Abstrak: Penelitian ini bertujuan untuk mengetahui jenis dekomposer yang efektif pada pembuatan kompos dan varietas yang tepat terhadap pertumbuhan dan hasil tanaman bawang merah serta interaksi diantara kedua faktor tersebut. Penelitian dilaksanakan dilahan Unit Pelaksana Teknis Dinas (UPTD) Balai Benih Hortikultura Saree, Kecamatan Lembah Seulawah Kabupaten Aceh Besar yang berlangsung dari bulan Juli sampai dengan Desember 2013. Lokasi penelitian memiliki jenis tanah Andisol. Penelitian ini menggunakan rancangan acak kelompok (RAK) pola faktorial dengan 3 ulangan. Ada dua faktor yang diteliti yaitu jenis dekomposer dan varietas bawang merah. Faktor jenis dekomposer terdiri dari empat taraf yaitu: kontrol, dekomposer Trichoderma harzianum, dekomposer Pseudomonas fluorescens P60, dan dekomposer cacing tanah Lumbricus rubellus. Faktor varietas terdiri dari dua taraf yaitu: varietas Trisula dan Vietnam. Analisis laboratorium dilakukan pada tanah sebelum diaplikasi kompos hasil dekomposer Trichoderma harzianum, Pseudomonas fluorescens P60, dan cacing tanah Lumbricus rubellus. Pengamatan dilakukan terhadap pertumbuhan dan hasil tanaman bawang merah, yang meliputi: tinggi tanaman umur 15, 30 dan 45 hari setelah tanam (HST), jumlah umbi per rumpun, berat umbi per rumpun dan potensi hasil per hektar.Hasil penelitian menunjukkan bahwa jenis dekomposer mempengaruhi tinggi tanaman bawang merah pada umur 30 HST dan jumlah umbi per rumpun. Tinggi tanaman bawang merah dan jumlah umbi per rumpun yang terbaik didapat dari perlakuan pemberian jenis dekomposer Pseudomonas fluorescens P60. Berat umbi per rumpun dan potensi hasil per hektar cenderung lebih baik dijumpai pada perlakuan pemberian jenis dekomposer Trichoderma. Varietas bawang merah mempengaruhi tinggi tanaman umur 30 HST, 45 HST dan jumlah umbi per rumpun. Tinggi tanaman dan jumlah umbi per rumpun yang terbaik dijumpai pada varietas Trisula. Sedangkan berat umbi per rumpun dan potensi hasil per hektar juga cenderung lebih baik pada varietas trisula. Tidak terjadi interaksi antara pemberian jenis dekomposer pada kompos dengan varietas terhadap pertumbuhan dan hasil tanaman bawang merah. Effectiveness of Decomposer Types on Compost to Increase Growth and Yield of Two Shallots Varieties (Allium cepa var ascalonicum) Abstrac: This research aimed to determine the most effective type of decomposer in making effectivecomposts and to reveal the variety of shallot that can boost its growth and yield, as well as the interaction between the two factors. This research was conducted inthe land of the Service Technical Implementation Unit (UPTD) of the Saree Horticulture Seed Center, Lembah Seulawah District, Aceh Besar Regency, from Juli to December 2013. The research location had Andisol soil. The research was accomplished using the factorial Randomized Block Design (RBD) with three repetitions. There were two studied factors, namely decomposer types and shallot varieties. The decomposer type factor consisted of four levels: control, Trichodermaharzianum, PseudomonasfluorescensP6, 0 decomposer, and earthworm Lumbricusrubellus. The shallot variety factor consisted of two levels: Trisula and Vietnam. Laboratory analysis was performed to analyze soil nutrient contents and organic fertilizer analysis. Observations were done on the growth and yield of shallots. The observed growth components were: plant height at 15, 30, and 45 days after planting (DAP), wet anddry stover weight, amount of bulbs per clump, bulb weight per clump, and yield potential per hectare. The results indicated that the types of decomposer had very significant effects on the plants’ height at 30 DAP and amount of bulbs per clump, significant effects on the weights of dry and wet stovers, and no significant effects on the plants’ height at 15 and 45 DAP, bulb weight per clump, and yield potential per hectare. The tallest shallot plant was a result of the Pseudomonas fluorescens P60treatment. It also recorded the best dry and wet stover weights and amount of bulbs per clump compared to other treatments. Slightly better bulb weight per clump and yield potential per hectare can be found in plants treated with Trichoderma. The shallot varieties had very significant effects on the plants’ height at 30 and 45 DAP and amount of bulbs per clump, significant effects on wet stover weight and no significant effect on the plants’ height at 15 DAP, dry stover weight, bulb weight per clump, and yield potential per hectare. Trisula produced the tallest plant at 30 and 45 DAP. It also produced the most wet stover weight, while its dry stover weight was slightly better than that of Vietnam’s. A better amount of bulbs per clump can be found in Trisula. It had slightly better bulb weight per clump and yield potential per hectare compared to Vietnam’s. The results showed that there was no interaction between the types of decomposers in compost and the varieties in the growth and yield of shallots.