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Laboratory Manual

LABORATORY 2 SOIL TEXTURAL CLASS

Soil texture is concerned with the size of mineral particles in the soil. The textural class name given to a soil is based on the particles in the A-horizon or topsoil. Sands are dominant in coarse-textured soils, whereas fine-textured soils silts and clays dominate. These soils because of their different sized particles differ vastly in chemical composition and physical properties, and hence, must be managed differently. The coarse particles are largely silica (quartz) in humid temperate regions, and the fine particles are silicate residues, more highly weathered than sands but higher in iron, aluminum, calcium, magnesium, sodium, and potassium. Differences in organic matter content, water-holding capacity, erosion, and drainage are among many soil properties related to texture.

Because management practices are related to soil textural classes, it is essential that one be able to identify broadly the various textural classes.

Soil Separates

Soil texture refers to the relative proportions of the various sized grains in a mass of soil. Specifically, soil texture refers to the proportions of sand, silt, and clay present in the soil. Sand, silt, and clay are called soil separates.

Sharp distinctions between the three groups are more or less arbitrary. However, the size groups have been arrived at after many trials in developing classes that can be used consistently and conveniently to define soil classification units.

The size ranges of the soil separates are as follows:

SOIL SEPARATE

SIZE RANGE (mm)

SANDS

very coarse sand

2.0 to l.0

coarse sand

1.0 to 0.5

medium sand

0.5 to 0.25

fine sand

0.25 to 0.l0

very fine sand

0.l0 to 0.05

SILT

0.05 to 0.002

CLAY

Below 0.002

Fragments less than l0 inches if rounded or l5 inches if flat are considered part of the soil mass. The presence of coarse particles larger than very coarse sand are used as modifiers of the textural class names. For example, particles larger than 2 mm but less than 3 inches in diameter are called gravel. Those particles larger than 3 inches but less than l0 inches in diameter are cobbles, and particles with diameters greater than l0 inches are termed stones or boulders. Hence, soils with many large particles may be named: gravelly sandy loam, cobbly sandy loam, stony sandy loam. Specific names are also used for the flat fragments (e.g., flaggy).

Soil texture is used in soil classification because it represents the most permanent characteristic of a soil. Structure of a soil horizon can be modified by management, such as tillage, but soil texture is not changed by management.

Soils rarely, if ever, consist wholly of one separate; therefore classes of soil texture are based on the percentages of sand, silt, and clay. Gravel and stones are removed by picking them out or by sifting the soil before textural classification is begun.

GENERAL GROUPINGS OF SOIL TEXTURAL CLASSES

General Terms

Basic Class Names

A. Sandy Soils (coarse-textured)

l. Sand

2. Loamy sand

B. Loamy Soils (moderately coarse- textured, medium-

l. Sandy loam

textured, and moderately fine-textured)

2. Fine sandy loam

3. Very fine sandy loam

4. Loam

5. Silt loam

6. Silt

7. Clay loam

8. Sandy clay loam

9. Silty clay loam

C. Clayey Soils (fine-textured)

1. Sandy clay

2. Silty clay

3. Clay

EFFECT OF SOIL TEXTURE ON SOIL PROPERTIES

Due to the differences in the size of the particles in the various soil classes, a number of other soil properties may be affected by the textural class.

PORE SPACE: Pore space increases with decreases in size of particles.

AERATION: Coarse textured soils because of larger pore sizes tend to be better aerated than fine textured soils, although the total air space is greater in the fine textured soils.

DRAINAGE: Coarse textured soils, other factors not being involved, are better drained.

ORGANIC MATTER CONTENT: All other factors being constant, fine-textured soils contain more organic matter and nitrogen than coarse-textured soils.

AGGREGATION: Fine textured soils will contain more aggregates than sandy soils.

BULK DENSITY: The weight per unit volume decreases as the size of the particles decreases. This is because of the greater air space in the finer textured soils. Note, however, that clays are referred to as heavy soils because of their difficulty in tillage and often stickiness, whereas sandy soils are referred to as light soils.

WATER HOLDING CAPACITY: Finer textured soils will hold more moisture than coarse textured soils. All of this water is not available to the plant.

EXERCISE

Soil Profile

Draw below a typical soil profile and label.

Classification Terms Popular Terms

A - Horizon Topsoil

B - Horizon Subsoil

C - Horizon Parent Material

http://www.soils.umn.edu/

At the address above, select Undergraduate Programs, then select Course 2125, and then Unit 3, or click on the address below.

http://www.soils.umn.edu/academics/classes/soil2125/doc/slab3hzn.htm

Visit this site for information on formation of soil: http://www.soils.umn.edu/academics/classes/soil2125/doc/slab1.htm

Draw soil profile here:

WATER HOLDING CAPACITY

Visit this site for information of soil water: http://www.soils.umn.edu/academics/classes/soil2125/doc/slab8h20.htm

Four 100 ml graduated cylinders were filled with 100 g of soil, and 12 ml of water were added. The soil and water were allowed to equilibrate overnight.

l. Record the total volumes of soil and the volume wetted by the water.

