The objective of this study was to evaluate the effect of ash from carbonized rice husk on the incidence and damage of thrips, downy mildew severity, yield, onion postharvest performance and chemical characteristics of the soil in organic system. The study was conducted in the field at Epagri, Experimental Station of Ituporanga, Santa Catarina State, Brazil. The cultivar used was Epagri 362 Crioula Alto Vale. The treatments were doses of carbonized rice husk ash 5 t ha-1, 10 t ha-1, 15 t ha-1 and untreated check. Randomized blocks design with five replications was used. The treatments do not affect the incidence and damage of thrips, downy mildew severity, leaf potassium content, yield and onion postharvest yield. Doses of ash influence the content of potassium in the soil in a positive quadratic relation and does not change the pH, organic matter, P, Ca, Al, Mg, and Na.