Overlooked Influence of Indian Hemp (Cannabis sativa) Cultivation on Soil Physicochemical Fertility of Humid Tropical Agroecosystems: Lowland Soils

Abstract

One agricultural practice that may be depleting plant nutrients in wetland soils of the humid tropics is cultivation of Indian hemp (Cannabis sativa), also called Marijuana. Though Nigerian Law, adopted from International Conventions on narcotics, prohibits handling of any part of cannabis plant, it is still illicitly cultivated. This practice may be undermining the quality of wetland agroecosystems. To support these concerns with empirical data, the influence of Cannabis cultivation on soil physicochemical fertility of wetland agroecosystems was assessed at a representative location in southwestern Nigeria. The study compared four land-use options; land not used for Cannabis cultivation (NUC), land currently under Cannabis cultivation (CCC), farmlands converted from Cannabis to alternative use (CAU), and Cannabis farmlands abandoned or seized (ABS). Soil data from the pedogenetic horizons under these land-use options were averaged and analysed. There were significant differences in soil bulk density, with low values in NUC (1.36 Mg m^–3) < medium values in CCC (1.55 Mg m^–3) < high values in both CAU and ABS (1.62-1.66 Mg m^–3). The highest value in the ABS (1.66 M  m^–3) is slightly above the critical limit (1.60 Mg m^–3) for root growth. Soil compaction in Cannabis farmland thus worsened even after discontinuation of cultivation. Soil pH, soil organic C, total N, exchangeable Ca, exchangeable Mg, apparent and effective cation exchange capacity also differed thus NUC ≥ CCC ≥ CAU ≥ ABS, while base saturation showed an inverse trend. Available P was, however, higher in CCC (14.32 mg kg^–1) than the rest, with lowest values in ABS (5.83 mg kg^–1). Micronutrients (Mn, Zn and Cu), excluding Fe which was unaffected, followed the trend of soil pH. It is concluded that continuous cultivation of Cannabis in humid tropical lowlands compacts the soil and drains soil nutrients except available P whose status is rather elevated. The practice thus poses a threat to food security and ecological well-being.