Although it is commonly believed that plants require soil to grow, most plants do not. In fact, it is not the soil itself that causes a plant to grow, but rather the nutrients found in the soil.
For a plant to grow, a few basic things are required. Plants need nutrients, support, protection against the elements, moisture and oxygen for the roots. These things are usually found in soil, but plants can grow properly in other ways as well. In fact, most plants grow well in hydroponics systems, growing systems that use only nutrient-rich water, when the necessary support and nutrients are provided.