Quantifying nutrient cycling in island agricultural and taro production systems and undertaking field trials to highlight the importance of budgeting for soil fertility management and increasing yield


Pacific Island nutrient cycles have been increasingly modified since human settlement 2000 years ago. In the last 50 years, agricultural intensification has resulted in further changes in the island nutrient flows. Low-input farming systems have been found to result in soil nutrient removal through weathering, leaching, and crop removal. Recent research has identified that the depletion of soil nutrients has contributed to yield decline of agricultural production across many Pacific Island nations.

This research quantified nutrient cycling in island agricultural taro production systems and undertook field trials to highlight the importance of budgeting for soil fertility management and increasing yield. The research involved:

  • Calculating supra-national scale soil nutrient balances for Fiji, Tonga, Samoa, Kiribati, and Tuvalu.
  • Undertaking field trials, soil sampling and extension in Fiji, Tonga, Samoa, Kiribati, and Tuvalu.
  • Calculating nutrient constraints for the field trials.
  • Measuring biological function, water flux and nutrient losses.
  • Calculating nutrient constraints for each soil type using data collected from the field.
  • Engaging in research extension with farmers, extension, and policy makers.

Key Results

Supra-national scale soil nutrient balances

Based on country scale agricultural land nutrient (nitrogen, phosphorus, and potassium) budgeting from the Food and Agriculture Organisation (FAO) country data (1964-2018) there were nutrient imbalances in the five studied countries, with long-term removal of soil nutrients over the last 50 years (Table 1). The average potassium balance was negative for all five countries and crop removal exceeded deposition and manure additions on the atolls and fertiliser additions on the volcanic and raised atolls. There were no synthetic fertiliser additions in Kiribati and Tuvalu. The budgeting calculations did not consider nutrient losses via erosion, leaching and run-off, or denitrification, and the net nutrient fluxes may be greater than calculated.

The use of animal and treated human waste would help off-set this imbalance, however, additional macro- and micro-nutrients would need to be added for balanced plant nutrition. While increasing fertiliser inputs could 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 could augment the fertiliser applications.

Table 1: Time weighted nitrogen (N), phosphorus (P), and potassium (K) additions, removal and mass balance (kg ha-1 yr-1) for the agricultural areas of Kiribati, Tuvalu, Tonga, Fiji and Samoa (1964-2017). Source: FAO country data.

Country Element Output Inputs Mass Balance
Crop Removal Deposition Fertiliser Manure
Kiribati N 3.63 1.36 0 0.85 -1.43
P 0.69 0.23 0 0.18 -0.29
K 7.60 0.36 0 0.18 -7.06
Tuvalu N 7.14 1.36 0 8.05 2.27
P 0.36 0.23 0 1.68 1.55
K 2.90 0.36 0 1.68 -0.86
Tonga N 27.58 1.36 16.98 0.65 -20.86
P 1.11 0.23 3.01 0.13 0.08
K 14.11 0.36 22.25 0.13 -7.43
Fiji N 10.17 1.36 4.56 3.31 -4.23
P 1.31 0.23 0.73 0.69 -0.19
K 12.41 0.36 1.21 0.69 -11.02
Samoa N 26.83 1.36 0.38 2.76 -22.61
P 1.47 0.23 0.16 0.57 -0.63
K 16.91 0.36 0.25 0.57 -15.91

Figure 1: Organic carbon content in soils collected during historical sampling (Gibbs 1976, Potter 1986, Cowie et al. 1991) and 2018 soil survey Tongatapu, Tonga.

In the last 30 years, soil organic carbon stocks have reduced (30 – 40%) in taro and sugar cane cropping areas in Fiji. The reduction in soil carbon stocks has been linked to declining soil fertility and production. The decline in soil organic matter is largely a result of intensification, inappropriate soil management and a failure to adequately fertilise the crop and the soil organic matter system.

In low input farming systems, the soil organic matter pool is critical for supplying macro nutrients, such as nitrogen, phosphorus, potassium, sulphur, and micronutrients. The decline of soil carbon represents a decline in the organic matter pool of nitrogen, phosphorus, and sulphur.

During 2018, Tonga soils were sampled at locations previously sampled in the last 35 years. Like in Fiji, soil organic carbon declined in Tongan soils, with a loss of between 30-50% in the last 35 years (Figure 1). This would have resulted in the subsequent decline in the soil nitrogen, phosphorus and sulphur pools.

Click here to see the nutrient cycling and field trials presentation.

Click here to see the country nutrient budgets and portal application presentation.

Field trials and extension in Fiji, Tonga, Samoa, Kiribati, and Tuvalu


Cowie, J., P. Searle, J. Widdowson and G. Orbell (1991). “Soils of Tongatapu, Kingdom of Tonga.” DSIR Land Resources, Lower Hutt.

Gibbs, H. S. (1976). Soils of Tongatapu, Tonga.

Potter, L. W. (1986). Tongan Soils: Site characteristics and management practices. Occasional Paper 7. South Pacific Smallholder Project. Armidale, NSW, University of New England.