SOIL-WATER RELATIONSHIPS:
PERMEABILITY AND
DRAINABILITY
Student Learning Objectives. Instruction in this lesson should result in studentsachieving the following objectives:
1 Define water holding capacity and gravitational water.
2 Explain how water holding capacity and permeability is affected by soil type.
3 Define permeability.
4 Describe the effects of soil type on permeability.
5 Evaluate the drainability of soils.
Anticipated Problem: What is water holding capacity and what is gravitational water?
I. Water holding capacity is the amount of moisture that is held by the soil available to the plants.
A. Gravitational water is free water that moves downward through the soil due to gravity.
Illinois Physical Science Applications in Agriculture Lesson B1–7 • Page 2
B. Soils have different levels of water holding capacity largely determined by the texture ofthe soil.
C. Soil Texture is the relative amount of sand, silt and clay in the soil.
D. Hydraulic conductivity is a measure of the rate of water movement through the soil.
E. The soil texture also affects pore space, a portion of the soil not occupied by solid materialbut filled with air or water. The larger the soil particle the larger the pore spaces.
Small particles have more spaces but they are smaller in size.
Anticipated Problem: How is water holding capacity affected by soil type?
II. Soils of different types affect the water holding capacity.
A. Soils with fine texture are composed mainly of clay and hold water very firmly. There isless gravitational water with fine textures.
B. Soils with medium texture have intermediate amounts of sand and clay, with a large
representation of silt. Medium texture soils hold water well but still allow movement
downward.
C. Soils with coarse texture represent those soils that are largely composed of sand. Coarsesoils will not hold water very firmly and have fast permeability.
Anticipated Problem: What is permeability?
III. Permeability is the movement of water through the soil, synonymous with hydraulic conductivityand drainability.
A. Permeability allows movement of water and in many cases nutrients through the soil.
B. The movement of the nutrients through the soil can be significant if the soil has much
permeability.
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Anticipated Problem: How is permeability affected by soil type?
IV. Soil type can affect permeability.
A. Medium texture allows for moderate permeability.
B. Coarse soils have rapid permeability.
C. Fine soils have slow permeability and do not drain well.
D. Drainage is the movement of excess water through the soil, removal of water from the
soil.
Anticipated Problem: How can you evaluate the drainability of soils?
V. Soil drainability is measured by the speed of which a given amount of water passes throughthe soil.
A. In a laboratory situation one may take two different soils and set up an experiment to
measure the drainability of each.
B. On field conditions the drainability may be done by measuring the time it takes a soil toabsorb 10 gallons of water as compared to another soil. This procedure is done often in
landscaping situations to determine if amendments need to be added. Soil in the field is
difficult to amend.
SOIL-WATER RELATIONSHIPS:
PERMEABILITY AND DRAINABILITY
Part One: Matching
Instructions: Match the word with the correct definition.
a. Drainage c. Hydraulic conductivity
b. Permeability d. Water Holding Capacity
______1. Measure of water movement through the soil.
______2. Amount of moisture able to be held by the soil and is available to plants.
______3. Movement of water through the soil, through pore space.
______4. Rate at which water moves through the soil.
Part Two: Fill-in-the-Blank
Instructions: Complete the following statements.
1. The smallest of the soil separates is ______.
2. A soil that has a large amount of clay is considered to be ______texture.
3. The largest of the soil separates is ______.
Part Three: Multiple Choice
Instructions: Write the letter of the correct answer.
______1. The amount of moisture that is held by the soil and is available to plants is called ______.
a. water holding capacity
b. hydroscopic water
c. cohesioned water
d. saturated water
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______2. Which soil texture is most permeable?
a. sand
b. silt
c. clay
d. loam
______3. The removal of gravitational or free water that is not directly available to the plant is called ______.
a. hydraulic conductivity
b. saturation
c. drainage
d. permeability
______4. As soil textures become more coarse, permeability rates _____.
a. increase
b. decrease
c. stay the same
d. increase and then decrease
______5. Permeability, hydraulic conductivity, porosity and soil texture are related to ______, ______,and ______of subsurface drain tile or pipe.
a. spacing, height, sizing
b. spacing, width, sizing
c. spacing, frequency, sizing
d. spacing, depth, sizing
______6. Which of the following types of soil has the slowest permeability?
a. coarse sand
b. silt
c. fine sand
d. clay
______7. Which of the following types of soil has the fastest rate of permeability?
a. coarse sand
b. silt
c. fine sand
d. clay
______8. Water in excess of capacity will drain from the soil due to ______.
a. hydraulic capacity
b. gravitational pull
c. percolation
d. saturation
Part Four: Short Answer
Instructions: Answer the following questions.
1. What is water holding capacity?
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2. What is permeability?
