To grow onions in poor soil, you can improve the soil quality by adding organic matter such as compost or well-rotted manure to provide essential nutrients for the onions. Additionally, you can choose onion varieties that are more tolerant to poor soil conditions. Make sure to space the onion plants accordingly to allow for proper growth and development. Water the onions regularly, but be cautious not to overwater as onions do not like waterlogged soil. With proper care and maintenance, you can successfully grow onions in poor soil conditions.