A manual of soil physics

Front matter

THE BIFLEX BINDER NOTEBOOK

Number

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Preface

The exercises described on the following pages are intended to give the student an understanding of the origin, composition, and physical properties of soils and to show the relations of these properties to methods of soil management. The work outlined in the manual is sufficient for two semestprs, but may be completed in less time if more than two laboratory periods are held per week.

While these exercises are the outcome of ten years' experience in teaching the important physical properties of soils, they are not all original with the authors. Material has been drawn from many sources and arranged to give the best applications to important principles of soil management.

In connection with each exercise, references have been given and questions asked. It is hoped that this method of study will enable the student to obtain a great amount of information relative to the application of each exercise.

THE AUTHORS

University of Nebraska, College of Agriculture Lincoln, Nebraska

[iii]

Contents

PAGE

Introduction 1

KXERCISE

II No. 1

No.

Fig. 3.

The two designs of compacting machines used in these exercises hygroscopic moisture content is not known, it must be determined with a separate sample (see Exercise XII).

Calculation. To obtain the apparent specific gravity of the soil, divide the weight of the water-free soil by the weight of the same volume of distilled water.

Per cent pore spaee = 100 - Apparent specific gravity x m

Absolute specific gravity

Tabulate the results in a form similar to the following :

Soil

Weight of Empty Tube

Weight of Filled Tube

Weight of Soil

Hygroscopic

Water

Weight of Water-free Soil in Tube

Sand

Volume of Cylinder

Weight of lcc. Water-free Soil

Apparent Specific Gravity

Absolute Specific Gravity 1

Soil

Space

Per Cent Pore Space

Sand cc.

From results obtained in this exercise, compute in pounds the weight per cubic foot and weight per acre foot of each soil.

Soil

Weight per Cubic Foot in Pounds Pounds per Acre Foot

Sand

1 The average absolute specific gravity of most soils is 2.65.

VOLUME DENSITY AND POROSITY References :

King: Physics of Agriculture, pp. 108-117.

Wiley : Principles and Practices of Agricultural Analysis, Vol. I,

1. Explain the difference between absolute specific gravity and

apparent specific gravity.

2. Given a soil having an apparent specific gravity of 1.1 and a

pore space of 58.5 per cent, what is the absolute specific gravity ?

3. Given three soils, having pore spaces of 65, 55, and 45 per

cent respectively and an absolute specific gravity of 2.65 each, what is the apparent specific gravity of each soil?

4. The apparent specific gravity of soils in the field may be

taken as an approximate indication of the tilth of the soil, Explain.

5. What factors influence the apparent specific gravity of

soils ?

a

EXERCISE IX

NUMBER AND SURFACE AREA OF SOIL PARTICLES

Object. The object of the exercise is to ascertain the number and surface area of the particles in a given volume of different soil separates.

Procedure. Considering the particles in each separate to be perfect spheres and arranged in a simple columnar structure, find the average number and surface area of the particles in a cubic foot of each of the soil separates, from coarse sand to clay inclusive. In obtaining the average diameter of a separate, use the average of the largest and smallest diameters. Indicate the method used in making the computation and tabulate your results in a form similar to the following:

Soil Separate

Number of Particles per

Surface Area per Cubic

Cubic Foot, in Millions

Foot, in Acres

Coarse sand

Medium sand

Fine sand

Very fine sand

Silt

Clay

King: Physics of Agriculture, pp. 109, 117-122.

King : Soil Management, pp. 161-187.

Widtsoe : Principles of Irrigation Practice, pp. 8-13.

1. What are the difficulties in determining the surface area of

soil particles accurately ?

2. Would the number of soil particles per cubic foot be in

creased or decreased if the soil had a single, or composite, structure? Explain.

3. What relation exists between the number of soil particles

and the total surface area of all soil particles in equal volumes of different soil classes ?

4. Is there any relation between the total surface area of all

soil x>articles per volume of soil and the water-retaining capacity? Explain.

5. Is there any relation between the total surface area of all

soil particles per volume of soil and the rate of chemical solution by which the plant-food constituents contained in the mineral particles become available for plant use ? Explain.

6. What influence does the total surface area of all soil par-

ticles per volume of soil have upon the quantity of food materials retained therein in a semiavailable form ? Explain.

EXERCISE X

FLOW OF AIR THROUGH SOILS

Object. The object of the exercise is to study the flow of air through different classes of soil.

Materials needed. Three percolation tubes ; aspirator; large bottle ; graduate ; sand ; surface soil ; subsoil.

