Procedure for Determining Soil Texture

Lab #4

Soil Texture

Today’s lab will determine the soil texture of your soil sample and understand the procedure to accomplish this task.

Additionally you will teach yourself how to do the ribbon test to determine the soil’s texture.

Place each of your soils (your soil from the surface and soil from 2 feet below) into two straight-sided jars filling them up approximately 60% of the jars capacity.

Additional lab requirements:

You will also make a grid of each student’s soil texture and the location of their soil on a map of the greater Stanislaus area. Be sure to note any tendencies based on the soil’s location.

What did we learn from looking at all of the findings together?

Munsell color chart your two soil samples.

Remember to bring your 3-4 cups of oven dried soil to class next week!

PROCEDURE FOR DETERMINING SOIL TEXTURE

1. Select a straight-sided bottle and fill approximately 3/5 full of soil.
2. Add water up to 1 inch from the top. Put a strip of masking tape up one side of the bottle.

1. Add 5-6 drops of ammonium hydroxide (household ammonia) or 20 ml (4 teaspoons) of an 8% solution of Calgon to separate the sand, silt and clay. (To make an 8% Calgon solution, mix 6 tablespoons of Calgon per quart of water.)

1. Mix for 20 minutes by rotating through 180.

1. Set the bottle down, let it stand for 45 seconds, and mark the sediments level. It represents the sand.

1. At the end of 30 minutes mark the sediment level in the bottle this is the silt line. Note how slowly the soil particles are settling. The clay is still in suspension.

1. After two or three days, study the soil in the bottle carefully. The sand will be easily identifiable because if you can see the individual particles then it is sand it will be very near the 45-second mark. The silt should form after 30 minutes however, it may have already settled. The clay will be a thin third layer (in our sandy soils) lying on top of the silt. Mark these layers.

1. Measure the total depth of soil in the bottle, in millimeters (mm). Then measure the depth of each layer. If the clay fraction is still in part suspended (as indicated by cloudy water), add 1-2 mm to the clay reading depending on the density of the suspension. Record these readings.

1. Using the following bulk density factors calculate the % by weight of each of the primary soil separates.

Sand= 1.5 bulk density

Silt= 1.3 bulk density

Clay= 1.2 bulk density

To calculate % by weight:

1. mm of depth of separate layer x bulk density = weight relationship (WR).

1. Add the 3 weight relationships to obtain a total weight relationship (TWR).

1. (WR  TWR) x 100 = % by weight of each separate.

(Please turn page for sample problem solution.)

1. Using these percentages by weight for each separate, determine the soil class by use of the textural triangle.

SAMPLE PROBLEM SOLUTIONS:

I.Sand layer measured 75 mm

Silt layer measured 12 mm

Clay layer measured 5 mm

1. mm of depth of separate layer x bulk density = weight relationship (WR)

Sand 75 mm x 1.5 =112.5 WR

Silt 12 mm x 1.3 = 15.6 WR

Clay 5 mm x 1.2 = +6.0 WR

134.1 TWR (Total Weight Relationship)

1. Add 3 WRs to obtain TWR = 134.1 TWR (Total Weight Relationship)

1. (WR  TWR) x 100% = % by weight of each separate

Sand (112.5  134.1) x 100% = 83.9%

Silt ( 15.6  134.1) x 100% = 11.6%

Clay ( 6.0  134.1) x 100% = 4.5%

100.0%

The soil class is LOAMY SAND.

II. Sand layer measured 26 mm

Silt layer measure 21 mm

Clay layer measured 19 mm

1. mm of depth of separate layer x bulk density = weight relationship (WR)

Sand 26 mm x 1.5 = 39.0 WR

Silt 21 mm x 1.3 =27.3 WR

Clay 19 mm x 1.2 = 22.8 WR

89.1 TWR

1. Add 3 WRs to obtain TWR = 89.1

1. (WR  TWR) x 100% = % by weight of each separate

Sand (39.0  89.1) x 100 =43.8%

Silt (27.3  89.1) x 100 = 30.6%

Clay (22.8  89.1) x 100 = 25.6%

100.0%

The soil is class LOAM

TEXTURE LAB Worksheet

Questions and activities that will be completed by the laboratory participant.

1. Measure jars to determine amount of Sand, Silt and Clay. (Use mm.)

1. Calculate percent Sand, Silt and Clay.

1. Plot percentages on soil triangle to determine their texture.

Top Soil ______Sub soil ______

1. Make a determination of Soil Texture of both.

a)Describe the feel of your soils- Then what is the texture using the ribbon test.

