Activity 2: Soil Texture, Structure, and Consistence You will learn how to determine soil texture and judge the cohesive and adhesive properties of a soil sample. Background Soil texture is determined by the ratio of sand, silt, and clay in the sample. Sand, silt, and clay are all mineral components of soil, and are defined by their particle size. Particles with a diameter greater than 0.05 mm are considered sand; between 0.002 mm and 0.05 mm, silt; and less than 0.002 mm, clay. (By definition, organic matter does not contribute to soil texture. Soil scientists group soil into three broad classes based on texture: the sands, the clays, and the loams (a mixture of sand, silt, and clay. A common field-test method to determine texture is the ribbon test (see Soil Analysis Card. In this test, a small amount of soil is moistened, formed into a ball, then squeezed and pinched to form a ribbon. The behavior of the sample during the test (for example, whether it forms a ball or a ribbon and, if so, how long a ribbon determines its classification. You will be trying this field method in this exercise. Soil Structure Primary soil particles (sand, silt, and clay are arranged into secondary units called peds. The shape of the peds and the way in which they aggregate in a soil is referred to as soil structure. Soil structure affects how easily air, water, and plant roots move through soil. Human activity such as repeated trampling or plowing when wet can alter it. Soils that separate easily into rounded peds are called granular. Granular soils have high permeability and therefore do not pack tightly. They are usually found near the soil surface where organic matter is abundant. Granular soils are particularly suitable for plant growth, because their structure permits air, water, and plant roots to easily penetrate the soil. and loamy soils often have blocky peds, which are angular and somewhat irregular in shape. Their irregularity ensures that soils composed of blocky peds contain pores that permit passage of air and water Soils with plate-shaped peds, which can resemble stacked sheets of ice, are tightly packed and difficult for air and water to penetrate. Platy soils usually have a high clay content and tend to be found in frequently flooded areas. These soils are often called "clay-pan." On the other hand, sand itself is a structureless soil; the primary particles do not aggregate but instead fall apart. Soil Consistence The degree to which soil resists pressure is referred to as its consistence. Farm and construction machinery and even a herd of cattle can put a great deal of pressure on the soil, so consistence is important when considering how land should be managed. The terms sticky, plastic, loose, friable, soft, firm, very firm, and hard are used to describe the consistence of the soil and how well the soil resists effects of wind, water, and machinery. Student Instructions 9
Purpose Hypothesis Procedure, Part A Determine the texture of an unknown soil sample through both ribbon analysis and particle size distribution, and compare the results of each test. Predict which soil texture your unknown soil sample has. A. Determination of Soil Texture Your teacher will assign your group a specific soil to test. You will be testing this soil throughout the next 2 labs. Put separate samples of clay, sand, and your assigned soil into cups and take them to your work station. Fill the spray bottle with tap water. Read the procedure on the side of the Soil Analysis Chart to determine soil texture. Go through the procedure with the clay, sand, and soil samples. Texture by Feel Analysis Take a handful of soil in the palm of your hand. Mist it with tap water from the spray bottle. Soak up any excess water by adding more soil a pinch at a time. Squeeze the sample. Does it form a ball? If no, add more water. If no again, you have sand. Once you have formed a ball the size of one or two large marbles, knead it until all aggregates are broken up. Then, make a ribbon by squeezing the dirt flat between your thumb and forefinger. At the same time, push upward with your thumb until if forms a ribbon. The ribbon should run past your forefinger and may be between 1 cm and 5 cm in length. Measure the ribbon and record its length in your notebook. Use the clay sample for practice. It should form a ribbon that is at least 5 cm long before it breaks, if you have trouble getting a 5-cm ribbon, make sure your ribbon is uniform in thickness. If you have a thick area of soil at the end of the ribbon, it will serve as a weight and cause the ribbon to break prematurely. Similar problems may occur if you make an area in the middle of the ribbon thinner than the rest. Practice with the clay until you can confidently make a ribbon; then use this technique on your soil sample. Record the texture in your data table. B. Particle Size Distribution You will analyze the distribution of particle size within your soil samples to determine the percent clay, sand, and silt present in each using the particle size distribution technique. Then, compare and contrast the results from Part B with the results from Part A. Hypothesis Procedure, Part B Based on the results of your ribbon test in Part A, make a general prediction about the particle size distribution you expect to see in your sample. In this procedure, your soil sample must settle overnight for interpretation the following day. Someone in your class will set up tests with sand and clay samples to serve as interpretation guides. In addition to looking at your own sample, you will be looking at all the other samples after they have finished settling. 1. After breaking up any large clods, fill a graduated cylinder with soil. 2. Add tap water up to 25 ml. 3. Cover the cylinder and shake it for 30 seconds. Add 1 drop of dish detergent to the cylinder of soil to settle the particles. 4. Allow the sample to settle overnight.
