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

· Broken up pieces of rock (Regolith)

· Humus

· Microorganisms and some larger organisms

· Air

· Water

Lets take a look at each one.

Broken Up Pieces of Rock

The primary ingredient in most soils is broken up pieces of rock. Broken up rock is also known as regolith. Rock is continuously broken down by a process known as weathering. 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 later!

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 or percentage 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 soils has a high 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. The water ends up going through the soil because it is permeable. Eventually though, you reach bedrock (solid rock) which will not let any water through. The water sits on this layer and flows very slowly underground. 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.

Weathering

We learned that soil was made up of several things including water, air, microorganisms, and humus. But most soil, including the soil right outside our science room window, is made mostly of broken up bits of rock. Sand, silt, and clay are actually minerals that have been taken away from much bigger rocks. In fact, they may once have been part of Mt. Everest sized mountains or towering volcanoes that used to be here millions of years ago. But where could such enormous mountains go?

The earth’s surface is constantly changing. We usually don’t notice the change because it typically happens very slowly. Over millions of years, even the tallest mountains can disappear. But how? The answer is weathering and erosion. A lot of people get these two terms confused, but they are different processes. Weathering is the break down of rock. Weathering is what breaks little bits of rock off of mountains. Most weathered particles are carried away by water. Erosion carries the smaller bits away, so erosion makes the mountain disappear. We will look at erosion later.

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

granite into clay

plant acid

acid rain

Finally, there are a few factors that affect how quickly something weathers. The first is climate. Hot, wet climates have more weathering than other climates, especially dry climates. The second factor is surface area, the more surface area a rock has, the quicker it weathers. A broken rock weathers quicker than a solid one because it has more surfaces to weather. Composition is a factor. Tombstones made of limestone weather much quicker than those made of granite. Granite is more stable than limestone. Time is also a factor. The more time there is to weather the rock, the more there is.

Definitions to know.

Regolith

Weathering

Porosity

Permeability

Humus

Horizon

Leaching

Don’t forget to look at your dirt on dirt homework sheets and the homework sheets you did on Tuesday. Good luck on the test.