Procedure. Fill the three percolation tubes, one with each class of soil, using the compacting machine in filling the tube, allowing the weight to fall three times from the 12-inch mark for each small measure of soil that is added to the tube.

After filling the bottle of the aspirator with water, attach the tube that ends near the top of the bottle to the tube containing the sand. By means of the other tube, siphon the water from the aspirator into the large bottle, which should be placed on the floor. Record the number of cubic centimeters of air that will flow through the sand in a given time. The amount of water, in cubic centimeters, that flows from the bottle is equal to the amount of air that is drawn through the soil.

Fig. 4. Bottle aspirators, and apparatus for studying flow of air through soils

In a similar manner, determine how many cubic centimeters of air will flow through the surface soil and through the subsoil in the same length of time.

Record your results in a form similar to the following :

Sul L

Sand

Cubic Centimeters of Air in 10 Minutes

King : The Soil, pp. 229-232.

King: Physics of Agriculture, pp. 125-137.

1. How do you account for the air flowing through the sand

so much faster than through the subsoil ?

2. What kinds of soils are naturally well aerated?

3. What kinds of soils are naturally poorly aerated?

4. Name several methods by which aeration of soils that are

naturally well aerated may be reduced.

5. Name several methods of increasing aeration in compact,

fine-textured soils.

6. What relation does aeration have to growth of organisms

in the soil ?

EXERCISE XI

PERCOLATION OF WATER

Part I

Object. The object of the exercise is to note the rate of flow of water through different classes of soil.

Materials needed. Three percolation tubes; three beakers; graduate ; rubber tubing ; sand ; surface soil ; subsoil.

Procedure. Fill the percolation tubes to within 3 inches of the top, one with each class of soil. In filling the tubes, use compacting machine No. 1, allowing the weight to fall three times from the 8-inch mark for each small measure of soil added to the tube. Place about an inch of gravel on the top of each soil, to prevent puddling.

Connect the tubes by means of short pieces of rubber tubing, and, by means of a longer piece, connect to the

water cock. Arrange the tubes so that the water will flow over the top of the soil in each tube, the surplus being conveyed into the drain. Before taking the readings, allow the water to percolate through each soil a week.

Determine the number of cubic centimeters of water that will flow through each soil in one half hour.

I m m i

Fig. 5. Apparatus for studying percolation of water through soils

Reduce the results of Exercise X to a ratio and compare with the ratio of water movement.

Tabulate your results in a form similar to the following : M

Kind of Soil

Sand

Cubic Centimeters in 30 Minutes

Relative Ratio of Water Movement

Relative Ratio of Air Movement

Part II

Materials needed. Three 8-foot percolation tubes ; plotting paper ; loam ; silt loam ; clay. (The tubes should be prepared by the laboratory assistant.)

Procedure. Take readings, every period, of the rate of percolation in each tube, recording your results in a convenient form. After the water has percolated through the entire column, take readings on the amount of water that percolates through each soil in twenty-four hours.

After all the data have been collected, plot curves showing the rate of water movement through each soil.

Hilgard : Soils, pp. 221-228.

Widtsoe: Dry Farming, pp. 111-116.

Hall : The Soil, pp. 75-79.

King: The Soil, pp. 170-173.

1. What factors influence the flow of water through soils ?

2. What objections to a sandy subsoil does this exercise show ?

3. In what practical way may a clay subsoil be improved ?

4. Will water percolate most rapidly through a dry, a moist,

or a wet soil ? Explain your answer in full.

EXERCISE XII

DETERMINATION OF HYGROSCOPIC MOISTURE

Object. The object of the exercise is to make a determination of the amount of hygroscopic moisture in air-dry soil.

Materials needed. Three tin boxes; balance; drying oven; sand ; surface soil ; subsoil.

Procedure. Put a 20- or 30-grain sample of each air-dry soil into a weighed tin box, and heat in a drying oven at 110° C. until a constant weight is reached. (Constant weight is usually reached, with most air-dry soils, in two and one-half hours.) The loss in weight is due to the loss of hygroscopic water.

Determine the per cent of hygroscopic moisture in each sample of soil, using water-free soil as the basis of calculation.

Tabulate the results in a form similar to the following :

Weight

of Can

WeightofCan and Soil

Weight ok Hygroscopic Water

Water-free Soil

Per Cent of Hygroscopic

Water

Soil

Before heating

After heating

Sand

(iniins

Hilgard: Soils, pp. 195-201.

1. Define hygroscopic water.

2. Name the soil and climatic factors that influence the hygro-

scopic capacity of soils.

