Soil Textures or Fractions Teacher Instructions

Transcription

1 Soil Textures or Fractions Teacher Instructions Introduction: The relative proportion of the various mineral grain sizes, such as sand, silt, and clay, in a soil is called the soil texture or the soil fraction. These mineral grains, derived from rock, are added to soils through the processes of weathering and erosion. To describe soil texture, names such as loamy sand, silt loam, clay loam and silty clay are used, as is shown in the background information section below. Since the particles vary in size, each type of particle will settle at a different rate when suspended in solution and this provides a basis for a quantification of the percentage of sand, silt, and clay in soil. Because sand particles are the largest, they will settle out quickly and the intermediate particle size of silt causes it to settle out before the finely divided and easily suspended clay particles. The chart in the background information section below summarizes the general sizes of sand, silt, and clay particles. In this protocol you will be using A horizon soil only. There are two methods provided in this protocol. You will need to select between Method A and Method B. Both methods are outlined in the student section, and both require approximately the same amount of time. This protocol in collaboration with the other Physical Soil Protocols allows students to gain a better understanding of the soil composition of their study site. Objectives: 1) Follow the selected soil texture protocol 2) Determine the proportion and type of soil particles in the soil sample 3) Classify soil sample according to particle size Students Required: Partners Estimated Time: 3 class periods (Method A: Day one 50 minutes, day two 50 minutes, day three minutes) (Method B: Day one 50 minutes, day two minutes, day three minutes) Overview of Tasks: 1) Select the method the students will be using, and provide all needed materials and copies. A) Method A has students measuring amounts of each type of soil particle. This method requires you to prepare a 5% Calgon solution by massing 5 g Calgon (sodium hexametaphosphate) and dissolving in 95 ml of distilled water. There will be a 30 minute waiting period where supplemental material can be used. B) Method B has students following a flow chart to determine amounts of each type of soil particle. 2) Review protocol and background information with students prior to the protocol. 3) Assign partners and allow students to follow the selected protocol. 4) Have students write a lab report in whatever format works best for your class.

2 Soil Textures or Fractions Student Instructions Introduction: The relative proportion of the various mineral grain sizes, such as sand, silt, and clay, in a soil is called the soil texture or the soil fraction. These mineral grains, derived from rock, are added to soils through the processes of weathering and erosion. To describe soil texture, names such as loamy sand, silt loam, clay loam and silty clay are used, as is shown in the background information section below. The best soils generally contain 10 to 20% clay and have approximately equal amounts of silt and sand. Additionally, healthy soils also have a fair amount of organic matter. However, organic matter is not a mineral component of the soil and is not measured in this protocol. Since the particles vary in size, each type of particle will settle at a different rate when suspended in solution and this provides a basis for a quantification of the percentage of sand, silt, and clay in soil. Because sand particles are the largest, they will settle out quickly and the intermediate particle size of silt causes it to settle out before the finely divided and easily suspended clay particles. The chart in the background information section summarizes the general sizes of sand, silt, and clay particles. Your instructor will select which method you will be using A or B. This protocol in collaboration with the other Physical Soil Protocols allows students to gain a better understanding of the soil composition of their study site. Materials: Method A #10 sieve *5 % Calgon Solution (used as a dispersing agent) Dropper Distilled Water Two- 25 ml graduated cylinders with stoppers Stopwatch or clock Soil sample Method B Soil sample Water Texture flow chart Balance

3 Procedures: Each group will follow these procedures. You will aggregate your results. Day 1: Methods A and B (50 minutes) 1) Break up into groups of two students. 2) Review selected protocol and background information first 3) Select the study site locations where each group will extract soil. 4) Go to study site(s). 5) Record any observations about the site (e.g. types of plants around, development, soil moisture, amount of leaf litter). 6) Follow the Soil Sampling Strategies Instructions (which can be found at: to collect your soil samples. Be sure to collect enough soil at each site to be able to follow your selected method: Method A: You will need at least 5 ml of soil for each group. Method B: You will need at least 25 g of soil for each group. 7) Back in the classroom dry your soil samples over night using an oven set to 150 F or under a warm desk lamp Day 2: Method A (50 minutes will have 30 minutes of down time): 1) Grind and sieve the soil through a #10 sieve if it has not already been done. 2) Fill one 25 ml graduated cylinder (#1) to the 5 ml mark with soil. 3) Add 1 ml of 5 % Calgon solution to graduated cylinder (#1). 4) Add distilled water to graduated cylinder (#1) to the 15 ml mark. 5) Put in stopper and shake the soil sample for 5 minutes. 6) After shaking the sample for 5 minutes, allow the sample to settle for exactly 30 seconds. 7) After 30 seconds, gently pour ALL the soil solution from the first graduated cylinder (#1) into the second graduated cylinder (#2) (sand particles will be settled at the bottom of the cylinder and will not pour do not force them out of cylinder #1). 8) Allow the soil solution in graduated cylinder (#2) to settle for 30 minutes. 9) At the end of 30 minutes, gently pour the remaining soil solution in graduated cylinder (#2) into the trash can or outdoors (again particles will be settled at the bottom of the cylinder and will not pour do not force them out of cylinder #2). 10) Measure the volume of soil left in graduated cylinder #1 (sand which settled in 30 seconds). Record on data sheet under the sand category. 11) Measure the volume of soil left in graduated cylinder (#2) (silt which settled in 30 minutes). Record on data sheet under the silt category 12) Using the data sheet below, calculate the amount and percentages of soil particles. Classify your soil sample.

4 Method B (20-50 minutes) 1) Place 25 g of soil in the palm of your hand. 2) Add water one drop at a time and knead to break down soil aggregates. Add enough water so that the clump of soil feels like putty. 3) Proceed through the soil classification flow chart below to determine the texture classification of the soil. Day 3 (20-40 minutes): Methods A and B 1) Each group should share their findings/data with the class. This should include a site description. 2) Interpret results and discuss. The following questions are designed to help you make meaningful conclusions. a. Which site had the highest percentage of clay? Sand? Silt? b. If you conducted the Soil Moisture Protocol: Was there a relationship between the soil type and the soil moisture content? Please describe. c. Do you think the soil type impacted the surrounding area? (e.g. types of plans present, types of micro organisms present). Please explain your answer.

5 Background Information: Vocabulary: sand, silt, clay, organic matter, and mineral Soil Texture Chart: Soil Size Chart: Type of Particle Diameter # of Particles in 1 gram Surface Area in 1 gram Sand 2 mm cm3 Silt 0.2 mm 90,000, cm3 Clay mm 9x ,000 cm3

6 Name Method A Data Sheet Amount of Original Soil Sample (ml) SAND (ml) 5 ml ml SILT (ml) ml CLAY (ml) (amount of original soil sample) (sand + silt) ml Percentage of Soil Particles (%) (Use the above Data) SAND % = (SAND) / (Amount of Original Soil Sample) x 100 = % SILT % = (SILT) / (Amount of Original Soil Sample) x 100 = % CLAY % = (CLAY) / (Amount of Original Soil Sample) x 100 = % Soil Classification (Use Soil texture Pyramid from the Background Information section) My Soil sample is classified as

7 Method B Soil Classification Flow Chart My soil sample is classified as