Lesson

					Colorado Agriscience Curriculum

Section:              Plant & Soil Science

Unit:                 Soils

Lesson Number:        PS U6 L4

Lesson Title:         Soil Structure & Texture

Colorado Agricultural Education Standards:
      Ag. II 10.9: The student will demonstrate an understanding of soil fertility and its effect
                   on crop production.

Colorado Science Standards:
      4.1: Students know and understand the composition of Earth, its history, and the natural
      processes that shape it.

Student Learning Objectives (Enablers)
      As a result of this lesson, the student will …

        1. Identify soil texture and structure.
        2. Demonstrate soil texturing through the ribboning method.
        3. Identify the various types of soil structure.

Time: Instruction time for this lesson: 50 minutes.

Resources
      AgriScience Lesson Plan Library
      Colorado Land Classification Manual

Tools, Equipment, and Supplies
        Overhead projector or computer with projector
        PowerPoint: PS U6 L4 Instructor PowerPoint
        1 copy per student of PS U6 L4 Student Job Sheet
        1 copy per student of Soil Descriptions
        1 copy per student of Soil Texture Flow Chart
        1 copy per student of Student Evaluation
        Student Notebooks
        Soil for texturing
        Area for texturing
        Paper towels
        Water bottle
        Writing tools

                                                                                                    1
Key Terms: The following terms are presented in this lesson and appear in bold italics:
       Loam                                               Consistence
       Aggregation                                        Clods
       Humus

Interest Approach

Display slide #2 of PS U6 L4 PowerPoint. This slide should be unlabeled to begin the class. Ask
the students to look at the slide and think about the various sizes of soil textures. After giving
them a few seconds to think it over, ask someone to tell you what all three kinds of texture are.
Continue to work with the class to get sand, silt and clay. List those off to the side. Then ask them
to use those three texture names to label the various size circles on the overhead. Largest is
sand, then silt, then clay. Discuss the size differences and how those differences affect overall
structure and texture in the soil.

Who can tell me the three types of soil textures? Good ideas. That is correct. The three sizes are
sand, silt and clay. Now, let’s think about how these three textures relate to each other in size.
Which texture goes with the largest circle? That is correct – sand is the largest soil texture. What
is the next largest texture? Correct – Silt is the middle size and then clay is the smallest. Very
seldom do you find a place where there is pure sand, silt or clay – but it is the mixture of these
three sizes of texture that make up various soil textures. Today we are going to look at soil
texture and structure and how it all relates to each other.

Summary of Content and Teaching Strategies

Utilize PS U6 L4 PowerPoint to teach the lesson and have the students take notes. A student
problem sheet is included at the end of this lesson plan that could also be used to guide the class
discussion.

Objective 1: Identify soil texture and structure. & Objective 3: Identify the various types of soil
structure.

Slide # 3
Soil Texture
  Soil texture = proportions of sand, silt and clay
  Property of the soil controlled by the size of individual grains or particles
  Soil is usually made up of particles of widely varying sizes.
  Soil texture expresses the average or combined effect of all these grain sizes

Slide #4
3 Soil Textures
  Sand
  Silt
  Clay


                                                                                                      2
Slide #5
  Sand: In the moist condition sand should feel gritty and will be loose and single grained.
     Squeezed when wet, it will fall apart when the pressure is released
  Clay: when moist is quite plastic and sticky when wet. When the moist soil is squeezed out
     between the thumb and fore finger, it will form long flexible ribbons.
  Silty Soil: when dry and pulverized will feel soft and floury. When wet the soil readily runs
     together and puddles. When squeezed between the thumb and finger it will ribbon but the
     ribbon will appear checked and cracked.

Discuss the soil triangle on slide #6. If you have given the students handouts of the triangle, it is
very effective to ask them to identify a point on the triangle by percentage numbers you write on
the board. This will ensure that the students are reading the triangle correctly.

Slide #7
Fine Textured Soils
  Clay
  Sandy Clay
  Clay Loam
  Silty Clay
  Silty Clay Loam

Slide # 8
Medium Textured Soils
  Sandy Clay Loam
  Loam
  Silt Loam
  Silt

Slide #9
Coarse Soils
  Sand
  Loamy Sand
  Sandy Loam

Slide #10
Soil Structure
the combination or arrangement of primary soil particles into secondary particles, units, or peds
(which are separated from adjourning aggregates by surfaces of weakness)

Slide #11
Soil Aggregation
The cementing or binding together of several soil particles into a secondary unit, aggregate, or
granule

**clods are different – they are caused by some disturbance such as plowing or digging
Discuss local issues about timely tillage here.

                                                                                                    3
Slide #12
Consistence
The resistance of a material of deformation or rupture – the degree of cohesion or adhesion of the
soil mass

If available – have samples ready to show students as you talk about the types of structure.

