This research was conducted on post-eruption land area near Mount Merapi in Central Java Indonesia with the addition of goat manure of 30 tons/ha using factorial experiment prepared in Randomized Block Design (RAKL), with two treatment factors, and the treatment was repeated three times. The first factor was the source of potassium: no potassium, KCl, KNO 3 . Second factor source was Sulfur: without sulfur, ZA, Petro-Cas. The data were analyzed by variance, if there is real difference followed by LSD test 1%. The results show that ( 1) Fertilization of KNO 3 and ZA(NH 4 )2SO 4 in onion plants shows the highest yield on fresh weight of onion tubers per clump and per square meter; (2) Fertilization of KNO 3 without the addition of sulfur to obtain dry weight of the lowest weight of onion tubers per clump and per square meter; (3) Fertilization of KCl and ZA (NH 4 )2SO 4 on onion plant causes the highest water content on dried onion tubers; and (4) Fertilization of KCl and Petro-Cas (CaSO 4 ) 2H 2 O on onion plants yields the highest number of essential oil.