Soil Textural Triangle Name: _________________________________________________ Period : _______ Date: _________ Essential Question: How do I classify the types of soil using the soil triangle? A soil texture triangle is used to classify the texture class of a soil. The sides of the soil texture triangle are scaled for the percentages of sand, silt, and clay. Soil Texture is one of the first things determined when a soil is examined. It is related to weathering and rocks as parent materials. The differences in horizons may be due to the differences in texture of their respective parent materials or rocks where soil formed. First, look at the orientation of the percentages on the sides of the triangle. The numbers are arranged symmetrically around the perimeter. On the left the numbers correspond to the percentage of clay, and on the right the numbers correspond to the percentage of silt. At the bottom of the triangle chart are the percentages of sand. To classify a soil sample, you find the intersection of the three lines that correspond the three proportions. On the chart, all of the percents will add up to 100%. Example: Classify a soil sample that is 30% clay, 15% silt, and 55% sand. First locate 30% on the clay axis, and draw a line horizontally from left to right. Next, locate 15% on the silt axis, and draw a line going down diagonally to the left. Finally, locate 55% on the sand axis, and draw a line going up diagonally to the left. The intersection is in a region called Sandy Clay Loam. See figure on the right. In some environments clay aggregates form that are so strongly cemented together that they feel like fine sand or silt. In humid climates iron oxide is the cement. Humid air can hasten the process of rusting, which an example of chemical weathering. Chemical weathering occurs when the minerals within the rocks are chemically altered. Oxidation represents a form of chemical weathering where oxygen combines with elements in the rock to form oxides. Iron-rich rocks provide a simple example of this: Oxidation results in a rusting effect similar to the rust found on exposed iron. In desert climates silica is the cement and in arid regions lime can be the cement. It takes prolonged rubbing to show that they are clays and not silt loams.Some soils derived from granite contain grains that resemble mica but are softer. Rubbing breaks down these grains and reveals that they are clay. These grains resist dispersion and field and laboratory determinations may disagree. Clarifying Questions: 1. What is a soil texture triangle? ____________________________________________________________________________ ____________________________________________________________________________ 2. What is one of the first things determined in doing soil analysis or examination? ____________________________________________________________________________ 3. Describe the comparative sizes of gravel, sand, silt and clay. ____________________________________________________________________________ ____________________________________________________________________________ 4. What is the soil parent material? ____________________________________________________________________________ 5. How is soil classified? ____________________________________________________________________________ 6. Why do farmers determine the texture and pH level ( acid 1-5, base 7-14) of their farm soil? Think!_______________________________________________________________________ ____________________________________________________________________________ ____________________________________________________________________________ 7. How is iron oxide affected by humid climates? ____________________________________________________________________________ ____________________________________________________________________________ 8. What is the cement in rocks in the desert? ____________________________________________________________________________ 9. What is the cement in rocks in the arid or dry regions? ____________________________________________________________________________ Use the Soil Textural Triangle below to answer questions 10-13. 10. What type of soil is made of 10 % sand, 80 % clay, and 10% silt? ________________________________ 11. What type of soil is made of 60% sand, 10% silt, and 30% clay? ________________________________ 12. What type of soil is made of 50% sand, 40% clay, and 10% silt? ________________________________ 13. What kind of soil is made of 40% sand, 40% silt, and 20% clay. It is the best soil for most plants. ________________________________ 14. The three sides of the soil textural triangle are represented by the percent (%) of sand, ___________, and _____________ http://soils.usda.gov/education/resources/lessons/texture/ Video: http://www.youtube.com/watch?v=-KNcUNGaou0 http://www.had2know.com/garden/classify-soil-texture-triangle-chart.html Answer Key: 10. clay 11. sandy clay loam 12. sandy clay 13. loam