Calculate the weight of the soil which was wetted by the water.

SOIL

TOTAL VOLUME

VOLUME WETTED

DRY WEIGHT OF WETTED SOIL

1.

_______________________

______________________

_______________________

2.

_______________________

______________________

_______________________

3.

_______________________

______________________

_______________________

4.

_______________________

______________________

_______________________

2. Calculate the percentages of water by weight and by volume and bulk density.

SOIL

BY WEIGHT (%)

BY VOLUME (%)*

BULK DENSITY, g/cc

1.

_______________________

______________________

_______________________

2.

_______________________

______________________

_______________________

3.

_______________________

______________________

_______________________

4.

_______________________

______________________

_______________________

*Referred to as field capacity.

Visit this site for more information on bulkdensity: http://www.soils.umn.edu/academics/classes/soil2125/doc/slab7bd.htm

AGGREGATION

Which soil has the greatest number of aggregates? _____________________________________________________

What particles in the soil are capable of forming aggregates?______________________________________________

SOIL COLOR

Describe the colors of the four soils (wet and dry)

1. WET___________________________________________DRY____________________________________

2. WET___________________________________________DRY____________________________________

3. WET___________________________________________DRY____________________________________

4. WET___________________________________________DRY____________________________________

SOIL TEXTURE BY FEEL (Consult page 8)

Link:http://www.soils.umn.edu/ At this link select Education then Courses. Select course 2125, then WEB Laboratories then Unit 6, or click on the address below.

http://www.soils.umn.edu/academics/classes/soil2125/doc/slab6tex.htm

Using the descriptions on the attached sheet, determine the soil textural class of the four soils.

1. ______________________________________________________________________________________

2. ______________________________________________________________________________________

3. ______________________________________________________________________________________

4. ______________________________________________________________________________________

Which of these soils would you rather have in your garden?________________________________

Indicate the advantages this soil would have and its problems you might encounter in its management.

________________________________________________________________________________________

1. Will an increase or decrease in the amount of organic matter in a soil change its textural class? Explain.

________________________________________________________________________________________

2. Explain why organic matter content in soils may vary according to soil textural class.

________________________________________________________________________________________

3. Will tillage of a soil change its texture? Explain.

________________________________________________________________________________________

4. What effect does organic matter have on the water holding capacity of a soil?

________________________________________________________________________________________

5. Indicate what is meant by the following terms and indicate whether the term describes a sandy or clayey soil. (Glossary)

a. Earliness ______________________________________________________________________________

________________________________________________________________________________________

b. Heavy ________________________________________________________________________________

________________________________________________________________________________________

c. Puddling [soil compaction] http://www.soils.umn.edu/ Select Education, then courses; at course 2125 , select WEB laboratories thenUnit 7.

http://www.soils.umn.edu/academics/classes/soil2125/doc/slab7bd.htm

______________________________________________________________________________

________________________________________________________________________________________

d. Well-drained ___________________________________________________________________________

________________________________________________________________________________________

e. Friable ________________________________________________________________________________

________________________________________________________________________________________

6. What is good soil tilth? ____________________________________________________________________

________________________________________________________________________________________

How would one create and maintain good soil tilth? ____________________________________________

________________________________________________________________________________________

References

Brady, N. C.. 1969. The nature and properties of soils, Nineth Edition. Macmillan, New York, 9th or any later edition.

Kohnke, H. and D. P. Franzmeir. 1995. Soil science simplified. Fourth Edition. Waveland Press, Prospect Heights, Illinois.

Rodale, J. I. 1971. How to grow vegetables and fruit by the organic method. Rodale Press, Emmaus, Pa.

Russell, E. W. l961. Soil conditions and plant growth. Ninth Edition. John Wiley, New York, or Tenth Edition, 1973.

Soil Survey Staff. U.S. Dept. of Agriculture, 1937. Soil Survey Manual. USDA Handbook No. 18, Washington, D. C.

GUIDE FOR CLASSIFYING THE BASIC TEXTURAL SOIL CLASSES

In the field texture is determined by the feel of the soil mass when rubbed between fingers. The following statements give the obvious physical characteristics of the basic textural grades:

SAND. Sand is loose and single-grained. The individual grains can readily be seen or felt. Squeezed in the hand when dry it will fall apart when the pressure is released. Squeezed when moist, it will form a cast, but will crumble when touched.

SANDY LOAM. A sandy loam is a soil containing much sand but which has enough silt and clay to make it somewhat coherent. The individual sand grains can readily be seen and felt. Squeezed when dry, it will form a cast which will readily fall apart, but if squeezed when moist a cast can be formed that will bear careful handling without breaking.

LOAM. A loam is a soil having a relatively even mixture of the different grades of sand of silt and clay. It is mellow with a somewhat gritty feel, yet fairly smooth and slightly plastic. Squeezed when dry, it will form a cast that will bear careful handling, while the cast formed by squeezing the moist soil can be handled quite freely without breaking.

