SOIL TEXTURE & COMPOSITION 6TH GRADE CREATED BY STEPHANIE

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Soil Texture & Composition

Soil Texture & Composition

6^th grade

Created by:

Stephanie Hines & Robert Vest

Ball State University

Sponsored by:

SOIL TEXTURE & COMPOSITION 6TH GRADE CREATED BY STEPHANIE

Indicators addressed:

Organize information in simple tables and graphs and identify relationships they

reveal. Use tables and graphs as examples of evidence for explanations when writing essays or writing about lab work, fieldwork, etc.

6.3.15 Explain that although weathered rock is the basic component of soil, the

composition and texture of soil and its fertility and resistance to erosion are greatly influenced by plant roots and debris, bacteria, fungi, worms, insects, and other organisms.

6.3.19 Investigate that materials may be composed of parts that too small to be seen

without magnification.

Objectives: Students will be able to

Name the three soil particles
State the relative size of soil particles
Explain that individual grains of silt & clay are not visible to the eye, but sand particles are
Explain that most soil samples are not comprised of just one particle, but are a combination of particles
Identify soils by their texture into general categories
Explain that soils are comprised of not just rock particles, but other components as well, such as organic debris, microorganisms & macroorganisms

Materials Required:

Paper lunch bags – one/student, plus one for each of the soil samples (approximately 50 bags)
3 different sized (one small, medium & large) & textured snack foods, such as sunflower seeds, nerds, or mini chocolate chips or M&Ms (small); raisins, peanuts, chocolate chips (medium); pretzel nuggets, animal crackers, popcorn, grapes (large) – enough for a few handfuls for each student. **If not using food, three different coins will also work
Samples of clay, silt & sand (4 cups of each, divided into a number of samples and placed into the paper bags, which depends upon how many are working together; for example 5 groups of students, each group should have one bag with clay, one with silt, one with sand, at least two mixed soils, for a total of 5 bags). **In the lesson, these samples will be referred to as the “pure” samples; although they may not be perfectly pure, they will simulate it closely.
A number of samples (2-4) of mixed soils. (4 cups of each, divided into a number of samples and placed into the paper bags, which depends upon how many groups there are) **In the lesson, these samples will be referred to as “mixed”. Try to get soils that contain organic debris and vegetation.
(For places to find the different soils, call your local Soil & Water Conservation office or Extension office; in general, sand can be acquired from a stream bed or bought as play sand; and clay can be found in most agriculture fields or wetlands in Indiana)
Small squirt bottles (3-4)
Hand lenses
Attached handouts: Soil Texture & Composition sheet, Texture by Feel guide, Textural Triangle

Lesson Introduction:

The major component in soils is rock particles: sand, silt & clay. These particles form over time from weathering and erosion. The particles are classified by size: sand is the largest (.05-2.0mm) and the individual grains can be seen by the naked eye; silt is smaller (.002-.05mm) and the individual grains cannot be seen the naked eye; nor can individual clay particles, the smallest particle (<.002mm). (Anything over 2.0 mm is considered a pebble.)

The composition of these three particles in an area is determined by five factors: parent material of the area, climate, vegetation, time and topography. Because all of these factors vary, soils, even in the same region, also vary.

Because two of the particle types are too small to be seen without microscopes, soil scientists have come to rely on different ways to determine particle composition: texture, or the ‘feel’ of the soil, and gravimetric weight, determined by how soils settle in a water column. Gravimetric weight must be done in a lab, with at least 24 hours to run the test and will not be addressed in this activity. Texture can be determined in the field. Though texture is somewhat less accurate and more subjective, is it a good way to get a basic understanding of a soil composition.

How a soil feels is determined by the composition of particles – sand, silt, and clay - in the soil. Sand feels grainy or gritty; silt feels floury and forms short ribbons (< 2.5 cm long)when squeezed and pushed between and through the thumb and index finger; and clay feels sticky, forms a ribbon more than 2.5cm long, and remains balled up overnight when wet. Soils are rarely composed of only one or the other of these particles, but contain a combination of the three. A perfectly balanced combination is called a‘ loam.’ Most soils are weighted toward one of the particles, which is reflected in the name, such as sandy loam, clay loam, silty clay, etc. For the purposes of this activity, no one is expected to be able to identify soil types with exactitude. The activity is more to provide a general of idea of the different types of particles and soils and to help identify the major particle in each sample.

The three rock particles create the bulk of soil (about 45%). The other components of soil include organic matter (about 5%): micro and macroorganisms, (such as mites, nematodes, and earthworms), vegetation; other organic matter (such as pebbles, and twigs), water (about 25%), and air (about 25%). The particles in a soil determine the amount and types of the other components. Particles determine the type of vegetation and organisms that can be supported, the amount of water and air it can hold, and its suitability for different purposes.

