Coconut-gliricidia mixed cropping systems improve soil nutrients in dry and wet regions of Sri Lanka

Agroforestry systems are alternative solutions for production and management of agricultural systems which may improve soil quality. In this study, we evaluated the potential of coconut Cocos nuciferabased Gliricidia (Gliricidia sepium) systems to improve soil quality of coconut lands in Sri Lanka in dry and wet regions. A three-year field experiment was conducted in a randomized complete block design with three treatments T0, T5 and T20, being respectively the control, five and twenty years aged Gliricidia intercropped coconut-based mixed systems. Three replicates of soil samples were taken at 0–15, 15–30 and 30–45 cm and differences in soil physical and chemical properties were evaluated among treatments and sites. We found significant effects of mixed system treatments on the soil chemical properties. In particular, organic matter, soil exchangeable potassium, total nitrogen and available phosphorus contents showed higher values in most coconut-gliricidia mixed systems’ soils, with highest values obtained for T20. Cumulatively for all soil depths, organic matter content (22%) and available phosphorus content (20%) were higher on the wet site, and total exchangeable potassium content (69%) higher on the dry site for T20. The pH, bulk density, microbial respiration and electric conductivity did not vary among treatments, but were influenced by the site characteristics, with the dry site showing higher values for pH and the wet site showing higher values for bulk density (5%), microbial respiration (33%) and electric conductivity (2%) in T20 treatment. The study demonstrates that the systems with Gliricidia differed in their soil chemical attributes and had higher levels of soil nutrients when compared to coconut monocrop even at early ages, underlying the potential of Gliricidia for the rehabilitation of coconut growing soils.