factors influencing wider adoption. This survey found that on average, growers that use intensive cacao tree pruning and grafting practices on at least 25% of their farm in the previous 15 years had higher yields than farms only using standard pruning. While further research is needed to provide greater confidence in the size of the yield potential, the analysis revealed that tree rejuvenation practices have potential to improve yields. The potential cost of three strategies to support growers to implement rejuvenation practices were assessed. These included providing clonal seedlings, training and tools for on-farm grafting, or an integrated approach that combines the first two strategies. The integrated strategy showed promise, offering a balance between genetic improvement of trees and grower development, while maintaining cost-effectiveness. However, achieving this will require a national strategy that integrates technical, institutional and economic support mechanisms.