Organic, All-Natural & Non-GMO Sourced Onion Fertilizer (3-4-3)! The Urban Farmer Onion Fertilizer will increase onion size, health and taste. This 3-4-3 (NPK) fertilizer ratio is specially formulated to get the most out of your onions. This fertilizer gives the onions exactly what it needs at the right time. Our fertilizers also contain several other beneficial vitamins, minerals .