Education Standards
Ribbon Technique
Soil Ribbon Method - Brandy
Soil Skeleton Notes
Texture worksheet
Video - Soil/Dirt Difference
What is Soil - pptx
Soil Types and Texture
Overview
This Lesson will examine the definition of Soil using written and video references. The lesson will also demonstrate how to identify different soil types by examining the texture of different soil samples. The lab portion will allow students to collect, evaluate and determine soil texture and type using the hands-on ribbon method. The lesson is meant to be conducted over a period of two to three days depending on class period length.
What is Soil?
Class will begin with an inquisitive discussion about what students think soil is and if there is a difference between dirt and soil.
Watch youtube video over the difference between dirt and soil.
The PowerPoint will be viewable to the class and we will discuss and fill out skeleton notes as we go. Students may also pick 1 fact from each slide to write on their "The more you Know" index cards. Information on these cards is used for entry/exit tickets each day.
This lesson will serve as a predecessor for examining soil types and texture.
MAIN IDEA: What is soil, how did it form and what does it do that is important to human life on earth?
The term "soil" should not be hard to define. It is what you find in fields and gardens where crops are grown. However, the most accurate definition of soil should cover both what it is and what it does.
- A soil scientist defines soil based on what it is, as "organized natural bodies similar to different types of ecosystems."
- A plant scientist, or agronomist, defines soil in terms of what it does, as "the unconsolidated upper part of the earth's crust capable of supporting terrestrial plant life."
WHAT SOIL INCLUDES
Soil is a very complex mixture of mineral matter, air, water, decaying organic matter and living organisms. All of these are necessary for soil to support the growth of plants and trees.
- Minerals found in soil are inorganic compounds in the earth's crust. Most rocks from which soils were formed are a mixture of minerals.
- Organic matter is material of plant or animal origin that decays in the soil. When dead plants and animals decompose, they become humus. Humus is a gelatin-like substance that binds soil particles together.
- The living organisms found in soil include decomposer microorganisms that break down organic matter and convert nutrients into forms plants can use.
- Air and water are extremely important elements of soil, and should make up about half of the soil mixture on a volume basis.
For ideal plant growth, a soil will be made up of about 45% minerals, 25% air, 25% water and five% organic matter.
HOW SOIL IS FORMED
The soils found on earth were formed over millions of years. In practical terms, lost topsoil is impossible to replace. Soil formation takes place by a process known as weathering. This process includes the decomposition of minerals and organic matter by rain, heat, cold, freezing, thawing, glaciers and landslides.
Many different types of soil are formed by this process. The type of soil and rate at which it is formed is influenced by:
- The type of original or parent material, such as limestone, glacial deposits or from alluvial deposits.
- The climate, including average temperatures and rainfall.
- The type of organisms that live on and in the land, including plants, humans and animals.
- The topography of the land, whether it is steep or flat.
- The length of time that parent material has been weathered.
Given time, the weathering process is a powerful force. It can break up huge rocks and grind them down into a rich soil mixture. The process starts when water collects in tiny cracks found on a rock.
When water freezes, it expands, causing the crack to get larger. When thawing occurs, more water enters the crack, it freezes, and the crack is again enlarged. Freezing and thawing over time break rocks apart. More cracks are formed and more freezing and thawing continues to break rocks into smaller parts.
As weathering takes place, plants begin to play a role in the processes. Small cracks in rocks begin to collect soil and seeds. Once the seeds start to grow, the roots of the plants further break down the rocks.
In some cases, the seed itself aids in the breaking of rocks. This takes place when a seed falls into the crack in a rock. Once there is enough water for it to start to grow, it pushes against the rocks and breaks them. As soil is blown, some will land on the growing seed, allowing it to continue to grow.
Frequently you may see a tree growing out of the cracks in rocks. As the tree grows, the roots continue to break apart the rocks, making a place for other plants to grow.
THE FUNCTIONS AND PURPOSE OF SOIL
The main purpose of soil is to provide a way for plants to grow. To do this, soil must provide four functions:
- Soil serves as an anchor for plant roots.
- Soil supplies water to plants.
- Soil provides air for plant roots to breathe.
- Soil furnishes plants with nutrients they need to grow.
Soil is also used for a number of other purposes in addition to supporting plants. As a geologic substance, soil serves as a roadbed, an aggregate for concrete and asphalt, a foundation for structures and bodies of water, and is used in landfills.
Soil is vital to human life. All of the crops we grow and the livestock we raise depend on soil and soil fertility. All terrestrial life, including wildlife, prairies and forests, depends on soil and the products of soil.
In addition, soil acts as a filtration system for earth's hydrologic cycle. This makes it possible for fish and other life to exist in lakes and streams.
BOTTOM LINE: Soil is a key factor in what makes life on earth possible.
Exploring Soil Texture
We will use the PowerPoint to discuss and use as a reference to fill out the worksheet, however this section will focus mainly on the LAB.
We will start the lab by collecting soil samples from 3 different areas using a soil probe. We will use 1 sample that has been collected from eastern Nebraska.
Students will have access to the video on the "Ribbon Method" on location. The lab will be conducted on the sample site. Samples will also be taken back to the classroom and the students will conduct the ribbon method to determine the makeup of the different soil types and determine what type of soil it is. Our focus will be primarily on the soil's structure. It may take a while to get the hang of ribboning the soil so we will spend most of the class period practicing in groups of 2.
Supplies needed for the lab: 1. Soil probe or probes for each group 2. Straw, squirt bottles of water for moistening the soil. Plastic baggies and sharpie markers for containing and labeling the soil samples for future use.
