Residual Effect of Compost on Ethanol Production of Sweet Soghum (Sorghum bicolor (L.) Moench) Varieties and Soil Organic Carbon at Dryland Farming Area in Bali, Indonesia

Abstract Increasing ethanol production of sweet sorghum have been receiving much attention under this era of finding new sustainable sources of bioenergy. Compost did not significantly affect biomass and ethanol production of four sweet sorghum varieties planted in 2012, therefore the objective of the 2013 experiment was to study the residual effect of compost applied in 2012 on ethanol production of sweet sorghum [Sorghum bicolor (L.) Moench] varieties and on soil organic carbon. The experiment was designed as randomized complete block with three replications and two treatment factors (rate of compost residues and sweet sorghum varieties) were in factorial arrangement. The compost residues were in the rates of 0, 10, 20, 30 t ha-1 (similar to rates applied in 2012), while the varieties were Super Sugar, Sugar Graze, SG-1A and Local Belu NTT. Super Sugar and Local Belu were the varieties used in 2012. The experiment was conducted from June until November 2013 at dryland farming area of Jimbaran, Badung Bali, Indonesia. Results of the experiment showed that residual effect of compost and the interaction effect between the two factors were not significant on ethanol content, sugar yield, juice production, ethanol production ha-1 and on soil organic carbon. Those parameters were only affected significantly (P<0.05) by variety. The highest ethanol production (1314.87 l ha-1 ) was given by SG-1A, which was not significantly different from those of Super Sugar and Sugar Graze varieties (920.41 l ha-1 and 1257.92 l ha-1). The highest stem ethanol content (91.31%) was produced by SG-1A, and the lowest (90.40%) was given by Local Belu variety. Low (<1.0%) soil organic carbon content was found in whole experiment. In conclusion, that high ethanol production could not be expected from residual effect of compost alone especially under low soil fertility.