3. What influence does the porous condition of the various

soil materials have upon the amount of hygroscopic water in a soil?

4. Is hygroscopic moisture of utility to planl growth? l)i-

at length.

EXERCISE XIII

CAPILLARY RISE OF WATER IN SOILS

Object. The object of the exercise is to make a detailed study of the upward movement of the water in different classes of soil when in different conditions.

Part I

Capillary Rise of Water in Loose Soils

Materials needed. Three 6-foot sections of 2-inch tubing; sand ; surface soil ; subsoil ; ruler ; muslin ; plotting paper.

Procedure. Fill three tubes, one with each class of soil, exercising care to compact the soils as little as possible. After placing the tubes in the tube rack and adding water, take readings every half hour during the period. Also take readings every day for a week and then once a week until the end of the exercise.

Part II

Capillary Rise of Water in Compact Soils

Materials needed. Three 6-foot sections of 2-inch tubing; sand ; surface soil ; subsoil ; ruler ; muslin ; plotting paper.

Procedure. Fill three tubes, one with each soil. Compact the tubes by tapping the sides of the tube six times after each measure of soil has been added. The water should be added to these tubes at the same time it is added to the tubes in Part I. Take all readings as directed hi Part I.

Part III Determination of the per cent of Capillary Water

Materials needed. Soil containers ; balance ; drying oven.

Procedure. When the capillary water has risen as high as it will by capillarity or has reached the top of the tube, obtain samples from each tube representative of each six inches from the top of the water table to the top of the moist column. Determine the per cent of moisture in all six tubes for every 6-inch section.

Part IV Capillary Rise of Water in Loam, Silt Loam, and Clay

Materials needed. Three 8-foot tubes; loam; silt loam; clay; plotting paper. (The tubes should be prepared by the laboratory assistant.)

Procedure. Take readings, every period, of the rate at which the water rises in the three classes of soil, recording your results in a convenient form.

After all the data for Parts I, II, and IV have been collected and tabulated in a convenient form, plot curves showing the capillary movement of the water as observed in each part.

Hilgard : Soils, pp. 201-216.

Widtsoe : Dry-Farming, pp. 106-111.

Hall : The Soil, pp. 97-102.

Lyon and Fippin : Soils, pp. 169-189.

1. In which soil was the water rising most rapidly at the end of

the first hour ? the 48th hour? the 112th hour? Explain.

2. What effect does compacting a soil have upon the capillary

movement of the water ?

CAPILLARY RISE OF WATER IN SOILS

8. What factors influence the amount of water drawn up in soils ?

4. What factors influence the rise of capillary water in soils?

5. What is the effect on capillarity if wet from the top as in

rains? Explain in full.

6. Is the quantity of water in the moist column uniformly

distributed? Explain in full.

7. Why is the daily rise of water not uniform ?

EXERCISE XIV

EFFECT OF ORGANIC MATTER AND DRY CLODS UPON CAPILLARITY

Object. The object of the exercise is to note the effect of organic matter and dry clods upon the capillary rise of water

in soils.

Materials needed. Stand with three glass tubes ; three beakers ; ruler ; muslin ; surface soil ; clods ; compost ; sawdust.

Procedure. Tie a piece of muslin over the bottom of each tube and fill each with 12 inches of surface soil. Place a 1-inch layer of clods in one tube, a 1-inch layer of compost in another, and a 1-inch layer of sawdust in the third ; then fill the remainder of each tube with surface soil. Place the tubes in the stand and fill the beakers about half full with water. Every thirty minutes for the remainder of the period take accurate readings of the rate at which the water rises in each tube.

Horns

Snl LS n o

Surface soil and compost . . .

Surface soil and sawdust . . .

[ncht s fitches

Fletcher: Soils, pp. 89-96.

1. What effect does a layer of coarse organic matter have upon

the capillary rise of water?

2. What may be the effect of plowing under coarse organic

matter just prior to planting corn ?

3. What may be the effects upon capillarity if land which is

to be planted to corn is plowed the previous fall ?

4. Is there any reason suggested in the exercise for plowing

the wheat land several weeks before sowing the Beed? Explain.

*

EXERCISE XV

POWER OF SOILS TO RETAIN WATER AGAINST PERCOLATION

Object. The object of the exercise is to study the water-retaining power of soils in different degrees of compactness and containing various amounts of organic matter.

Materials needed. Two stands of six tubes each ; sand ; surface soil ; subsoil ; compost or sawdust ; 4-gallon jar ; scales.