Slide #13
Structureless soil
Soil where the particles of coarse soil fail to cling together, when fine soil breaks into large clods,
or when the soil is massive, a single compacted substance

Slide #14
Platy Structure
Soil aggregates developed along the horizontal direction: flaky

Slide #15
Prismatic Structure
A soil structure type with a long vertical axis that is prism shaped, vertical faces are well defined,
without rounded caps

Slide #16
Columnar Structure
Vertically oriented, round-topped structural prisms – rounded caps

Slide #17
Granular Structure
A natural soil ped or aggregate – have plane or curved surfaces which have slight or no
accommodation to the faces of surrounding peds

Slide #18
Types of Soil Structure
  Platy
  Prismatic
  Columnar
  Angular Blocky: Block-like – three dimensions of same magnitude
  Subangular blocky: same as angular except the vertices are more rounded
  Granular
  Crumb: similar to granular except the peds are porous

Slide #19
Class of Structure
Size of individual ped




                                                                                                     4
Slide #20
Good Soil Structure
  Good Soil Structure:
  Necessary for good water penetration into the soil
  Water holding capacity
  Ease of working the soil
  Good root penetration
  Favorable movement of soil air
  Availability of plant nutrients
  Good internal drainage

Slide #21
Binding Agent in the soil
  Organic matter converted to humus is the chief binding agent for stable soil structure.
  Continuous cultivation and never plowing under any organic matter tends to destroy
     structure


Review/Summary

Have several soil samples (bags ready) – in the shop or classroom, have the students texture
several kinds of soil using the ribboning method. Have them work through the chart and decide
on the soil texture.

Now that we have learned about the different types of soil texture and structure, let’s see how
you can do. Please gather around this table. We are going to use the flow chart that tells you how
to determine texture. You all need to texture both of these soil types and determine what they are.
Does anyone have any questions? Okay – let’s get started. Do a demonstration and then work
them all through the activity.


Application
Extended classroom activity:
      Visit a local area to texture the soil and view structure.

FFA activity:
      Compete in the Land Classification Career Development Event.

SAE activity:
      Have the student evaluate some of their soil at home for texture and structure. You can
      also have them all bring in a sample bag of soil. If this was assigned during an earlier
      lesson – it can now be used.


Evaluation
      See the attached sample evaluation.

                                                                                                 5
Answers to Assessment:

   1.   Sand, Silt, Clay
   2.   Clay, Sandy Clay, Clay Loam, Silty Clay, Silty Clay Loam
   3.   The binding together of soil particles in a secondary unit
   4.   Not bond together naturally – caused by a soil disturbance
   5.   Platy, prismatic, columnar, angular, subangular blocky, granular, crumb
   6.   Answers will vary




                                                                                  6
                                                                              PS U6 L4 Student Job Sheet


Unit: Plant & Soil Science                                                    Name:
Job: Recognizing Soil Texture & Structure                                     Date:

1. What is soil texture?

2. What are the 3 textures of soil?

3. How is field determination of texture made?

4. What should sand, silt, and clay feel like in the moist condition?

5. Fine textured soils include which textural classes?

6. Medium textured soils include which textural classes?

7. Coarse textured soils include which textural classes?

8. Define the following terms:
     A. Soil Structure                                C. Consistence
     B. Soil aggregation                              D. Structureless soil

9. List the seven soil structure types and briefly explain each.

10. What does class of structure refer to?

11. Why is good soil structure important?

12. What binds soil structure together?




                                                                                                      7
                                                                                                                   Soil Texture Flow Chart
Soil Texture

        Soil texture refers to the proportions of sand-, silt- and clay-sized particles in the soil. Soil
texture is determined in the field by moistening a soil sample and feeling with the fingers. The
following flow diagram may be used to determine soil texture by feel. (source:Thien, SJ. 1979. A flow
diagram for teaching texture-by-feel analysis. Journal of Agronomic Education 8:54-55)



                       START: Place approximately 3 tablespoons of soil in palm.
                       Add drops of water and knead the soil to break down
                       aggregates. Soil is at the proper consistency when it is
                       plastic and moldable. If soil is too dry, add more water; if it
                       is too wet, add more soil.



                                                                       NO
                                        Does soil remain in a                            SAND
                                        ball when squeezed?



                       Place ball of soil between thumb and forefinger, gently pushing the soil with the
                       thumb and pushing upward to form a ribbon. Allow the ribbon to emerge and
                       extend over the forefinger, breaking from its own weight.

                                                      NO

              Does soil form a ribbon?                                   LOAMY
                                                                          SAND


                    Is the ribbon                NO             Is the ribbon         NO            Is the ribbon >2
                        1 inch                                    1-2 inches                          inches before
                     long before                                 long before                            breaking?
                      breaking?                                   breaking?




                                 Excessively wet a pinch of soil in palm and rub with forefinger.