Top Soil- ______

Sub-Soil-______

Now take additional sample of your soil and do the ribbon test and see if you come up with the same answer.

Check other students’ soils obtain as many different textures as possible and compare their ribbon tests to their actual soil textures. Be sure to record your observations as to how accurate your ribbon testing is!!

Person’s soil ribbon length feel Soil texture Actual Texture

a. ______

b. ______

c. ______

d. ______

e. ______

1. Explain what type of agriculture your soil texture could support.

1. Discuss any soil management problems that might result with use of this soil and their solution.

1. Which soil sample in #4 has the greatest surface area?

1. Complete the color tests on your soils using the Munsell color chart on the top-soil and sub-soil and record your findings.

Munsell Color

Topsoil ______

Sub-soil______

9. How does organic matter affect the texture of soil?

10. How might the following be affected by soil texture?

a. Tillage practices–

1. Erosion –

c. Drainage -

11. Explain how the soil textures found in a soil profile can be both an inherited trait from the parent material and an acquired trait as a result of the soil development process?

12. Determine the surface area (cm2/gm) for sand, silt and clay based on the data provided in table 2-1 of this handout found on page 8.

13. Red soils are commonly referred to, as clayey and dark soils are considered high in organic matter. Does soil color determine soil qualities or vice a versa? What is the relationship between soil color and the soil’s characteristics?

14. Given what you have just learned how will you apply this knowledge to your future agricultural endeavors?

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LAB # 4

Texture of all student’s Soil

Name Location Texture

Top Sub

Soil Texture

Each soil is a porous mixture of mineral particles, organic matter, air, and water. Its properties depend on the nature and amount of each constituent. The mineral particles in a typical surface soil in good condition for plant growth will comprise about one-half of a soil’s volume. A useful way to describe the mineral fraction of soil consists of separating particles into three size categories; sand, silt, and clay, called soil separates. Sand (2.0—0.05 mm) is the largest, silts (0.05—0.002 mm) are intermediate, and clays (less than 0.002 mm) are the smallest mineral particles. Soil texture describes the proportions of sand, silt, and clay in a soil. Their abundance or scarcity affects nearly every aspect of soil use and management.

An outstanding feature of a soil is the tremendously large surface area that the many small particles provide. A single handful of soil may expose as much as 5 to 10 acres of surface area. Surface area is important because it provides places for chemical and physical reactions to occur. For example, available nutrients and water are absorbed, stored, and transferred on soil particle surfaces. Surfaces provide sites for soil particles to attach to each other and form stable aggregates that determine soil porosity and govern many water and aerations. Soils that expose large amounts of surface will also weather more rapidly.

As the average particle size decreases, the amount of surface area per unit weight, called specific surface area, increases. Sands exhibit a very low specific surface area and silts only slightly more. However, the layered arrangement of clay crystals create multitudes of interior surfaces and contributes to very large specific surface area values. Table 2-1 shows the representative distribution of surface area in three different soils. It should be obvious that it takes only a small amount of clay to completely overwhelm and dominate the surface area phenomena in a soil.

Table 2-1.

Distribution of surface area in a 100 g soil sample of three different textures.

Texture and percent composition (sand-silt-clay)
Sandy loam Silt loam Silty clay loam
(65-25-10) (20-60-20) (15-55-30)
Sand
Silt
Clay / -- cm2 --
1,950 600 450
37,500 90,000 82,500
30,000,000 60,000,000 90,000,000

Soil texture is a rather permanent feature of a soil, unless subjected to rapid erosional depositions or removals. Texture properties are an inherited trait reflecting the mineralogical composition of the parent material, but do undergo slow changes during profile development. The most pronounced change is associated with an illuvial accumulation pf clays in the B horizon. Clays that are formed in the surface horizons where weathering is most intense may be moved within the profile by percolating water. A complete characterization of a soil includes a texture determination of the various horizons.

Each soil manager needs to be aware of soil texture. A summary of water, tillage, erosion, and chemical factors of soil management related to soil texture is given in table 2-2.

In addition, soil texture affects the suitability of soil for building and construction use, urban planning, pollution control, and forest, range, or recreational utilization.

This exercise demonstrates the use of the textural triangle for designating soil textural class names and presents a method for determining soil texture without the use of laboratory equipment.

Table 2-2.

