Abstract

Maintaining soil fertility while enhancing cassava (Manihot esculenta Crantz) productivity remains a major challenge on alluvial soils, which are inherently prone to nutrient depletion and physicochemical degradation because of their young, weakly aggregated mineral composition and fluctuating drainage regime. This study evaluated the effectiveness of integrated organic–inorganic fertilisation using humic acid, compost, and NPK (Phonska; 15% N–10% P₂O₅–12% K₂O–10% S) on post-harvest soil chemical properties and cassava agronomic performance, through a 13-month field experiment on alluvial soil in Kertosono, East Java, Indonesia, arranged in a Randomised Block Design with six treatments (T1: humic acid; T2: compost; T3: Phonska; T4: humic acid + Phonska; T5: humic acid + compost; T6: untreated control) and three replications. Soil quality was assessed using pH, electrical conductivity (EC), and redox potential (Eh), while agronomic performance was measured through plant height and tuber yield. Integrated fertilisation significantly enhanced cassava growth and productivity relative to single-fertiliser applications and the control. The humic acid + compost treatment (T5) produced the highest tuber weight (11.10 kg plant⁻¹, a 38.8% increase over the control’s 8.00 kg plant⁻¹) together with the highest soil EC (0.249 mS cm⁻¹) and Eh (123.7 mV), while humic acid combined with NPK (T4) increased tuber weight by 31.3% over the control and produced the tallest plants at three months after planting (127.7 cm, +30.7%). However, prolonged fertilisation reduced soil pH below 5.0 in every fertilised treatment except T1, falling as low as 4.1 under T5, indicating a risk of progressive acidification. Overall, synergistic organic–inorganic fertilisation can improve cassava productivity while supporting soil functional stability, with pH, EC, and Eh serving as practical, field-deployable indicators for precision fertilisation in sustainable cassava cultivation on alluvial soils.