Can fertilization be a driver of rubber plantation intensification?

Improving the productivity of existing rubber plantations is necessary to cope with the growing demand for natural rubber (NR) while limiting the environmental and social impacts linked to the expansion of rubber cultivation at the expense of natural ecosystems and food crops. The effect of fertilization on NR yield is still unclear and is poorly documented particularly in Thailand, the world leader in NR production. Hence, the main objective of this study was to re-assess the possible effects of fertilization on the yield of intensively managed rubber plantations in Thailand. Our main hypothesis was that the effect of fertilization would be higher with intensive latex harvesting practices (high tapping frequency with ethylene stimulation). To test this hypothesis, we set-up a split-plot experiment with four fertilization doses (T1, no fertilization, T2, T3, T4, respectively low, medium and high doses of NPK fertilizer) and two tapping systems (S/2 d2 with and without ethylene stimulation). Here, we present the results of the first three years on dry rubber yield, latex metabolism assessed with the latex diagnosis method, and latex and rubber properties related to their technological properties. Our results showed a positive effect of fertilization on yield from the first year, but the effect was only statistically significant (p < 0.05) in the third year. The maximum effect of fertilization (+13%) compared to the control treatment (T1) was obtained with the highest dose of fertilizer (T4). Cumulatively over the 3-year period, the increase in yield was +5% with the T2 dose, and +8% with the T3 and T4 dose. Contrary to our main assumption, we observed no interaction between fertilization and ethylene stimulation. Latex diagnosis revealed that the effect of fertilization on yield was linked with a direct effect on latex metabolism mainly through an increase in inorganic phosphorus content (Pi). Finally, we observed no detrimental effect of the fertilizer treatments on latex and rubber quality. We can thus conclude that increasing latex yield through fertilization combined with intensive tapping does not involve any identified risks for the quality of rubber. Taken together, our results demonstrate that fertilization can help increase the yield of rubber plantations where intensive latex harvesting is practised. However, in a context of low NR prices for rubber farmers, the economic return of a +8% increase in yield is questionable. Our trial is continuing to assess the long term effects of the fertilization on yield as well as on the growth and nutritional status of the trees, and the nutrient balance of the plantation.