3. How is water holding capacity affected by texture?
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Assessment
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TS–A
Technical Supplement
SOIL-WATER RELATIONSHIPS:
PERMEABILITY AND DRAINABILITY
TEXTURE describes the relative proportion of the various sizes of mineral particles,
(sands, silts, and clays). Texture can be determined by passing a sample of soil through a
sieve to determine the percentages of sand, silt, and clay. Soil texture controls water
movement, water holding capacity, and availability and retention of nutrients.
Separation of soil particles may be accomplished by shaking a soil sample in a tall cylinderof water, then allowing the particles to settle. Compare the volume occupied by each ofthe soil separates to the known volume of the original sample, and determine a percentageof each particle type. See TM–C to determine soil textural type.
AVAILABLE SOIL MOISTURE is the difference between the amount of water in the
soil at field capacity and the amount of water in the soil at the permanent wilting point.
Field capacity (FC), permanent wilting point (WP), and available soil moisture (AM) varyby soil texture. See the table of moisture holding capacities of different soils.
Moisture Holding Capacities of Soils
Soil Texture FC% WP% AM%
Sand 09 02 07
Loamy sand 14 04 10
Sandy loam 23 09 14
Sandy loam + organic matter 29 10 19
Loam 34 12 22
Clay loam 30 16 14
Clay 38 24 14
Well structured clay 50 30 20
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STRUCTURE is the way soil particles are arranged. Single grained, granular, platy, and
blocky describe structure. Structure influences moisture relationships, water movement,
availability of nutrients, etc.
TILTH is the physical condition of the soil. Soils in good condition are mellow, crumbly,
easily worked, and take water readily.
BULK DENSITY is the ratio of a soil’s mass to its volume (g/cm3 or lb/ft3). To obtain the
bulk density of a soil, collect a sample and measure its volume. Oven dry the sample to
remove all water, and determine the dry weight. Dry weight divided by volume is the bulkdensity. Bulk density is sometimes referred to as apparent density—the real density of themineral particles themselves is approximately 166.67 lb/ft3 (specific gravity = 2.67).
POROSITY, the percentage of the soil volume that is pore space, can be determined from
real and bulk densities:
Pore space % = 100(l-As/Rs)where, As = Bulk density
RS = Real density
Porosity (pore space) may also be determined by filling the void space in a soil sample
with a known volume of water.
INFILTRATION RATE is the rate at which water enters the soiL.it is a function of structure,tilth, density, porosity, and moisture content. Infiltration rate places limitations onirrigation design. If water application rates exceed the infiltration rate, runoff and erosionmay occur.
HYDROSCOPIC MOISTURE is water held tightly by soil particles; does not move by
capillarity and is usually unavailable to plants.
CAPILLARY MOISTURE is water held in pore spaces by the surface tension between
the water and the soil particles; the primary source of water for plants.
GRAVITATIONAL WATER is free water that moves downward due to gravity; most
soils are at field capacity if allowed to drain freely after saturation for 24 hours. Soil moisturecontent near of at field capacity is ideal for most plants (provides a balance betweensoil moisture tension and aeration). See soil moisture curves, on the following page.Little water will be lost from bare soil after it has drained to field capacity. Plants willremove the moisture by transpiration.
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PERMANENT WILTING POINT occurs when soil moisture drops to where plants
wilt and remain wilted...normal growth and transpiration ceases.
WETTING PATTERNS—When water is applied slowly to the soil at a single point, it is
acted upon by the forces of gravity (downwards) and capillarity (radially outwards), producinga wetted pattern characteristic of the soil type and application rate.
Sandy soils are characterized by large voids between soils particles. These large voids exertweak capillary forces, but offer little resistance to gravitational flow. Lateral and upwardwater movement is limited; downward water movement is rapid. The wetting pattern fora sandy soil will therefore be deep with little lateral spread, and upward water movementwill be minimal.
At the other extreme, a heavy clay soil exerts strong capillary forces, but resists downwardwater movement by gravity. The wetted pattern in a heavy clay soil will tend to be broadand of moderate depth.
Application rates will also affect the shape of the wetted pattern. For example, 5 gallons ofwater applied to a soil in 5 hours will probably produce a wider, shallower wetter patternthan 5 gallons applied over a 10-hour period. Higher application rates tend to produce awider zone of saturation under the emitter, assisting horizontal movement.
Water Movement in Various Soils
Soil Type Radius Wetted Area (Ft)
Coarse sand 0.5–1.5
Fine sand 1.0–3.0
Loam 3.0–4.5
Heavy clay 4.0–6.0
Light sandy soils require higher water application rates. Heavy clays and clay loams often
benefit from a lower water application rate. This low rate avoids surface ponding and runoff,and promotes deeper water penetration. Water movement in soils determines emitterplacement.
Prepared by Philip Buriak, Associate Professor, Agricultural Engineering, University of
Illinois
References
Hardie Irrigation Micro Irrigation Design Manual, Michael J. Boswell, Third Edition,
April, 1986. Copyright—James Hardie Irrigation.
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