Plants do not need soil itself as an ingredient in photosynthesis. They need light, water, carbon dioxide and functioning cells, plus mineral nutrients for healthy growth. Many plants can grow hydroponically without traditional soil.
What soil normally provides
Soil anchors roots and stores water and mineral nutrients. It also supports organisms that influence plant health. These benefits are important but distinct from the carbon-fixing chemistry that happens mainly in green tissues.
How hydroponic growing works
In hydroponics, roots receive water and dissolved nutrients through a managed system. Leaves still use light and carbon dioxide to make sugars. The water and nutrient supply must be controlled carefully because the soil buffer is absent.
Where the plant’s bulk comes from
Much of the dry mass built by photosynthetic plants comes from carbon originating in carbon dioxide in the air. Water contributes hydrogen and oxygen; mineral elements are essential but usually a smaller fraction of dry mass.
What this means for houseplants
Good potting mix is important for root health and drainage, even though photosynthesis occurs mainly aboveground. Soggy roots can weaken the entire plant and indirectly reduce its ability to photosynthesize.
Frequently asked question
Can a plant survive in water forever?
Some species can grow in properly managed water or hydroponic systems, but plain water alone may not provide the required nutrients and oxygen for roots.
Sources and further reading
See this university, government or veterinary source for more information.
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