Development of sustainable nutrient management strategies for taro growers on Taveuni Island, Fiji : 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
Taro (Colocasia esculenta) is a major component of the socio-cultural, dietary and economic livelihood of Pacific Island countries, including Fiji. However, Fiji’s total taro exports have substantially declined over the last decade, mainly due to a reduction in production from Taveuni Island. A trend of decreasing taro yields has been further exasperated by recent extreme climate events, which have reduced the availability and affordability of planting material. The combination of lower yields and increased costs, have reduced farmers’ returns on existing farm land, which has contributed to further deforestation of forest reserves. The overall aim of this study was to develop improved nutrient management practices on existing farm land, to help growers to achieve sustainable taro yields and financial returns.
This study included a survey of 73 taro farmers, which involved a quantitative analysis of taro yields and soil fertility. Fresh taro corm yields were very low, being an average of 6.9 t/ha. About 75% of the farms surveyed had low soil Olsen phosphorus (P) levels of < 10 mg/kg and 65% had low soil exchangeable potassium (K) levels of < 0.4 me/100g. Fertiliser nutrient inputs were also low, with semi-commercial farmers using an average of 31 kg nitrogen (N), 17 kg P and 27 kg K/ha, and commercial growers using an average of 41 kg N, 26 kg P and 40 kg K/ha per crop. The survey also identified that fertiliser placement and application timing practices may also reduce nutrient use efficiency by the crop.
Two successive field experiments were repeated over three sites, on Taveuni Island, to evaluate a range of nutrient management strategies to improve soil fertility and taro productivity. When no N and P fertiliser was applied, average fresh taro corm yields were very low (6.2 t/ha), irrespective of K and sulphur (S) fertiliser inputs. The addition of up to 200 kg N/ha and 120 kg P/ha, resulted in a substantial average yield increase to 14.9 t/ha. Taro yields were also responsive to K fertiliser use, with significant increases in yield up to 200 kg K/ha, the highest rate of K assessed.
There was a strong linear relationship between N fertiliser use and taro sucker population up to 300 kg N/ha. In the second experiment, sucker numbers increased with increasing N fertiliser rate up to 280 kg N/ha. Further increasing N rates up to 360 kg N/ha did no further increase sucker production. When no fertiliser was applied, average taro sucker
population across the three sites was about 20,500 suckers/ha, which increased to about 122,500/ha at 280 kg N/ha. The response of sucker numbers to N was not influenced by P fertiliser use, but there was a small effect with increasing K fertiliser rate, from 100 to 200 kg K/ha. Following natural disasters, when sucker numbers are limited, the use of N fertiliser may be an effective short-term strategy to help re-establish the taro industry and minimise inflated sucker prices. The use of the legume mucuna, as a green manure crop intercropped with taro, reduced N fertiliser requirements. Mucuna provided a benefit, for both corm yield and sucker numbers, equivalent to N fertiliser applied at a rate of approximately 80-100 kg N/ha. Some of the benefit may also be due to improved weed suppression and a mulching effect from mucuna.
Mixing P fertiliser in the planting hole increased corm yields by 38%, compared to the when P fertiliser was placed at the bottom of the hole. Despite the high rates of P fertiliser used in the first experiment there were no significant P carry-over effects on taro yields in the second experiment. This was due to the higher soil P status from P fertiliser application being confined in close proximity to the planting holes.
The Soil Plant Analysis Development (SPAD) chlorophyll meter was evaluated as being effective at predicting N status of taro crops and yield potential, when other major nutrients were non-limiting. When SPAD readings were greater than 65, at 8 and 12 weeks after planting (WAP), then the taro yields were mostly high (i.e. > 12 t/ha). SPAD readings less than 40, at 8 and 12 WAP, would indicate that plant N status is likely to limit taro yield and that a review of N fertiliser use is required.
The recommended nutrient management strategy for taro farmers with farms with degraded soil fertility, involves the use of inter-cropping taro with a mucuna green manure crop in combination with fertiliser at the following nutrient rates; 120 kg N, 120 kg, 200 kg K/ha and 80 kg S/ha. At current taro corm and sucker prices, this recommended strategy is expected to provide a net income of NZ$35,835, which is 247% higher than for the average grower practice. The recommended practice also produces 74% more taro suckers than the current grower practice. Increasing sucker production is an important strategy to enable taro growers to increase production quickly following a natural disaster, helping farmers be more resilient to the effects of Climate Change.