SOIL, HOW IT IS FORMED

Soil

By Moira Whitehouse PhD

Hey, listen to this. Have I got the dirt on....

The continents have a layer of solid rock, the crust, covering the hot stuff in the mantle.

The continental crust, then, is mostly covered with thick layers of soil.

Here we see a slab of Earth taken out of the crust with the soil on top showing where we live.

Bedrock (crust of the Earth)

USDA

Soil, from the bedrock to the top, is our subject.

There are different layers of soil—similar in characteristics such as composition, texture, and color.

Thankfully, soil covers most of the Earth’s solid crust (bedrock); however, in some places it is thin or nonexistent.

Plant growth, that allows us to live, occurs on the top layer of soil.

bedrock D

layers of soil

http://www.nrcs.usda.gov

Below that layer are several other layers, some that provide minerals and ores for our use.

Why do we care?

Soil is made of.....?

Speaking mainly of top soil and the upper soil horizons

Four things:

• weathered pieces of rock made up of minerals (All rock is a mixture of minerals.)

• organic material (remains of dead plants and animals)

• air (containing oxygen)

• water

The other half is made up of many, many, interconnecting holes between the rock pieces and organic matter.Depending on location, the time of year or rainfall, these holes, called pores, are filled with either air or water.

The weathered rock pieces (minerals) make up about half of the total mass of most soil. Less than 10% is organic matter-- dead plants and animals.

Soil

WaterTiny pieces of weathered rock

Remains of dead plantsand animals

Air with oxygen

In pores, spaces between the pieces of weathered rock

airwater

Those ingredients that make up soil are necessary for the plants and micro-organisms that live there.

• The weathered pieces of rock and the organic material provide many of the nutrients such as iron, nitrogen, potassium that plants need to grow and to carry out their life processes.

• The weathered pieces of rock anchor the plants roots so a plant doesn’t blow or wash away.

• Plant roots and soil microorganisms get the oxygen and water they need to live from the spaces (pores) in the soil.

In this presentation we will focus mainly on the solid parts of soil—weathered rock and the humus—the organic material (dead plants and animals). Soil:

Weathered rock(tiny pieces of rock)

Humus—organic matter (remains of dead plants and animals)

First we will explore the weathered rock part of the soil.

Weathered rock is formed through the process of weathering which breaks rock into smaller and smaller pieces.

Weathering is caused by agents in nature (wind, water, temperature variations) that break rock down into smaller pieces.

In review, ...so what is weathered rock?

Pieces of rock that have been broken down into smaller pieces by the forces of nature—water, wind, ice, acid water, plant roots, etc. are called weathered rock.These pieces of rock may be the size of a boulder or a grain of sand.

Some of these pieces of rock may be small that we can only see them under a microscope.

The smallest pieces of weathered rock is called soil.

The weathered rock in soil probably started out as a huge boulder.

In the process of being broken down, the size of the particles of rock become smaller and smaller—boulders to large rocks, to smaller pieces of rock to pebbles to sand, silt and clay.

Over hundreds, maybe thousands of years, it could have happened something like this.

SoilSoil

Looking again at the solid part of soil, first we will consider the three types of soil that are the result of weathering:

Soil:

Humus(remains of deadplants & animals)

Weathered rock

Sand ClaySiltlargest smallest particles

Later we will discuss humus.

1. Sand—largest particles

2. Silt—medium sized particles

3. Clay—smallest particles

These three types of soil, sand, silt and clay are identified largely based of the size of their weathered rock pieces.

The following guidelines are use to talk about the different sized particles of sand, silt and clay.

Size of Particles of Rock (Diameter)

• coarse fragments such as pebbles > 2 mm• sand < 2 mm to 0.05 mm

• 2 m boulders

• clay < 0.002 m

• silt < 0.05 mm to 0.002 mm

Wikipedia Commons

Still looking at the solid part of soil, we now examine the humus:

Soil:

Humus(remains of deadplants & animals)

Weathered rock

Sand ClaySiltlargest smallest particles

Humus is formed when dead plants and animals decay.

What is humus?

How is it formed?

The organic part of soil—that which was once living.

