What do you think of when you hear the word dirt? Dirt is usually thought of as something bad. Whoever does laundry at your house probably wishes that there was no such thing as dirt, but in fact we are all very lucky that dirt exists. Almost all living things depend on dirt, which is really known as soil. Soil is that pesky stuff that you have to shovel through if you are trying to make a hole in your yard. But how did it get into your yard in the first place? To find this out, you must first know what soil is made out of.

Soil is actually a mixture of things. Some of the things in soil are obvious, like bits of rock, but did you know that soil is part alive? Soil is made up primarily of the following items.

Soil Composition

· Regolith-Broken up pieces of rock (minerals)

· Humus

· Microorganisms and some larger organisms

· Air

· Water

Let’s take a look at each one.

Broken Up Pieces of Rock

The primary ingredient in most soils is broken up pieces of rock. Rock is continuously broken down by a process known as weathering. There is more on weathering at the end of this study guide, but it is basically a process where the weather and/or climate breaks rock down. A lot of rain or heavy wind can take big rocks and turn them into smaller rocks. If you break rock down far enough, it begins to look just like soil. Soil size is called soil texture. You can usually tell the type of soil by the way it feels when you feel it between your fingers. There are a few basic sizes of soil that you should know about.

· Gravel - the large sized particles in soil. Usually pea sized and sometimes called pebbles.

· Sand -typically made of quartz. Gritty feeling.

· Silt - very small size. Leaves fingers stained when you rub it between your fingers.

· Clay - the smallest size. Feels sticky. Kitty litter is made of it.

Rocks are made up of different minerals. You will be learning about them next week!

Humus

Soil is not made of broken up rock alone. If you take a look outside, you will see that the ground is covered with leaves. What happens to those leaves? Decomposers begin to break down the organic matter in the leaves, twigs, or dead bugs that live in the area. What is left is a rich, very dark, nutrient rich material called humus. Humus is what all those gardeners are after when they throw all their scraps in a compost pile. Many plants rely on a good amount of humus in the soil in which they grow. Soil that contains a lot of humus is called fertile soil, because it can produce a lot of plants. Since plants use some of the nutrients in the humus, new humus has to be created to keep the soil fertile. When farmers need to replenish the humus in their soil, they fertilize the soil by adding decomposing plant material. What is that material they use…. that’s right-manure.

Microorganism and Some Larger Organisms

We said that decomposers break down the organic matter into humus. We talked about decomposers in the past, and most of you think of mushrooms. Mushrooms are certainly a decomposer, but they only make up a small percentage. Microscopic fungi is abundant in most soils. They break down a lot more organic material than do mushrooms. The king of all decomposers happens to be the dominant life form on Earth—Bacteria! If it wasn’t for bacteria we would be up to our armpits in dead things. There are lots of bigger organisms in the soil as well. Moles, earthworms, and insects dig through the soil and breaks it down even further. When they do this they add the next constituent to soil which is……….

Air

That’s right. When we are talking about the make-up of soil, we cannot forget to talk about air. Without air, the larger organisms could not survive in the soil. Organisms can’t breathe dirt! The more organisms in the soil, the more air there is. The more air there is, the more the organisms can break down the organic matter into humus. Did you ever wonder why a farmer has to plow their fields each year. They are mixing air into the soil to help humus production. Plant roots need air as well, so plants grow better in soil with lots of air in it. The air spaces give the roots room to grow as well. The air is important for another reason too……………..

Water

What does air have to do with water? The answer is porosity. Porosity is the amount of pore space (air space) in soil. The porosity depends heavily on the texture or size of the sediment grains in the soil. The more air spaces, the higher the porosity. High porosity often means that a lot of water can fit into the soil. Sometimes, however, the air spaces don’t connect very well and water can’t move very well through the soil. Soil permeability is the ability of a soil to let water travel through it. If you have a gravely soil, there are often a lot of empty spaces, so it has a high porosity and high permeability. The permeability is so high that water drains right through it. Plants won’t grow well in gravely soils because the water drains away before the plant has a chance to have a drink. Soils that are mostly sand drain quickly as well. Clayey and silty soils have a medium porosity but a low permeability. They allow water in, but the water won’t travel through them. This is why kitty litter is made out of clay. It holds water because it is porous, but it is not permeable so it won’t let it go. The best type of soil to have is a mixture. Can you guess why?

By the way. All the water that does drain through the soil must wind up somewhere. If we went outside and dug a ten foot hole, I bet we would find water. Water that builds up under ground is known as groundwater. You shouldn’t think of groundwater as an underground river because it only flows at about one foot a year or less. It’s more like an underground reservoir. Groundwater supplies rivers and lakes with their water.

Climate and Soil

We went outside a week or so ago and took soil samples with an auger. We saw a few pebbles and sand, and we noticed that the soil stained our fingers a lot. We also discussed how heavy rains sometimes cause huge puddles to form on the soccer fields. This tells us that the soil outside our building is low in permeability because it has lots of clay and silt in it. Is the soil like that everywhere in the world? Of course the answer is no. Florida’s soil is much different than ours. The soil there, as one would expect, has lots of sand in it. It drains much more quickly there, so the soccer fields rarely flood. Why do these two places have much different soil? The answer is climate! Climate is considered to be the most important control of soil formation. Since biomes are closely related to climate, we can look at soils in different biomes. Tropical rainforests have lots of rain and are very hot. The rain breaks down bedrock through the process of weathering, creating lots of smaller, broken up rock. The hot climate also is good for decomposers, who decompose the organic matter quickly. Deserts are very dry. Only wind breaks down rock there. There is so much wind that small particles (clay and silt) blow away. Gravel gets sand blasted by the wind. The only thing left is sand. Tundra is very cold. Ice breaks apart big chunks or rock making a gravely soil. There is often a lot of humus here though, because decomposer work very slowly.

Time is the second most important control of soil composition. The longer the soil has been exposed to weathering, the more it will be broken down. Mature soil is soil that is old has been weathered a lot. Immature soil has had little time to be weathered.

Weathering is a process that breaks down rocks. The word weathering is a little misleading. While it is true that weather is sometimes responsible for breaking down rocks, things like people, animals, and gravity (which have nothing to do with weather) are also involved with weathering. There are two types of weathering-mechanical and chemical.

· Mechanical weathering is the breaking down of rocks into smaller pieces without changing the chemical make-up of the rock.

Simply put, the rock goes from being big, to being small. It’s just a smaller version of itself. A good way to think about it is to think of a log. If you chop it up, you may be left with a toothpick. The toothpick is still wood. It can burn, float, give you splinters. It’s basically a miniature log. Nothing has changed but the size.

· Chemical weathering is the break down of rocks into smaller pieces where the rock has changed chemically.

Simply put, the rock gets smaller, but it is no longer the same type of rock than before. It’s something new. Let’s stick with the log example. If you take a log and throw it into a fire, you wind up with ash. Ash is not wood. You can’t get splinters from it. It can’t burn. You can’t make a canoe out of it. It is wood that has changed into something new.

Here are some examples of weathering that take place in nature.

Mechanical rocks into pebbles

frost wedging

root pry

dynamite

Chemical iron into rust (oxidation)

granite into clay

plant acid

acid rain