Soil management factors related to texture

Sandy Soils Loamy Soils Clayey Soils
Water Relations
Infiltration—entry of surface water into Rapid Medium-slow Rapid, if cracks
the soil. Opposite of runoff present very slow,
potential. if no cracks
Percolation—internal water drainage Excessive Good Fair-Poor
and leaching.
Water Storage—available for plant use Very low Medium High
Aeration—movement of oxygen into Very good Moderate Poor
rootzone
Tillage and Erosion
Tillage Power Required—drawbar pull Low Medium High
Tillability—ease of seedbed preparation Easy Medium Difficult
Erosiveness—Wind detachment and Moderate High Low, if more than
transport 27% clay
Erosiveness—Water detachment and Low High Low—Medium
transport
Chemical Relations
Fertility potential—nutrient absorption Low Medium High
Chemical Recommendations—Rates per Low Medium High
acre

Part I. The soil textural triangle and its use

Soil mixtures that have similar distributions of sand, silt, and clay, and hence have similar properties, are grouped into the same soil class. Twelve soil textural classes are recognized and their compositions are designed on a textural triangle (fig. 2-1). Soil class names consist of the terms sand, silt, clay, and loam used as nouns, adjectives, or both. It is essential to commit to memory these twelve class names for future reference in this course. When the percentages of sand, silt, and clay in a soil sample are known, the class name can be determined by plotting these values on a textural triangle using the following technique.

1. Procedure for using the textural triangle to determine soil class name.

When plotting values on a triangular graph, special note must be taken about the proper procedure. The rule for plotting values is as follows: The value is projected from an axis along a line parallel to the line which meets the zero point of the axis. For a soil that contains 55% clay, 32% silt, and 13% sand the procedure would be:

1. Locate 55% on the clay axis and project a line parallel to the bottom of the

triangle in the manner shown by the dotted line in figure 2-1.

2. Locate 32% on the silt axis and project a line parallel to the clay axis as

shown in figure 2-1. The intersection of these two lines occurs in the area

designated “clay”.

3. As a check, located 13% on the sand axis and project a line parallel to the

silt axis. If all three lines intersect at the same place, the class name has

been determined correctly.

Soil Texture Triangle

Figure 2-3. Texture triangle showing the percentages of sand, silt, and clay in the textural classes. The intersection of the dotted lines shows that a soil with 55 percent clay, 32 percent silt and 13 percent sand has a clay texture.

Part II. Determination of soil texture by the feel method

It is possible to identify the texture of a soil by simply feeling or manipulating a moist sample. This method is used whenever exact proportions are not needed or when equipment needed to obtain an exact analysis is not available, such as in the field. It requires considerable practice to become highly proficient with this method, however, the novice can quickly obtain satisfactory results by practicing the following technique and making use of the simplified textural triangle (fig. 2—2).

A.The Modified Textural Triangle

I.Think of the textural triangle as modified in figure below. This version omits the rather rare silt class and combines the loamy sand class into the sandy loam category for simplicity. Other class boundaries are arranged in a symmetrical fashion that should be more easily visualized and remembered.

2.Basically the modified triangle consists of three tiers based on clay content: (I) the CLAYS, those soils high in clay as exhibited by being strongly cohesive and capable of being shaped into good ribbons; (2) the CLAY LOAMS which are intermediate in clay content, cohesiveness, and ribbon formation, and (3) the LOAMS, or soils low enough in clay content so as to exhibit little or no cohesiveness and mold into poor or weak ribbons. The sands do not form ribbons.

3.The three tiers described in step #2 are each subdivided into three classes. If silt is very prominent, the prefix silt or silty is added to form the categories on the right side of the triangle. If sand is very prominent, the prefix sandy is added to form the categories on the left side of the triangle. When neither sand nor silt predominate, the intermediate category designating the tier level is used without a prefix.

B.Determination of Texture by Feel

The texture by feel method utilizes two manipulations that allow soil class to be determined.

I.First, a moist soil sample is formed into a ribbon. When water is added to a soil, the clay becomes plastic and pliable and can be molded into a ribbon. The length and strength of this ribbon depends a great deal on the clay content, of the soil. This test delineates the tier of the modified textural triangle in which the sample belongs.

2.Second, the contribution of sand and silt to the sample is estimated by rubbing a small bit of wet soil into the palm. Sand particles are large enough to feel gritty while the smaller silts feel exceptionally smooth. This procedure determines which, or if any, prefix is added to the tier designation.

3.Now, follow the flow diagram in figure 2—5 and practice the technique first on samples for which the textural class is known. Refine your technique with practice, then determine the texture of some unknown samples. Record your results on the data sheet.

Fig 2-5

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