The bulbs of the onion plant grow underground. However, the leaves grow above ground. The conditions of the soil where the onions are grown is particularly important, because these conditions influence the flavor and size of the onions.
Onions, which are among the oldest cultivated plants, are best suited for growth in mildly acidic soil that contains a significant amount of organic matter. The bulbs of the onion plant require the right kind of soil to grow properly; rocky soil or dense clay soil can keep the bulbs from forming properly. Commercial farmers typically grow onions from onion seeds, but it is also possible to grow onions from small onion bulbs.