                                          SANDY
  SANDY         Does soil feel                               Does soil feel          SANDY                Does soil feel
                very gritty?
                                          CLAY               very gritty?                                 very gritty?
  LOAM                                    LOAM                                       CLAY

                                 NO                                           NO                                   NO

                                          SILTY
  SILT          Does soil feel                               Does soil feel          SILTY                Does soil feel
                very smooth?
                                          CLAY               very smooth?                                 very smooth?
  LOAM                                    LOAM                                       CLAY

                                 NO                                           NO                                   NO


                    Neither                                 Neither grittiness                           Neither grittiness
  LOAM            grittiness or           CLAY               or smoothness                                or smoothness
                  smoothness              LOAM               predominates            CLAY                 predominates
                 predominates




                                                                                                                                        8
                                                                                          Soil Descriptions
The following descriptions are helpful in making texture determinations.

        Sand – Sand is loose and single-grained; individual grains can be readily seen or felt.
Squeezed when dry, it will fall apart when the pressure is released. Squeezed when moist, it will
form a cast that will crumble when touched. Sands found in eastern Colorado usually adhere to the
hand and need to be brushed off.
        Loamy sand – Loamy sands generally leave more sand grains adhering to the skin and leave
more staining. Moist casts can be handled more freely. This soil texture does not ribbon, but will
break with the first push to form a ribbon.
        Sandy loam - A sandy loam contains more than 50% sand, but has sufficient silt and clay to
make it somewhat coherent. Individual sand grains can be readily seen or felt. Squeezed when dry,
it forms a cast that stands careful handling. A cast formed from moist soil can be freely handled.
Sandy loam ribbons should push out between thumb and forefinger 1 to 4 times.
        Loam – A loam is a soil that feels as if it has an even mixture of sand, silt and clay. It is
mellow with a somewhat gritty feel, yet fairly smooth and slightly plastic. Squeezed when dry, it
will forms a cast that will bear careful handling, while the cast formed by squeezing the moist soil
can be handled quite freely without breaking. Loam ribbons should push out between thumb and
forefinger from 4 to 8 times and the ribbon will show a slight glaze.
        Silt loam – A silt loam has more than 50% silt, a moderate amount of the fine grades of sand
and only a small amount of clay. When dry it may appear cloddy, but the lumps can be readily
broken. When pulverized it feels soft and floury. When wet the soil readily runs together and
puddles. Either dry or moist it will form casts that can be freely handled without breaking. Silt loam
ribbons should push out between thumb and forefinger 4 to 8 times, but will appear checked and will
be relatively weak. Ribbons may show a slight glaze.
        Sandy clay loam – A sandy clay loam has more than 50% sand and approximately 30% clay.
Individual sand grains can be seen and felt. Weak, dry casts can be formed and moist casts are
strong and distinctly sticky. Ribbons should push out between thumb and forefinger from 6 to 12
times and show a glaze.
        Clay loam – A clay loam has approximately 35% clay and usually breaks into clods or lumps
that are hard when dry. When moist, the soil is sticky and plastic and will form a cast that will bear
much handling. When kneaded in the hand it does not crumble readily but tends to work into a
heavy compact mass. Ribbons will push out between thumb and forefinger 8 to 12 times, have a
glazed surface, and retain finger or thumb prints. The ribbon is usually thin and will break readily,
barely sustaining its own weight.
        Silty clay loam – A silty clay loam differs from a clay loam in having less than 20% sand, a
smoother, more floury feel, and a lesser glaze on the ribbon. Ribbons will push out between thumb
and forefinger 8 to 12 times.
        Clay – A clay contains more than 50% clay-sized particles. The soil forms very hard clods
when dry and is very plastic and usually sticky when wet. Ribbons will push out between thumb and
forefinger at least 8-12 times, have a very distinct glaze, and retain distinct finger or thumb prints.
        Sandy clay – A sandy clay has less than 20% silt and more than 35% clay. The soil is very
similar to clay but feels gritty. Ribbons will push out between thumb and forefinger 8-12 times, and
grains will stand out from the glazed surface.
        Silty clay – A silty clay has less than 20% sand. Because it contains more than 40% silt, it is
smoother to the touch and less sticky when moist than clay. Ribbons will push out between thumb
and forefinger 8-12 times and show a glaze.
                                                                                                         9
                                                                                          Soil Triangle




                                              FINE




                     COARSE
                                                              MEDIUM




        For the purpose of the land judging contest, soil textures will be grouped as noted on the
texture triangle, above, and written below.

                              TEXTURE CLASSES
______________________________________________________________________________

FINE           clay (C), sandy clay (SC), clay loam (CL), silty clay (SiC), silty clay loam (SiCL)

MEDIUM         sandy clay loam (SCL), loam (L), silt loam (SiL), silt (Si)

COARSE         sand (S), loamy sand (LS), sandy loam (SL)




                                                                                                   10
                                   Soil Texture & Structure
                                     Student Evaluation

Name: _______________________________________                         Date: _______________

   1. In order of size (largest to smallest), name the three types of soil textures.



   2. What is an example of a fine textured soil?


   3. What is soil aggregation?




   4. What is a clod?




   5. Name three types of soil structure.




   6. Why is good soil structure important?




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