Lesson B1 –6 • Page 1
Instruction in this lesson should result in students achieving the following objectives:
1
Describe the concept and importance of soil density.
2
Determine soil density.
3
Explain how to alter soil density.
4
Describe the effect of soil density on erosion, water holding capacity and flooding.
Anticipated Problem: What is bulk density?
I. Bulk density is a measurement of how tightly soil particles are pressed together.
A. Density is the mass of an object divided by the volume.
B. Texture , the relative proportion of soil separates in a soil, affects the density, as different separates have different masses and have corresponding pore space.
C. Structure , the arrangement of soil particles into aggregates or peds, also determines density because various soil arrangements will compact more easily than others.
D. The particle density of the soil is the mass per unit volume of soil particles excluding the pore space.
E. The soil consistence , the soil’s response to pressure at various moisture levels, can be described as hardness, friability, stickiness, and plasticity.
Anticipated Problem: How can you determine soil density?
II. Soil density is the mass of the soil particles divided by volume.
A. When removed from the earth’s crust the soil contains air and water that will affect the mass readings and do not constitute soil density.
B. The soil needs to be dried in order to find the true density.
C. Density is usually expressed in grams per cubic centimeter.

Anticipated Problem: How can you change soil density?
III. Soil density is affected by tillage and non-tillage systems.
A. The working of the subsoil by a subsoil tillage system can decrease the density of the soil by encouraging more pore space per volume.
B. Some seedbeds and fields have too little soil density so rolling the field or working the soil with a finisher may provide better seedbed density.
C. At construction sites it is important to have a compacted subsoil so settling will not occur once the building is completed.
Anticipated Problem: What effect does soil density have on various soil characteristics?
IV. Soil density affects soil characteristics.
A. With higher the soil density the amount of pore space decreases, thus decreasing the water holding capacity of the soil. This decrease in water holding capacity is especially significant in the larger pore spaces that are readily available to hold water which plants can utilize.
B. Erosion can also be affected by density, as water will not infiltrate the soil as readily and will stay in contact with the topsoil for a longer period of time.
C. Flooding will result from excessive runoff and decreased infiltration in wet years.
Puddling can result from localized compaction, as is the case in tractor tire ruts.

Part One: Matching
Instructions: Match the word with the correct definition. a. texture c. consistence b. structure d. density
_______1. The soil’s resistance to pressure at various moisture levels.
_______2. The arrangement of soil particles into peds.
_______3. The relative amount of sand, silt and clay in the soil.
_______4. The mass per unit volume of an object.
Part Two: Fill-in-the-Blank
Instructions: Complete the following statements with increase or decrease.
1. As compaction and soil density increases water holding capacity will ____________.
2. If a soil has more clay in it has a tendency to ___________ its soil density.
Part Three: Multiple Choice
Instructions: Write the letter of the correct answer.
_______1. Which of the following terms is used to describe how tightly soil particles are pressed together? a. texture b. structure c. bulk density d. particle density
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_______2. Which of the following terms describes the characteristics of a soil in its response or resistance to pressure, as described at various moisture content? a. texture b. structure c. bulk density d. consistence
_______3. Which of the following terms describes the relative proportion of soil separates in a soil? a. texture b. structure c. bulk density d. consistence

_______4. Sandy soils have a _________________ permeability. a. very slow b. slow c. moderate d. rapid
_______5. The ___________________ of the soil particle affects water-holding capacity and aeration. a. size b. shape c. color d. weight
_______6. Which of the following is the greatest cause of soil compaction? a. water b. soil tillage c. air d. insects
_______7. Which of the following does not help enhance soil pore development? a. root systems b. earthworms c. tilling at the same depth level every year d. adding organic matter
_______8. Which of the following is the correct order of soil particle in size, starting with the smallest particle size? a. silt, clay, sand b. sand, clay, silt c. clay, sand, silt d. clay, silt, sand
Part Four: Short Answer
Instructions: Answer the following questions.
1. How can soil bulk density affect plant growth?
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2. Which texture of soils generally has the largest bulk density and why?
3. What makes it difficult to improve the bulk density of soil?
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Technical Supplement
1. What is bulk density?
Bulk density describes how tightly the particles in soil are pressed together. Bulk density is calculated by dividing the dry mass of a given sample by the volume it fills.
Bulk density = oven - dry weight in grams/volume of cylinder in cubic centimeters
2. How does soil density affect the growth of plant roots?
In extremely dense soils such as clay, roots have difficulty penetrating the soil. Air and water do not circulate through the soil easily. High-density soil will restrict growth in a root crop such as carrots. In soils with low densities, such as sandy soil, most plants have problems with root structure. Water flow is rapid, and roots dry quickly.
3. How does soil density affect water infiltration?
The lower the density, the faster the movement of water throughout the soil. For example, water moves quickly through sandy soil, and slowly in dense soils such as clay soil.
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4. How does soil density affect flooding, erosion, and soil temperature?
Loosely packed (low density) soil:
 does not flood easily due to maximum water infiltration (thus giving less structural support to the plant)
 erodes easily
 changes temperature quickly, due to a great amount of air/water movement; water evaporates quickly
Tightly packed (high bulk density) soil:
 floods easily due to minimal water infiltration
 does not erode easily
 does not change temperature quickly
5. What is the effect of soil texture on the bulk density?
The size of the soil particles affects soil traits such as water holding capacity and air and water movement. Loose soils, such as sand, drain quickly and dry out quickly. In loose soils, temperature changes rapidly and root support is poor. Tightly textured soils, such as clays, retain water longer and impede air amount and movement.

6. How can bulk density be changed?
 Freezing and thawing action
 Changes in moisture and temperature
 Loosening soil by tillage practices, such as plowing, results in a bulk density of .8 to 1.5 grams/ cubic cm.
 Repacking soil using a harrow or disk; 4–5 trips over tilled ground can raise the bulk density as high as 1.4 g/cu. cm.
 Adding organic matter to return crop residue to the soil
 Planting deep-rooted crops such as alfalfa to make the soil less dense.
7. What is an ideal bulk density for growing field crops?
Most field crops grow best in soils with a bulk density of 1.1 to 1.4 grams/cubic centimeter, slightly greater than the bulk density of newly plowed land.
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References
Buriak, Philip and Edward W. Osborne. Physical Science Applications in Agriculture.
Danville, IL: Interstate Publishers, Inc., 1996.
Prepared by Lyle Pendarvis, Bob Howard, and Deb Boe
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