MAIN IDEA: What is meant by soil texture and soil structure, and why are they important?
Texture of soil is determined by the size of soil particles. Many times you can feel those differences by rubbing soil between your thumb and fingers. Sand has larger particles and feels coarse. Clay has fine particles and feeds smooth.
In most cases, a soil is made up of a combination of larger and smaller particles. The texture is then based upon the proportion of various particle sizes, such as sand, silt and clay, which are included in the mix.
Soil particle sizes are usually grouped into four size classes.
- Gravel particles are 2.0 mm or larger
- Sand particles are 2.0 mm to .05 mm
- Silt particles are .05 mm to .002 mm
- Clay particles are .002 mm and smaller
Notice that the largest sand particles are 1,000 times as large as the largest clay particles. It would require 40 large silt particles to equal one large sand particle.
Classes of soil based on texture include:
- Sandy soils contain 70% or more sand.
- Silt soils contain 80% or more silt.
- Clay soils contain 40% or more clay.
- Loam soils contain an intermediate mixture of sand, silt and clay.
WHY SOIL TEXTURE IS IMPORTANT
Soil texture determines the soil's ability to hold nutrients, store water and provide for plant root growth and development. All of these are related to particle size. That is because particle size determines the total amount of particle surface area in a given volume of soil.
Imagine filling a large cup with golf balls. It would not take many. Fill another cup with marbles and a third with BB shot. If you then filled each one with water, which do you imagine would have the most surface area in contact with the water? The BB shot will easily win. The same is true with soil particles.
The surface area of the particles in a soil is important in determining the water-holding capacity of the soil. Water in soil is held as a film on the surface of individual particles. Thus, the more surface, the more water and plant nutrients the soil is able to hold. The difference can be dramatic.
A handful of sand may have the surface area of a ping-pong table, while the surface area of the same size handful of clay would be closer to a football field. At the same time, water can soak into sand very rapidly, while it soaks into clay very slowly. This is referred to as the "permeability" of the soil. Permeability is also important. It is the ease with which gases, liquids or plant roots can penetrate or pass through the soil.
- Clay soils tend to have a high ability to hold water and nutrients; however, they are very low in permeability.
- Sandy soils tend to be very low in ability to hold nutrients; however, they are very high in permeability.
- Silty soils are moderate in both ability to hold nutrients and permeability.
WHY SOIL STRUCTURE IS IMPORTANT
Soil structure is defined as the arrangement of soil particles into aggregates, also called peds. Soil gets structure when soil particles are cemented together by some natural cementing agent such as clay, organic matter or iron and aluminum oxides.
Soil aggregates that occur naturally in soil are called peds, while clumps of soil caused by tillage are called clods. Some peds are quite large. They range in size from as small as a large grain of sand to several inches in size.
Space between clay particles is tiny, because the particles are so small. Because of this, clay is sometimes called a "tight" soil which lacks permeability. Drainage is poor. However, spaces between the peds may be large, which greatly improves a tight soil such as clay for plant growth. Good structure improves air and water movement and makes it easier for roots to grow. The spaces between the peds also improves the water-holding capacity.
COLOR AND SMELL
You can learn a lot about a soil by the way it looks and how it smells.
Black to brown soil:
Black to brown soils get their color from organic matter or from dark parent materials. You can learn more about their history from their smell.
- Organic soils are often produced by waterlogged conditions, such as a lowland or marsh. These soils have a sour, oily smell.
- Organic matter can also be high in well-aerated soil. These soils have the earthy smell of good soil.
- Soil from dark parent material often has a faint chalky odor.
White to light gray soil:
White to light gray color may indicate a soil that has been leached, removing chemicals and organic matter. Often these are sandy soils. White color also may be due to accumulations of lime, gypsum or other salts.
Yellow to red soil:
Yellow to red soils, often seen in warmer climates, get their color from iron oxide. This is the same compound found on rusty metal. The red color indicates good drainage because air with oxygen is required to form the oxide. Yellow is from an iron oxide that includes some water, producing the mineral limonite. Yellow means the soil is slightly less well-drained than red soil.
Bluish-gray soil:
Bluish-gray color is produced by unoxidized iron and indicates a lack of oxygen in the soil. This often is the result of waterlogging caused by poor drainage. This condition is sometimes called "greying" of soil.
Mottled soil:
Mottled soils show patches of different colors, often including spots of rust, yellow and gray. This mottled condition suggests a soil that is waterlogged for part, but not all, of the year.
SELECTING THE BEST SOILS
You can observe the condition of a soil in a number of ways. Color and smell are obvious conditions you can observe by going out to a plowed or cultivated field. Texture and structure are a little more difficult to determine.
Measuring soil texture:
An accurate measurement of soil texture requires the use of a graduated cylinder. Dry soil is filtered down through several screens which sort the soil particles by size, taking out the sand first, followed by silt, and allowing the finest clay particles to settle to the bottom.
You also can make an estimate of soil texture by kneading it in your hand and feeling it between your fingers. This is called the "ribbon test" or "feel method" of determining soil structure.
Observing soil structure:
Soil structure is more difficult to determine. It is best observed through a microscope. You also may be able to see the results of poor structure or good structure in a field. Rain soaks rapidly into soil with good structure and poorly into tight soil. Poor crop growth on end rows where there has been more tractor traffic is an indication of compaction or a breakdown of soil structure.
Ideal agricultural soil:
The ideal soil for agricultural purposes contains 10-20% clay, 40% sand and 40% silt. This is called a loam soil. The color should be dark brown or black indicating a fair amount of organic matter produced under conditions of good aeration. It also should have a good open, granular, or crumb structure that provides aeration and water-holding capacity.