Changes in soil pH water (pHw) were investigated on a sugarcane plantation in the Ramu Valley of Papua New Guinea. The plantation was established in 1979 from natural grassland and the dominant soils were Fluvents and Vertisols. Statistical analysis of the topsoil pHw data (n = 541) showed a significant (α = 0.01) decrease from 6.5 to 5.7 between 1979 and 1996. Based on samples from 80 fields at different sampling times, the average decrease in topsoil pHw was calculated to be 0.4 units after 10 years of continuous sugarcane cultivation (r2 = 0.481). Between 1986 and 1996, subsoil pHw also decreased significantly (α < 0.05) by 0.4 and 0.3 units in the 0.30–0.45 and 0.45–0.60 m horizons respectively. The acidification trend accelerated in the 1990s when trash-harvesting replaced pre-harvesting burning and sulphate of ammonia became the dominant nitrogen fertilizer. Between 1991 and 1995, average nitrogen application rates were 90 ka ha−1 a−1 which resulted in an annual addition of 11.6 kmol H+ ha−1. The soil buffer capacity was estimated to be 125 kmol H+ ha−1 pH−1 which implied that the pHw could further decrease by 0.5 unit after five annual applications of 90 kg N ha−1 pH−1 as sulphate of ammonia. Although these soils were young in pedological terms and they had been cultivated for less than 20 years, they had acidified significantly and this could affect sugarcane production adversely if such trend were to continue.