The CocoaSoils nutrient offtake model: preliminary results from onfarm trials

Current nutrient recommendations in cocoa vary per country and often do not include N. To improve recommendations, partners in the CocoaSoils consortium (www.cocoasoils.org) have set up large-scale fertiliser trials (Core Trials) and small on-farm trials (Satellite Trials) across the tropics. We developed a nutrient offtake model to determine fertiliser rates and tested selected treatments in farmers’ fields to answer the following questions: 1) Can farmer yields be improved by additional nutrient applications as recommended by the offtake model? 2) What are the differences between countries in terms of yields and responses to treatments? 3) Are the offtake model recommendations cost effective? With the offtake model we calculated the offtake in vegetative biomass and beans and converted these to nutrient recommendations. In 389 Satellite Trials, we compared farmer practices (control, T1), Good Agricultural Practices without additional nutrient applications (T2), GAP + national fertiliser recommendations (T3) and GAP + offtake model recommendations (T4). We monitored yields and calculated return on investment (ROI: USD earned per USD invested) for T3 and T4, assuming the following prices (in USD kg-1): dry cocoa beans 1.58; nitrogen 1.3; phosphorus 4.1; potassium 1.3. Preliminary yield results from the Satellite Trials suggest that yields in T4 plots were consistently higher (around 2150 kg dry beans ha-1 year-1) than yields in T3 plots (around 1920 kg ha-1 yr-1) in all countries but Nigeria. Yield patterns in Ghana, Cote d’Ivoire and Cameroon were similar; yields from Nigeria were substantially lower. Response patterns were similar in the main and light season. Adding nutrients according to the offtake model predictions was cost effective but ROIs were highly variable (1-5 USD USD-1). Estimated offtake quantities fall well within the range of recommendations listed by Van Vliet and Giller (2017); estimated N requirements are similar to other crops. There was a clear positive effect of T4 over T3 on yield. From the data we cannot tell which nutrient was responsible for the response: was it mostly the N, or the K, or a combination? The Core Trials will provide the answers to these questions when they enter the productive phase. In terms of costs, ROI may become negative when fertiliser prices increase or cocoa prices decrease, especially in Nigeria.