Automatic watering, school science project

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Fill a bottle with water and place it upside down and half-buried in soil in a flower box. An air bubble rises up in the bottle from time to time, showing that the plants are using the water. The water reservoir is enough for several days, depending on the number of plants and the weather. Water only flows from the bottle until the soil round it is soaked. It starts to flow again only when the plants have drawn so much water from the soil that it becomes dry, and air can enter the bottle. One notices that plants can take water more easily from loose soil than from hard.