Declining Soil Nutrient Status Can Constrain Agricultural Productivity and Food Security in Pacific Island Countries: A Country Scale Assessment

Since human settlement approximately 2,000 years ago, Pacific Island nutrient cycles have been increasingly modified. Modern agricultural intensification has resulted in further changes in the island nutrient flows. Country scale agricultural land nutrient (nitrogen, phosphorus, and potassium) budgeting in Tonga, Fiji, Samoa, Kiribati, and Tuvalu were calculated from FAO country statistic data (1964–2018). Nutrient input data from birds, atmospheric dust, and rainfall and human waste were calculated from literature values. Overall, there are nutrient imbalances in all countries and agricultural lands are exporting nitrogen, phosphorus, and potassium. The budgeting calculations did not consider nutrient losses via erosion, leaching and run-off or denitrification, and the net nutrient fluxes may well be greater than reported. The use of animal and human waste would help off-set nutrient imbalances, but additional macro- and micro-nutrients would need to be added for balanced plant nutrition and soil carbon sequestration. While increasing fertiliser inputs will improve the nutrient balance and potential primary productivity, trade-offs such as nutrient losses will need to be considered. Improving nutrient budgets would need a farming systems approach, whereby the use of cover crops, crop rotations and legumes would augment the fertiliser applications.