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Basic Soil Science: The Properties of Soil NotesDate _______________Topic___________________________________

Basic Soil Science BSS3 The Properties of Soil

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Soil Color:

Basic Soil Science: The Properties of Soil Notes


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Soil Structure:

Basic Soil Science: The Properties of Soil Notes


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Soil Texture:

Basic Soil Science: The Properties of Soil BSS3.1

Soil Matrix Color: Dark Brown, Very Dark Brown, Black

These colors are caused by accumulations of organic matter in soils. Humus coats the soil particles, giving them a dark color. Usually, the darker the color, the more organic matter the soil contains, and the more fertile and productive is the soil.

Dark colors are typical of A horizons (the surface layer of soil). In northern Missouri soils, nearly all A horizons have this color.

That is not the case in much of southern Missouri. Organic matter content is lower; therefore, the soils are lighter in color. In general, if the lighter-colored soils have been cultivated, and much crop residue has been mixed into the Ap horizon (plow layer), then the color is probably brown or very dark brown. If the soil has not been cultivated and there is not much native vegetation, then the A horizon is likely to have a light brown color.

Some soils have black colors extending well down into the subsoil. That is usually an indication of wetness. In wet soils, organic matter breaks down very slowly and the soil is darkened by the partially decomposed organic matter that accumulates.

Some very clayey, sticky soils may be black, too. In these soils, organic matter is mixed throughout the entire soil, and the soil is black, even though the organic matter content is not particularly high.



Soil Matrix Color: Light Brown, Brown, Yellowish Brown

These are the colors of well-aerated soils. Well-aerated means that air moves freely into and out of the pore spaces of the soil. As microbes and plant roots use up oxygen in soil pores, oxygen from the air above the soil moves in to replace it. Well-aerated soils, therefore, provide a healthy environment for plant roots.

Brown colors are due to iron oxide coatings on mineral grains (soil particles). Chemically, they are the same as a coating of rust on a piece of iron. These iron oxide coatings require plenty of oxygen in soil pores. If water should fill soil pores and remain there for a long time, oxygen cannot reach the iron coatings, causing the soil to turn gray. That is why brown colors indicate that the soil has good air-water relations and has not been saturated for long periods of time.

Brown colors are typical of B and C horizons (lower horizons) that are well-aerated. This is true all over Missouri. As long as there is not enough organic matter to darken the soil and there is plenty of oxygen to maintain iron oxide coatings, the soil will nearly always be brown or red.



Soil Matrix Color: Red, Reddish Brown

These colors are also caused by iron oxide coatings, and they also indicate well-aerated soil. The soil is red, rather than brown, only because the chemical form of the iron oxide is a little different.

Most red soils are very old soils and are very strongly weathered. They are more leached, more acidic, and less fertile than soils having brown colors.

Red soils occur on some of the uplands of southern Missouri. Except for the A horizon, all the other horizons in these soils usually are red. Red soils are rare in northern Missouri, but there are a few areas where the soils formed in limestone residuum and have red subsoils.



Soil Matrix Color: Dark Gray, Light Gray, White

Dark gray soils are wet soils. When soil pores are full of water, oxygen cannot get in. Gradually the yellow-brown coatings are removed from mineral grains and are leached away. The gray color is the natural gray color of the uncoated mineral grains, darkened a little by organic matter. Dark gray is typical of B and C horizons in wet soils.

E horizons (subsurface layers) are always lighter in color than the horizon above them because E horizons have lost organic matter and are leached. Some E horizons occur in wet soils. Iron is reduced and leached from the soil by water moving horizontally on top of a clayey subsoil. Other E horizons may occur in well-drained soils. In these soils, different chemical processes cause the loss of iron oxide coatings from mineral grains. These E horizons have bright-colored B horizons below them.

There are some parent materials (loess and glacial till) common in northwest Missouri that are naturally gray. It should be noted that this color is not a result of wetness.



Soil Matrix Colors

Dark Brown, Very Dark Brown, Black

Light Brown, Brown, Yellowish Brown

Red, Reddish Brown

Dark Gray, Light Gray, White



Soil Color Matching

Match the colors below to the four classes of soils:o Dark brown, very dark brown, black o Light brown, brown, yellowish brown o Red, reddish brown o Dark gray, light gray, white



Guide for Describing Mottles

Feature Description Classes

Abundance The percentage of exposed surface area occupied by mottles

Few: less than 2% of exposed surface Common: 2-20% of exposed surface Many: more than 20% of exposed surface

Size The approximate diameter of individual mottles

Fine: Diameter less than 5mm Medium: Diameter 5-15mm Coarse: Diameter more than 15mm

Contrast The relative difference between the mottle color and the matrix color

Faint: Mottles evident only on close examination. Mottle color and matrix color are very nearly the same.

