Rubber plantation management affects soil nitrogen availability and tree growth in smallholder farms: Evidence from a survey in Agusan del Sur, southern Philippines

Natural rubber (NR) is a critical industrial commodity, and its production in Southeast Asia is dominated by smallholder farmers. In the Philippines, rubber farming is a major source of income for rural communities. However, the link between field management, soil properties and rubber tree growth in the region remains poorly understood, particularly in the key production areas such as the Province of Agusan del Sur. This study investigated the rubber tree growth and soil properties – including soil type, ammonium, nitrate, soil pH/EC, soil organic carbon (SOC) and total nitrogen (TN) – and evaluated how these variables are associated with two common field management systems in Agusan del Sur: rubber monoculture (RM) vs. rubber-based intercropping (RI). We collected data from 36 smallholder rubber farms growing 9–10-year-old trees. The results showed field management significantly affected rubber tree growth and soil properties. Trees under monoculture system had a larger girth (57.3±11.9 (SD) cm) but with lower soil pH (pH 5.38 ± 0.67), compared to those under intercropping (51.4 ± 8.8 cm; pH 5.81 ± 0.73). Results showed that the monoculture systems were associated with higher tree girth, due to the higher TN and NH 4 ⁺ availability in the soil, and enhanced leaf N uptake. These findings highlight that improving ammonium availability while managing soil acidification may be critical to optimizing productivity and sustainability in smallholder rubber farming systems.