Dr. Universe

Dear Dr. Universe: Where does dirt come from? - Brian, student from Pullman, Wash.

In just a word, the story of soil goes something like this: “CLORPT!” It’s fun to say, and it helps explain how tough rock turns into the soft soil farmers need to grow food and feed the world.

Jim Harsh is a scientist at Washington State University and an expert on soil. He said a soil scientist, Hans Jenny, came up with five actors in the soil story: CLimate, Organisms, Relief, Parent material, and Time.

The texture, smell, and color of soil can often depend on the type of climate where the soil formed. Up in the cold, rainy mountains, soil will look and smell different than it does in the dry, hot desert. Red soils, black soils, white soils, yellow soils and even spotted soils can all be found around the planet.

Soil is bursting with life. When leaves, plants, grasses, small bugs, and animals fall into cracks of rocks, these organisms start breaking down and soil starts to form. It’s here where tiny microbes, such as bacteria and fungi, are busy working in the soil.

Usually when someone says the word “bacteria” it sounds like germs, but most microbes in soil are actually helpful.

Microbes have several jobs. First, they hang out on the surface of rock where they produce acid to break down rock. They eat nutrients in the soil and return them back to the earth. Second, they create sticky glue and thread-like strings to hold together the particles of sand, silt, and clay that make up soil. These are called aggregates. A good soil will have a lot of aggregates and create a solid place for plants to stick their roots. Microbes even eat earthworms’ waste. It’s a big, beautiful cycle.

The shape of land, or the relief, also impacts how soil forms. Soils on the sunny side of a hill are much warmer than those on the shady side of a hill. If the hillside is very steep, the soil can wash away as fast as it is formed, Harsh said.

Soil has to start somewhere and that’s why it needs parent material. Parent material can come from volcanic ash, sediments in rivers, or the rocks in your backyard. Even the Grand Canyon is parent material. Over time this rock breaks down through weathering and with the help of microbes.

Time is the other big factor. Some soils, especially the red ones, take hundreds of thousands of years to develop. Scientists have found fossilized soils and samples from African deserts that are more than 55 million years old. Now you really know what they mean when someone says “old as dirt.”

It isn’t officially part of CLORPT, but the last actor in the soil story is you. Farmers, scientists, and everyone who likes to eat dinner are stewards of soil. Just two feet of soil pretty much determines if the world has food. How we take care of the earth will determine what kinds of soils we have for the future.

Dr. Universe

WSU, Pullman