Changes in soil fertility and leaf nutrient concentration at a sugar cane plantation in Papua New Guinea

Abstract

Commercial sugar cane (Saccharum qfficinarum) cultivation in Papua New Guinea started in 1979 at a plantation in the Ramu valley where Udifluvents and Hapluderts are the dominant soil types. The sugar cane is not irrigated and receives only nitrogen (N) fertilizers (±90 kg N ha‐1 y‐1). Changes in soil chemical fertility were assessed by comparing soil fertility data from the mid‐1980s and 1990s and by comparing soil fertility data from sugar cane and adjoining natural grassland. Between the mid‐1980s and 1990s the topsoil pH had declined significantly (p<0.001) by 0.3 units and this was accompanied by a decline in cation exchange capacity (CEC) of 34 mmolc kg‐1. Total N levels in the topsoils declined (p<0.001) from 2.5 to 1.9 g kg‐1 and available P from 36 to 27 mg kg‐1 during the same period. Exchangeable potassium (K) also declined significantly (p<0.05) with 1.3 mmolc kg‐1, but changes in exchangeable calcium (Ca) and magnesium (Mg) were not significant. The decline in soil fertility was highest in the topsoil although significant changes occurred up to 0.6 m depth. Total N decreased in the 0–0.15 and 0.15–0.30 m soil horizons, but increased in the lower horizons, possibly because of nitrate leaching. A similar degree of soil fertility decline was observed when soils under sugar cane and adjoining natural grassland were compared. However, the interrow had a slightly lower fertility level in comparison to within sugar cane rows. The decrease in total N, available phosphorus (P) and exchangeable K in the soil coincided with a decrease in the leaf N, P, and K concentrations of the sugar cane over the past 10 years. It was concluded that soil fertility had markedly declined under sugar cane monocropping although levels remained favorable for sugar cane cultivation. For sustainable soil management, nutrient inputs as well as small applications of lime may eventually be needed.