Plants need soil?
No, but they do need the nutrients commonly found in the soil, and nitrogen fixing soil bacteria are extremely beneficial, though not technically necesary, to plants.
Marine plants do not need soil. Lichens can grow on rocks where there is no soil. However, they are slow-growing. Most land-based plants do need soil, however. Plants which normally grow in soil can be successfully grown hydroponically (in water only). This is achieved by providing nutrients (fertilizers) and air to the water which meet the nutritional needs of plants being grown.