Effects of Organic Fertilizer Substitutions on Latex Yield and Soil Fertility in Hevea brasiliensis Plantations: A Case Study in Southern Philippines

ABSTRACT Rubber plantations can be a major source of income for small landholder farmers in rural communities across Southern Asia. However, rising fertilizer costs and progressive soil degradation threaten long‐term productivity and economic viability. To address this challenge, we conducted fertilizer trials across two field sites (Acrisols and Cambisols) in the southern Philippines, to evaluate the effects of reduced chemical fertilizer rates and organic fertilizer substitution on latex yield and soil fertility in mature rubber plantations. Five treatments (Control—no fertilizer, recommended chemical fertilizer rate (RR), 50% of RR, 50% of RR with vermicompost, and 50% of RR with K‐humate) were included. Results show that over the one‐year period, latex yield increased by 23%–56% at the Marcelina site, and by 18–32% at the Mabuhay site, compared to the respective unfertilized control. Across both sites, latex yield responses showed no statistical difference among full recommended (RR) and reduced fertilizer rates (50% RR, 50% RR + vermicompost, 50% + K‐humate). Key soil properties—including pH, EC, dissolved nitrogen (N), NH 4 + ‐N, NO 3 − ‐N, plant available phosphorus, dissolved organic carbon (C), soil organic C and total N—were not significantly affected by fertilizer application, but varied with sampling time. These results highlight that (1) in the short term (e.g., one‐year), the locally recommended fertilizer rates can be reduced without compromising soil fertility or yield, enabling farmers to cope with periods of financial hardship; (2) substituting chemical fertilizers with organic fertilizers, particularly vermicompost, has better potential to sustain latex yield in plantations with varying soil textures (clayey soils and silty clay loamy soils). The conclusion drawn from our study should be validated in a longer timeframe in future work. Our findings provide evidence to inform region‐specific fertilizer recommendations that enhance both sustainability and profitability for smallholder rubber farmers.