Special organisms in the soil, called decomposers, cause dead plants and animals to decay or rot changing their bodies into the humus part of soil.

What causes these dead things to change into soil?

When plants and animals die, they become food for these decomposers--bacteria, fungi, arthropods, nematodes and earthworms.

Decomposers recycle dead plants and animals into nutrients plants need.

Bacteria are the smallest living organisms, and the most numerous of the decomposers.

A teaspoon of fertile soil generally contains between 100 million and 1 billion bacteria.

They carry out the majority of decomposing that occurs in the soil.

http://soils.usda.gov

Magnified bacteria found in the soil.

Fungi are not plants; they can't make their own food.

Fungi is the name for simple organisms including mushrooms, molds and yeasts.

Next to bacteria, fungi are the most efficient decomposers.

They absorb their nutrients from the organisms they are decomposing. In the process they release enzymes that decompose dead plants and animals.

http://www.flickr.com Benimoto

http://www.flickr.com Futurilla

http://www.flickr.com scoobygirl

Mushrooms growing on logs

http://www.flickr.com mill56

http://www.flickr.com photogirl7

Mushrooms growing on a forest floor

Mushrooms growing in dead grass

Fungus beginning to decompose leaf veins in grass clippings.

http://soils.usda.gov/

Other important decomposers found in the soil are numerous invertebrates—animals without backbones.

The initials “FBI” can be use to help us remember the three main decomposers types:—fungi---bacteria---invertebrates.

Invertebrate decomposers include worms called nematodes, mites, pillbugs and millipedes.

Nematodes, a group of invertebrate decomposers living in the soil are tiny non-segmented worms typically 1/500 of an inch in diameter and 1/20 of an inch in length.

One square yard of woodland or agricultural soil can contain up to several million nematodes. http://soils.usda.gov/

Nematodes magnified in soil.

http://www.flickr.com zimpdenfish

Pill bug

Other important invertebrate decomposers

Decomposing mites

Photo credit: Joseph O'Brien, USDA Forest Service,

Millipedes

Organisms such pill bugs, millipedes and mites are important to the soil because they stir up and churn the soil, mixing in air which is needed by other organisms in the soil habitat.

They shred organic matter into small pieces, assisting other soil organisms in the decomposition process.

The lowly earthworm is also an important decomposer.

Earthworms eat dead plants and animals, thereby, absorbing the nutrients that they need to survive. Earthworms excrete wastes in the form of casts which are rich in nutrients such as nitrogen and phosphorous that plants need.In addition to breaking down organic materials and adding nutrients to the soil, earthworms also help loosen the soil, thereby, creating space for the oxygen that plant roots and microorganisms need to live.

Decomposition creates soil that containsthe nutrients plants need in a form that they can use to carry out their life processes.

USDA

The cycle of plants absorbing minerals from the soil and and these minerals being returned to the soil through decomposition is repeated over and over in nature.

http://www.flickr.com/ angus clyne

Where there is lots of plants to decay and enrich the soil, such as in deciduous forests and grasslands, the soil is rich in humus and very fertile.

Wikipedia Commons

Where there is little or no vegetation to provide the organic debris, such as at the seashore or in the desert, the soil has little or no humus and is not very fertile.

Desert in Saudi Arabia The Chihuahuan Desert along the Rio Grande

Wikipedia Commons http://www.flickr.com/ Cory Leopold

In review, we learned that soil is made up of four main things. Can you remember them? (two solids, one liquid and one gas)

Soil

WaterTiny pieces of weathered rock

Remains of dead plantsand animals

Air with oxygen

In pores, spaces between the pieces of weathered rock

airwater

Next, what do we call tiny particles of weathered rock?Yes, we call them soil.

What are the three main types of soil that result from weathering of rocks?

Sand, silt and clay

What is the main feature that distinguishes sand, silt and clay?

The size of the particles, sand being the largest and clay the smallest

Humus is formed when dead plants and animals decay.

What is humus?

How is it formed?

The organic part of soil which was once living.

Weathered rock makes up one part of solid soil. What makes up the other part? Yes, it is humus.

When we love and honor it we call it soil.

This stuff is an important natural resource for man.

But when grownups don’t like it, they call it dirt or mud.