In this activity, students will utilize the Textural Triangle to classify soil. The triangle was devised by and for soil scientists in order to separate soils into a number of categories based upon texture and composition. The Texture Guide will assist students in determining into which category the soil samples fall. Again, the main purpose of this activity is not precision, but awareness of the composition of the particle sizes and of some of the tools that scientists use when studying soil.

Procedures:

Before students come to class, prepare a bag for each student with three different size snacks. Each bag should contain enough so that students can grab a couple of different combinations of handfuls of the mixture. Fold over the tops so that students don’t open them right away.
Pass out the bags to students, requesting them NOT to look in the bags.
1. Engage the students in the lesson by asking them to slowly open the bags and put one of their hands inside. What do they feel? Can they identify what is inside? Ask them to identify what they feel.
2. Exploring the objects inside the bag. Ask student to carefully pull out a handful and put it on their desks. Were they right in their blind identification?
3. Now ask students to sort what they have on their desks. How did they sort the objects? Why did they do it this way?
4. Leaving that pile, ask them to take out another handful. Again, sort this pile. Did they get same number of sorted categories? Did they draw out the same number of individual pieces? (Students should get different numbers of individual pieces) What if they were to draw another sample? Would they get the same number of individual pieces? Most likely not. So, each sample is a little different.
5. Have students combine and sort all of the objects into three different categories. By what do they classify? By color, shape, size, texture, etc.?
6. Once they state how they categorized everything, ask them to all categorize by size.
Explain that there are a variety of ways of categorizing different things. Ask students what are some other things they know that are categorized & how are they categorized? (Music, grade level, coins, foods, gender, place in the family, natural or human-made, animals, plants, etc.) Things can be categorized or sorted by an infinite number of ways. Even though students may not know it or think of it, soil is also sorted and categorized.
Extend the concept into soils.
1. Students will get the feel of the different particles by first feeling “pure” samples of the different types: clay, silt & sand. Working in groups, each group gets one sample of each of the three pure soils. Hand out ‘Soil Texture & Composition’ sheets. Ask students to write on their sheets how these soils feel (good adjectives). They may need to squirt a little bit of water on each of the samples. They should roll it around, feel the texture. What does each sample do in their hands? Answer the questions on the handout for each sample. The clay sample should feel very sticky and make a ribbon more than an inch long when rubbed between two fingers. The silt sample will feel soft and floury. It will form a ribbon between their fingers, but much shorter. The sand will feel very grainy and won’t hold together when rubbed between fingers.
2. Ask students to pull out individual particles. Can they do this? With sand they can, but not with clay or silt. These are too small to be seen without a microscope. Ask them to remember what was in their snack bags. How did the class eventually sort out everything? By size. Soil is also sorted this way. Which particle is the biggest? Sand. Which of the two remaining is smallest? Clay. But this we cannot determine today. So the order of descending size is: sand, silt, & clay.
3. These samples they are investigating now have just one type of soil. Ask students to recall when they pulled a sample out of their bags. Did they get just one type of snack? No, they had a mixture of the three types. And did they get the same mixture the second time? No, they got a different sample. Soil is similar. It is rare to get a soil sample that has just one particle in it. Typically soils are a mixture of the different types of particles. Ask students how they knew what was in their bags before they could see what the objects were. They could feel them. Again, soil is similar. Soil scientists identify soil by the feel, or the texture of soil.
4. Now that students are familiar with how the particles feel by themselves, they will investigate combinations of the particles. Pass out samples of the mixed soils to students. They should follow the Texture Guide in order to classify on the Triangle which soil they have. Are there other things they can identify in the soil besides the particles, other organic matter? While the rock particles are the main component of soil, it also contains a variety of other things. Ask students what are some things they see in and around soil. Vegetation, insects, earthworms, & twigs. There are also things that students can’t see with their eyes, just like they can’t see the individual particles of silt & clay. These are called microorganisms. Give students hand lenses to search for microorganisms.

All of these things together are called soil and add even more texture to it!

e. Have students complete another trial of classifying by Texture & Feel.

Wrap-up. Ask students to tell you how they classified their soil samples. Did

the different groups with the same soil get the same classification? (If not, why not? This is a subjective part of classification that soil scientists work a long time to get good at! Right now students are just learning and it requires a lot of practice to get accurate.) Ask students how they knew in which category to put their sample. (By texture!) What else did students find in their soils? Are these things a part of the soil?