Plants Grow from Air & Water

A plant builds almost its whole body out of thin air and water, not out of soil. Plants are special because they can take in carbon dioxide gas from the air through their leaves. At the same time, their roots pull water up from the ground. Inside the leaves, the plant uses energy from sunlight to join the carbon dioxide and water together to make sugar. That sugar is the plant's food, and the plant uses it to grow new leaves, stems, and roots. This process is called photosynthesis.

So what does the soil do? Soil holds the plant up and gives it water and small amounts of minerals, like nitrogen and potassium. Those minerals matter, but they are more like vitamins than meals. They make up only a tiny bit of the plant's mass. A tree that weighs as much as a car did not eat a car's worth of dirt. Most of its wood and leaves came from invisible gas in the air and from water.

Roots do three big jobs for a plant. First, they anchor the plant, holding it firmly in the ground so wind and rain cannot knock it over. Second, they absorb water and minerals from the soil and send them up to the stems and leaves. Third, many roots store extra food that the plant made during photosynthesis, saving it for later.

Notice what roots do not do. They do not eat soil. The water and minerals roots collect are important, but the water is what really adds up. A plant takes in huge amounts of water over its life, and that water becomes part of the food the plant builds. The minerals from soil are used in much smaller amounts, like a pinch of salt in a big pot of soup.

The tip of each root has special zones. One zone protects the tip as it pushes through the dirt, one zone is where new cells are made so the root can grow longer, and one zone is where tiny root hairs reach out to soak up water. Every root hair is like a little straw sipping for the whole plant.

Some plants do not need soil at all. Air plants, also called Tillandsia, grow hanging in the air on trees, rocks, or even wires. Instead of roots in dirt, they pull most of what they need straight from the air and from water that lands on their leaves.

People who keep air plants at home care for them by giving them a bath. About once a week, you fill a clean bowl or sink with room-temperature water and let the plants soak. After their bath, you shake off the extra water and set them somewhere with bright light and moving air so they dry out. If they stay soggy too long, they can rot, so drying matters as much as soaking.

Air plants are real-life proof of a big science idea. They grow bigger and heavier over time, and none of that new mass comes out of soil, because they never touch any. Their bodies are built from water and from things in the air around them.

Hydroponics is a way of growing plants in water instead of soil. The word even means working water. In a hydroponic garden, the plant's roots sit in water that has a small amount of mineral nutrients mixed in, and the rest of the plant grows up into the light and air.

Gardeners like hydroponics because plants often grow faster this way and give bigger harvests. A simple home system can be built from a tub of water, a pump to keep the water moving and full of oxygen, and baskets that hold each plant in place so its roots dangle down into the water.

Think about what this proves. A head of lettuce grown this way is built entirely without any soil at all. Its mass came from water, from the tiny bit of minerals dissolved in that water, and from carbon dioxide in the air. If soil were plant food, hydroponic plants would starve. Instead, they thrive.