SILT LOAM. A silt loam is a soil having a moderate amount of the fine grades of sand and only a small amount of clay, over half of the particles being of the size called "silt". When dry it may appear quite cloddy, but the lumps can be readily broken, and when pulverized it feels soft and floury. When wet the soil readily runs together. Either dry or wet it will form casts that can be freely handled without breaking, but when moistened and squeezed between thumb and finger it will not "ribbon" but will give a broken appearance.

CLAY LOAM. A clay loam is a fine-textured soil which usually breaks into clods or lumps that are hard when dry. When the moist soil is pinched between the thumb and finger it will form a thin"ribbon" which will break readily, barely sustaining its own weight. The moist soil is 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.

CLAY. A clay is a fine textured soil that usually forms very hard lumps or clods when dry and is quite plastic and usually sticky when wet. When the moist soil is pinched out between the thumb and fingers it will form a long, flexible "ribbon". Some fine clays very high in colloids are friable and lack plasticity in all conditions of moisture.

GRAVELLY OR STONY SOILS. All of the above grades of soil, if mixed with a considerable amount of sand, gravel or stone, are designated as sandy clay loams, sandy clay, etc., as gravelly sandy loams, gravelly clays, etc., or as stony sandy loams, stony loams, etc.

EXPECTED RESPONSES TO SOIL TEXTURAL CLASS as texture varies from:

COARSE -------------------------------------------> To-------------------------------------------------- > FINE

Sand

Loamy sand

Sandy loam

Loam

Silt loam

Clay loam

Clay

Soil surface area ___________________________________________________> Increase

Pore volume _______________________________________________________> Increase

Particle density _____________________________________________________> Same

Moisture retention __________________________________________________> Increase

Bulk density _______________________________________________________> Decrease

Field capacity ______________________________________________________> Increase

Wilting percentage __________________________________________________> Increase

Infiltration rate _____________________________________________________ > Decrease

Permeability _______________________________________________________> Decrease

Leaching loss hazards _______________________________________________> Decrease

Potential of aggregation ______________________________________________> Increase

Cation Exchange Capacity ___________________________________________ > Increase

EFFECTS OF ORGANIC MATTER

LOW -------------------------------------------> To-------------------------------------------------------> HIGH

Bulk density ________________________________________________________> Decrease

Moisture retention ___________________________________________________> Increase

Field capacity ______________________________________________________> Increase

Wilting percentage __________________________________________________> Increase

Available water _____________________________________________________ Increase

Cation Exchange Capacity ____________________________________________> Increase

Aggregation (structure) _______________________________________________> Increase

Infiltration rate ______________________________________________________> Increase

Leaching __________________________________________________________> Decrease

TEXTURAL TRIANGLE

SOIL PROFILE

WATER-HOLDING CAPACITY

V =volume of water added (ml).
This value is obtained by measurement of the volume of soil in the cylinder.

W = mass (weight) of soil added = 100 g.
100 g of soil were added.

v = volume of soil (ml) wetted by the water added.
This value is obtained by measurement of the wetted soil (identified by color)
in the cylinder.

w = weight (mass in g) of soil holding the water.
This value is calculated by direct proportions:

W/V= w/v

100 g/ measured volume = x g/ measured volume

CALCULATIONS OF WATER-HOLDING CAPACITY

VOLUMETRIC (by volume) = ml of water added/volume of soil holding the water.

GRAVIMETRIC (by mass or weight) = g of water added/g of dry soil holding the water.

Remember that l ml of water is 1 g of water (at 0 ^oC).

Percent by volume = (ml water added/volume of soil holding the water, v) x 100

= (12ml/v)

Percent by weight = (g water added/mass of soil, w) x 100

= (12g/w) x 100

TOP OF PAGE

Produced and maintained by Allen Barker
University of Massachusetts, Amherst.

SOIL SEPARATE		SIZE RANGE (mm)
SANDS	very coarse sand	2.0 to l.0
	coarse sand	1.0 to 0.5
	medium sand	0.5 to 0.25
	fine sand	0.25 to 0.l0
	very fine sand	0.l0 to 0.05

SILT		0.05 to 0.002

CLAY		Below 0.002

General Terms	Basic Class Names
A. Sandy Soils (coarse-textured)	l. Sand
	2. Loamy sand

B. Loamy Soils (moderately coarse- textured, medium-	l. Sandy loam
textured, and moderately fine-textured)	2. Fine sandy loam
	3. Very fine sandy loam
	4. Loam
	5. Silt loam
	6. Silt
	7. Clay loam
	8. Sandy clay loam
	9. Silty clay loam

C. Clayey Soils (fine-textured)	1. Sandy clay
	2. Silty clay
	3. Clay

SOIL	TOTAL VOLUME	VOLUME WETTED	DRY WEIGHT OF WETTED SOIL
1.	_______________________	______________________	_______________________
2.	_______________________	______________________	_______________________
3.	_______________________	______________________	_______________________
4.	_______________________	______________________	_______________________

SOIL	BY WEIGHT (%)	BY VOLUME (%)*	BULK DENSITY, g/cc
1.	_______________________	______________________	_______________________
2.	_______________________	______________________	_______________________
3.	_______________________	______________________	_______________________
4.	_______________________	______________________	_______________________