If you were to explore the ground in almost any terrestrial region on Earth, chances are you would find soil. This layer of Earth's surface is essential to life. It supplies, both directly and indirectly, what we eat and wear, as well as the oxygen we breathe. Even so, it's easy to take soil for granted. You might assume that the soil near your home or elsewhere has been there forever. However, the land surface has changed dramatically over geologic time. For instance, depending on its location, soil formation may have been interrupted by volcanic activity, glacial movement, or climate change. In some parts of the world, entirely new land masses are forming as you read this. So what is soil, how does it form, and how does it support life?
Soil formation begins with the weathering of rock through chemical and physical processes. When rock, such as that laid down by active volcanoes on the big island of Hawaiʻi, is newly formed, chemical and physical weathering begin almost the moment the lava cools. Rain, wind, and surf break large pieces of rock into smaller pieces, which are then broken into even smaller pieces. Soon, tiny organisms such as bacteria and fungi occupy small crevices in the rocks. Through respiration, photosynthesis, and chemosynthesis (the production of carbohydrates using chemical energy), these organisms produce gases and nutrients that support further soil development. As organisms die, they leave their remains behind. Each death contributes additional organic compounds to the soil. The nutrients, such as nitrogen and phosphorous, are stored in the soil, where other organisms use them. Eventually these organisms will contribute their own biomass to the accumulation of soil.
Environmental conditions affect the development of soil properties and characteristics. Climatic variables, especially temperature and precipitation, strongly influence the rate of the weathering of rocks and soil. The kind of soil that forms in a particular location also depends on the physical and chemical properties of the parent rock. Vegetation, as well as soil microorganisms, affects both the rate of soil formation and the composition of the soil. The topography, or the slope, shape, and direction of the land surface, affects how water travels through a landscape and determines the soil's resistance to erosion by water. Lastly, soil properties and characteristics are influenced by the length of time these processes have been in operation. So, when you see raw, unweathered, solidified lava, you are actually seeing the initiation of the soil-formation process.
To learn more about how life might have originated on Earth, check out Deep-Sea Vents and Life's Origins, Life Before Oxygen, and Caves: Extreme Conditions for Life.
To learn about some of the elements scientists think are essential to life, check out Ingredients for Life: Water, Life's Little Essential: Liquid Water, and Ingredients for Life: Carbon.
Explore environmental factors involved in evolution of plant and animal life on Hawaiian volcanic islands in this NOVA classroom activity.