Turning Volcanic Ash into Fertile Soil: Farmers’ Options in Coffee Agroforestry After the 2014 Mount Kelud Eruption

Abstract

Post eruption land reclamation consists of hoeing, mixing volcanic ash with soil, adding external organic and/or in-organic fertilizers and making infiltration-pits (‘rorak’). This study, after the 2014 eruption of Kelud volcano, aimed to evaluate: (a) soil physico-chemical fertility post eruption, (b) impact of organic inputs interacting with ash in infiltration pits on soil C and N underneath (1 st experiment), (c) biomass loss (decomposition) of local biomass ( Trema orientalis and Parasponia andersonii ) in a coffee agroforestry system (2 nd experiment). Measurements in the ash-affected (+Ash) Tulungrejo-village (Ngantang-Malang district) were contrasted with an area without recent ash deposits (-Ash) in Krisik (Gandusari-Wlingi district). The 1 st experiment (-Ash site) treatments did not lead to statistically significant influences on soil conditions just below the infiltration pits during 12 weeks of monitoring. The 2 nd experiment quantified rate of biomass loss from litterbags. In +Ash location, litter half-life time (t 50 ) was 19.5 weeks for coffee or Parasponia as single biomass source to 24 weeks for Coffee+Sengon+Durian. In -Ash location decomposition was slower, with t 50 of 24 weeks for Parasponia to 27 weeks for Coffee+Sengon+Durian biomass. Concentrations of soil NH 4 and NO 3 below the litterbags peaked between 4 to 8 weeks, with nitrification lagging behind on ammonium release.