[ Start I Soil Texture By Feel Flow Chart Place appcoximately two teaspoons of swi in youf palm. Add a few drops of water and Kneed soil to break down ^1 the aggregates Soil is at proper consistency when It feete plastic ar>d moldable, l^e moist putty. Add dry soil to soak up water Does the soil remain in a t>8il when squeezed? Is the soil too wet? Place bail of soil >etween thumb and forefinger, gently pushing the soil with your tjiumb, squeezing it upward irito a ritjbon. Form a ribbon of ihiiform Iti«cl<nes5 arvd wkjtfi. Allow the nbbori to emerge arkl extend over fixelwiger, breaking from its own weight. Does the soil fonm a r«3bon? Ho toamy Sand make a weak ribbon < V long before it tweaks? make a medium ribbon 1-2" iwig before it bfeaks? make a strong ribbon > 2" long before it bfeaks? HI i L % s A N D Na + Neitlier gritty nor smoolh? r + Excessively wet a small pinch of soil in your palm and rub it vrith your forefinger. Sandy Sitt ± Neitiier g^tty nor i f Sandy Does aay V } Sitty V soil Ho Neitiier gritty nor Mo Sandy LO ^ %CLAY HI Sitty From http://www.bigsiouxwaterfestival. org/teachers/28/index.htm
Name Group # Next Day te: Do not shake the container or otherwise disturb the layers. 1. Place a piece of white paper behind the cylinder and observe each layer. 2. Measure the volume of each layer in ml and record it in your data table. Use the control (sand/clay mix to determine boundaries between the layers. You may need to use magnification. 3. Calculate particle size distribution: Divide the volume of each soil layer by the total volume of the soil and record it in your data table. Multiply by 100. Calculate the percentages of clay, silt, and sand in your sample, and record in your data table. 4. Determine the soil type from the soil triangle. 5. Record the soil type in your data table. DATA TABLES Hypothesis: Texture/type of soil sample as determined by feel: Texture Determined by Particle Size Distribution Layer Silt Layer t. Sand Layer Total Volume % % Silt % Sand ml ml ml ml Soil Texture/type: Observations: Laboratory Questions 1. How did your results in Part B compare to the results in Part A? Which do you think is more accurate and why? 2. What happened to the soil particles in the cylinder overnight? Explain. 3. What conclusion can you draw? Be sure to write only one to two sentences and revisit your hypothesis. 4. Of all possible soil types on the soil triangle, which soil would be best to grow crops in? Why?
APES Name Lab: Soil Texture Prelab: Answer the following questions before starting the lab. 1. What is the purpose of this lab? 2. What determines soil texture? 3. What is the difference among sand, silt and clay? 4. What is the difference between soil texture and soil structure? 5. What is soil consistence? How does this relate to soil compaction? 6. If determining soil texture by measuring the percentages of the different types of particles seems so direct, why would one need to know how to determine soil texture by hand? 7. Write a hypothesis: How do you expect the results of the soil texture by feel and the soil particle distribution texture to compare? (Do you expect the results to be the same for both tests or different? 8. Why do you expect these results?