Horticulture Science Lesson 25Understanding Soil Textureand Structure
Horticulture Science Lesson 25Understanding Soil Textureand Structure
Interest Approach
Interest Approach
Provide students with various samples of soil. Onesample should be nearly all sand, one nearly all clay, andone nearly all silt. Ask students to determine how
the samples differ. Would each sample be equallyproductive? Indicate that the samples vary according tothe size of soil particles. Ask students how particle
size might affect various soil properties. Allowcomments to lead to a discussion of soil texture.
Student Learning Objectives
Student Learning Objectives
•Describe the concept of soiltexture and its importance.
•Determine the texture of a soilsample.
Student Learning Objectives
Student Learning Objectives
•Explain soil structure, itsformation, and importance.
•Differentiate various soilstructures.
Terms
Terms
• aggregates
• clay
• clods
• loam
• mineral matter
• peds
• permeability
• ribbon method
Terms
Terms
• sand
• silt
• soil structure
• soil texture
• soil workability
• textural triangle
• water-holding capacity
What is soil texture and why is itimportant?
•Soil texture is the fineness or coarsenessof a soil.
•The inorganic material in soil is calledmineral matter; it began as rock and wasweathered into small particles.
•Texture describes the proportion of threesizes of soil particles.
1. Sand is the largest mineral particle.
2. Silt is a medium-sized mineral particle.
3. Clay is the smallest mineral particle.
What issoil textureand why isitimportant?
What is soil texture and why is itimportant?
•Soil texture is important because it affectswater-holding capacity, permeability, soilworkability, and plant growth.
1. Water-holding capacity is the ability ofa soil to retain water for use by plants.
2. Permeability is the ease with which airand water may pass through the soil.
What is soil texture and why is itimportant?
3. Soil workability is the ease with whichsoil may be tilled and the timing of workingthe soil after a rain.
4. Most plants grow best in soils that havegood aeration and water-holding capacity.
–Some root crops like carrots and onionshave stunted growth in a fine-texturedsoil.
How is the texture of soildetermined?
•Soil texture may be determined in one oftwo ways.
•The percentages of sand, silt, and clay maybe tested in the lab.
•Once tested, you may determine thetextural class of the soil by referring to thetextural triangle.
How is the texture of soildetermined?
•There are 12 basic textural classes.
1. Silt
2. Silt loam
3. Silty clay loam
4. Loam, which contains some of all three soilparticle sizes
5. Sandy clay loam
6. Loamy sand
7. Sand
8. Sandy loam
9. Sandy clay
10. Clay loam
11. Silty clay
12. Clay
How is the texture of soildetermined?
•The relative amounts of sand, silt, and claymay also be determined in the field usingthe ribbon method.
•Five textural classes may be determinedusing the ribbon method.
1. In fine-textured soil, a ribbon forms easily andremains long and flexible.
2. In moderately fine-textured soil, a ribbonforms but breaks into pieces ¾ to one–inchlong.
How is the texture of soildetermined?
3. No ribbon forms in medium-textured soil. The sample breaksinto pieces less than ¾–inchlong. The soil feels smooth andtalc-like.
4. No ribbon forms in moderatelycoarse-textured soil. The samplefeels gritty and lackssmoothness.
5. No ribbon forms in coarse–textured soil. The sample iscomposed almost entirely ofgritty material and leaves littleor no stain.
What is soil structure, how doesit form, and why is it important?
Different soils have different soil structures.
•Soil structure is the arrangement of thesoil particles into clusters or aggregatesof various sizes and shapes.
•Aggregates that occur naturally in the soilare referred to as peds, while clumps ofsoil caused by tillage are called clods.
What is soil structure, how doesit form, and why is it important?
•Structure is formed in two steps.
1. A clump of soil particles sticks looselytogether due to:
a. Plant roots surrounding the soil and separating clumps
b. Freezing and thawing of soil
c. Soil becomes wet and then dries
d. The soil is tilled
e. Fungal activity
What is soil structure, how doesit form, and why is it important?
2. Weak aggregates are cemented to makethem distinct and strong.
–Clay, iron oxides, and organic mattermay act as cements.
–When soil microorganisms break downplant residues, they produce gums thatalso glue peds together.
What is soil structure, how doesit form, and why is it important?
•Soil structure is important for severalreasons.
1. It improves soil tilth.
2. It improves permeability.
3. It resists the beating action of raindrops,minimizing the formation of crusts thatreduce crop stands.
What are the various soil structuresand what do they look like?
There are eight primary types of structure.
•Granular aggregates are small, non-porous, and strongly held together.
•Crumb aggregates are small, porous, andweakly held together.
•Platy aggregates are flat or plate-like.Plates overlap, usually causing slowpermeability.
What are the various soil structuresand what do they look like?
•Prismatic aggregates are prism-like with thevertical axis greater than the horizontal.
–Prismatic has flat caps while columnar hasrounded caps.
•Columnar aggregates are similar to prismaticand are bounded by flat or slightly roundedvertical faces.
–The tops of columns, in contrast to those ofprisms, are very distinct and normally rounded.
What are the various soil structuresand what do they look like?
•Blocky aggregates are block-like with six ormore sides.
–All three dimensions are about the same.
•Structureless, as the name suggests,involves no apparent structure.
–It may be found in one of two forms:
1. In single grain, soil particles exist asindividuals and do not form aggregates.
2. The massive form involves soil particlesclinging together in large uniform masses.
Review/Summary
Review/Summary
•What is soil texture and why isit important?
•How is the texture of soildetermined?
Review/Summary
Review/Summary
•What is soil structure, howdoes it form, and why is itimportant?
•What are the various soilstructures and what do theylook like?