Effect of Nitrogen Forms and Rates on Yield and Performance of Young and Mature Cocoa Trees in Two Agro-ecological Zones in Ghana

Abstract

ABSTRACT Cocoa affectionately called ‗golden pod‘ is one of the few valuable tropical crops. Undoubtedly, cocoa is the most inevitable agricultural produce among several other crops produced in Ghana. It is the stronghold of Ghana‘s economy, being second leading foreign exchange earner after gold, worth about 30 percent of all revenue from export and responsible for about 57 percent of overall agricultural export. More notably, the cocoa sector offers jobs to about 2 million Ghanaians. Despite the significance of cocoa to the economic stability and as a key export crop for Ghana, yields of cocoa beans are stagnating well below average yields worldwide. Nitrogen (N) plays a critical role in the growth and development of crops. There have been several assertions by industry players on the impact of nutrient nitrogen on cocoa growth and productivity, yet there is limited empirical evidence on the impact of N on the productivity of cocoa in Ghana. Though some studies have focused on the impact of one of the nitrogen forms (i.e. either ureic, ammoniacal or nitric) on cocoa in Ghana, there is yet a comprehensive study to investigate the impact of all three forms of nitrogen on the growth and productivity of cocoa. In this project, two representative cocoa farms from two Agro-Ecological Zones (Coastal Savannah and Evergreen Forest) were evaluated for the impact of N forms (urea/ammonium/nitrate), N rates (low/optimum/high), and nutrient N recycling through litter fall production on the growth, development and productivity of young and mature cocoa trees. A multi- stage random sampling technique was used to collect data on soil-plant nutrition practices among cocoa farmers whilst a randomized complete block design (RCBD) was used for the on-farm experiment. The survey revealed that 79.2% of the respondents used inorganic fertilizer on their farms with a small proportion (20.8%), using organic fertilizer. Farmers used both liquid (foliar) and granular forms of inorganic fertilizers with the majority of such applications (72.5%) being done in liquid forms because the type of granular fertilizers used lacked N. The results from the on-farm trials showed increased levels of essential soil nutrients on fertilized plots than unfertilized plots. Cocoa yield increased with an increasing rates of N. Young cocoa trees were responsive to N supplied as nitrates whereas mature cocoa trees responded positively to N supplied in the form of urea. Additionally, young cocoa trees were more efficient in N use than mature cocoa trees whilst N resorption for mature cocoa trees was greater than N resorption recorded for young cocoa trees in both Agro-Ecological Zones. N content in the litter tissue of young cocoa trees was higher than that found in mature cocoa trees. Finally, at increasing N application rates, litter fall production, and litter tissue N content increased. The study concludes that although the yield effect was short term, there is the potential benefit to cocoa plants regardless of the age and location when N in the form of nitrate is added to fertilizers for young plants whilst urea could be incorporated into more mature cocoa plants nutrition in both Agro-Ecological Zones of Ghana. The most promising nitrogen (N) rates for an effective cocoa yield in terms of the pod and dry bean yield should be pegged at 20 to 30 kg N/ha. The incorporation of N in the formulation of granular inorganic fertilizers is recommended.