Distinct: Mottles are readily seen, though not striking. Mottle color and matrix color are different, though not widely so.

Prominent: Mottles are so conspicuous that they are the outstanding visible feature of the horizon. Mottle color and matrix color are widely different.

Color Described the same way as soil matrix colors.

Most common mottle colors are: Yellowish brown Dark reddish brown Red Gray



Types of Soil StructureST

RU

CT

UR

ED

Granular: Roughly spherical, like Grape Nuts™ cereal. The structure is usually 1-10mm in diameter. It is most common in surface horizons (A horizons) where plant roots, microorganisms, and sticky products of organic matter decomposition bind soil grains into aggregates.

Platy: Flat peds that lie horizontally in the soil. Most are less than 2cm thick. Not common, but occurs mostly in subsurface horizons or dense layers (E and Bx horizons).

Blocky: Roughly cube-shaped with generally flat surfaces. Divided into two types: angular blocky has edges and corners that remain sharp; subangular blocky has edges and corners that are rounded. Sizes commonly range from 5-50mm across. Typical in the subsoil (B and Bt horizons). They form by repeated expansion and contraction of clay materials.

Prismatic: Peds are taller than they are wide. They often have five sides. Sizes are commonly 10-100mm across. Most common in the lowest part of the subsoil (B and BC horizons). The prisms in some strongly developed soils have rounded tops because the tops have lost their corners by eluviation (downward movement of material). These prisms are called columnar.


Basic Soil Science: The Properties of Soil BSS3.5ST

RU

CT

UR

EL

ESS

Single Grain: In some very sandy soils, every grain acts independently and there is no binding agent to hold the grains together into peds. Permeability is rapid, but fertility and available water capacity are low.

Massive: Compact, coherent soil is not separated into peds of any kind. Massive, claylike soils usually have very small pores, slow permeability, and poor aeration.



Types of Soil StructureST

RU

CT

UR

ED

Granular: Roughly spherical, like Grape Nuts™ cereal. The structure is usually 1-10mm in diameter. It is most common in surface horizons (A horizons) where plant roots, microorganisms, and sticky products of organic matter decomposition bind soil grains into aggregates.

Platy: Flat peds that lie horizontally in the soil. Most are less than 2cm thick. Not common, but occurs mostly in subsurface horizons or dense layers (E and Bx horizons).

Blocky: Roughly cube-shaped with generally flat surfaces. Divided into two types: angular blocky has edges and corners that remain sharp; subangular blocky has edges and corners that are rounded. Sizes commonly range from 5-50mm across. Typical in the subsoil (B and Bt horizons). They form by repeated expansion and contraction of clay materials.


Basic Soil Science: The Properties of Soil BSS3.5 Prismatic: Peds are taller than they are wide. They often have five sides. Sizes are commonly 10-100mm across. Most common in the lowest part of the subsoil (B and BC horizons). The prisms in some strongly developed soils have rounded tops because the tops have lost their corners by eluviation (downward movement of material). These prisms are called columnar.

STR

UC

TU

RE

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Single Grain: In some very sandy soils, every grain acts independently and there is no binding agent to hold the grains together into peds. Permeability is rapid, but fertility and available water capacity are low.

Massive: Compact, coherent soil is not separated into peds of any kind. Massive, claylike soils usually have very small pores, slow permeability, and poor aeration.



Particle Size

Sand

Silt

Clay

Particles are large and can be seen with the naked eye.

Not sticky

Particles are very small and flat; can be seen with high-powered microscopes.

Less than 0.002 mm in size

Particles are smaller than sand; can’t be seen without hand lens or microscope.

Feels gritty

Range in size from 0.05 mm to 2 mm

High water holding capacity

Will not stick together when wet

Feels smooth (like flour or corn starch)

Range in size from 0.002 mm to 0.05 mm

Feels sticky; can be molded into ribbons or wires

Holds large amounts of water in a form plants can use

Low capacity for holding moisture and storing nutrients



Texture Triangle



Clues to the Feel of Textural Classes

Sand Moist sample collapses after squeezing Hands don’t get dirty working the sample

Loamy Sand Sample has very little body Moist soil barely stays together after

squeezing Just enough silt and clay to dirty hands

Sandy Loam Sand dominates noticeably Enough silt and clay to give the sample

body Moist soil stays together after squeezing Hardly forms any ribbon at all

Sandy Clay Loam Feels gritty and sticky Forms ribbon 1-2 inches (2.5-5 cm) long

Sandy Clay Feels definitely sandy Forms ribbon 2-3 inches (5-7.5 cm) long

Loam Sand noticeably present, but doesn’t

dominate Sample works easily between thumb and

fingers Contains enough silt and clay to give

sample good body Sample only forms short, broken ribbons

Silt Loam Feels smooth, like flour or corn starch Tends to be nonsticky Only forms short, broken ribbons

