Soil is a mixture of minerals, organic matter, water and air. The minerals come from bits of broken-down rock. Organic matter is material that is alive or used to be alive. It includes leaves, twigs, bugs and many microscopic, or tiny, organisms like bacteria and fungi. In fact, there are more microorganisms in a handful of soil than there are people on the planet. Soil is needed for life to exist on Earth. It contains many nutrients that help plants grow. Then the plants provide nutrients that help animals grow. Which of these is not a natural part of soil?