Clay Loam Noticeably gritty, but sand doesn’t

dominate Noticeably sticky Noticeably hard to work between thumb

and fingers Forms ribbon 1-2.5 inches (2.5-6 cm)

long

Silty Clay Loam Feels smooth and sticky Contains very little sand Forms ribbon 1-2.5 inches (2.5-6 cm)

long

Clay and Silty Clay Dry sample absorbs a lot of water before

it is moist enough to work Sample very hard to work between

thumb and finger Forms ribbon 2.5-4inches (6-10 cm) long



Estimating Textural Classes



Properties of Soil News Report Evaluation

In pairs, create a six-minute interview pertaining to the information learned in this lesson. Focus on how to determine the color, texture and structure of a given sample of soil. One person will be the news broadcaster and the other will play a role pertinent to the features of the interview.

15 points 10 points 5 points

Length 5-6 minutes 3-4 minutes 2 minutes or less

Content Factual information regarding how to determine the color, texture, structure of soil; brought in students’ personalities, humor, opinions

Factual information regarding how to determine the color, texture, structure of soil

Lacked factual information regarding how to determine the color, texture, structure of soil

Interviewee Identity

Identity of person being interviewed was appropriate for the content presented in the report

Identity of person being interviewed was not appropriate for the content being presented in the report

Overall Effect Like an interview on TV; students spoke clearly, were easily understood, showed knowledge of content

Somewhat like an interview on TV; students spoke clearly, were easily understood, showed knowledge of content.

Lacked resemblance to an interview on TV; and/or students needed to speak more clearly and show more knowledge of content

Comments:

Total __________________ / 45 points



The Properties of Soil Evaluation

Match the definition on the left with the term on the right.

1. ____ Splotches of colored soil in a matrix of different color

2. _____ A system used to identify soil color

3. ____ Structure is roughly spherical, like Grape Nuts; usually 1–10 mm in diameter

4. _____ Structure consists of flat peds that lie horizontally in the soil

5. _____ Structure consists of peds that are roughly cube-shaped with more less flat surfaces

6. _____ In the structure, the peds are taller than they are wide, often with five sides

7. _____ Every grain acts independently and there is no binding agent to hold the grains together into peds

8. _____ Largest of the soil separates, between 0.05 and 2.00 mm in diameter

9. _____ Medium-sized soil separate, particles between 0.05 and 0.002 mm in diameter

10. _____ The class of smallest soil particles, smaller than 0.002 mm in diameter


A. Sand

B. Blocky

C. Munsell color

system

D. Prismatic

E. Granular

F. Single grain

G. Clay

H. Mottles

I. Silt

J. Platy


List the four broad classes of soil color:

11. ___________________________________________________________________________

12. __________________________________________________________________________

13. ___________________________________________________________________________

14. ___________________________________________________________________________

Use the soil triangle to determine the correct name for the texture of a soil sample that is 40% sand, 15% clay, and 45% silt.

15. The soil is a ___________________________.



The Properties of Soil Evaluation

Match the definition on the left with the term on the right.

1. H ___ Splotches of colored soil in a matrix of different color

2. C ____ A system used to identify soil color

3. E ___ Structure is roughly spherical, like Grape Nuts; usually 1–10 mm in diameter

4. J ____ Structure consists of flat peds that lie horizontally in the soil

5. B ____ Structure consists of peds that are roughly cube-shaped with more less flat surfaces

6. D ____ In the structure, the peds are taller than they are wide, often with five sides

7. F ____ Every grain acts independently and there is no binding agent to hold the grains together into peds

8. A ____ Largest of the soil separates, between 0.05 and 2.00 mm in diameter

9. I ____ Medium-sized soil separate, particles between 0.05 and 0.002 mm in diameter

10. G ____ The class of smallest soil particles, smaller than 0.002 mm in diameter


A. Sand

B. Blocky

C. Munsell color

system

D. Prismatic

E. Granular

F. Single grain

G. Clay

H. Mottles

I. Silt

J. Platy


List the four broad classes of soil color:

(in any order)

11. Dark brown, very dark brown, black

12. Light brown, brown, yellowish brown

13. Red, reddish brown

14. Dark gray, light gray, white

Use the soil triangle to determine the correct name for the texture of a soil sample that is 40% sand, 15% clay, and 45% silt.

15. The soil is a loam.


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missouriffa.org · Web viewBasic Soil Science: The Properties of Soil Notes Basic Soil Science: The Properties of Soil Basic Soil Science: The Properties of Soil Notes Notes BSS3.11