Simulating Threshold Effects of Soil ph on Hydrangea Plants KIT# 320
The learning objectives of this activity are explored in more depth in SEPUP s 2-3 week module Thresholds & Toxicology. Please consider using SEPUP products such as full year courses, course units, modules, and LAB-AIDS kits with catalog numbers ending in S. Contents 12 Chemplates with stir stick 12 red/blue pencils 6 15-mL dropper bottles of agricultural lime 3 30-mL dropper bottles of plant mass 3 30-mL dropper bottles of soil aluminum 3 30-mL dropper bottles of plant nutrients NOTE: Safety eyewear, which is not included in the kit, should be worn when performing this activity. expand-a-kit This LAB-AIDS kit is designed for use with six groups of four, or at most five, students. If any of your classes have more students than this kit is designed to accommodate you can obtain extra components. Each 320EL kit contains enough materials for one additional student group. Cat. No. 320EL contains: 2 Chemplates with stir stick 2 red/blue pencils 1 15-mL dropper bottle of agricultural lime 1 30-mL dropper bottle of plant mass 1 30-mL dropper bottle of soil aluminum 1 30-mL dropper bottle of plant nutrients 2014 Lab-Aids. All rights reserved.
Activity Overview In this simulation activity, students model how hydrangea plants are affected by soil ph. Agricultural lime (mostly CaCO 3 ) is often used to adjust soil ph. Soil ph is a factor that influences the color of hydrangea flowers and the health of the plant. Below a positive threshold ph, hydrangea flowers are blue, above this threshold dose flowers are pink, and higher doses, above the negative threshold amount, cause health problems. The use of a simulation allows students to add agricultural lime to each plant and then immediately observe the plant s reaction. Students use the data obtained from the simulation to determine the positive and negative threshold doses for agricultural lime. They also reflect upon the results to decide whether they have sufficient data to make a recommendation for an optimal amount of lime to use if you want to produce healthy plants with pink flowers. Concepts, Processes, and Issues (with NSES 5 8 Content Standards Correlation) 1. Laboratory testing is often used to help establish positive and negative (toxic) threshold levels for substances. (Perspectives: 1, 4, 5) 2. Quantitative data derived from tests can be mathematically extrapolated. (Inquiry: 1, 2) 3. Several factors must be considered when extrapolating lab-derived data. (Perspectives: 1, 4, 5; History: 2) Relevant National Agriculture, Food, and Natural Resources (AFNR) Standards PS.02.02. P I: Prepare growing media for use in plant systems PS.02.02.01.c. Formulate and prepare growing media for specific plants or crops PS.02.03. P I: Develop and implement a fertilization plan for specific plants or crops. PS.02.03.01.b. Describe nutrient deficiency symptoms and recognize environmental causes of nutrient deficiencies. PS.02.03.01.c. Monitor plants for signs of nutrient deficiencies and prepare a scouting report. PS.02.03.02.a. Discuss the influence of ph and cation exchange capacity on the availability of nutrients. PS.02.03.02.c. Adjust the ph of growing media. PS.03.02. P I: Develop and implement a plant management plan for crop production. PS.03.02.02.a. Explain the reasons for preparing growing media before planting PS.03.02.02.b. Prepare soil for planting with the addition of amendments. PS.03.02.03.b. Apply preplant treatments required of seeds and plants and evaluate the results. Advance Preparation Use your downloaded pdf of the Student Worksheet and Guide to print out sufficient copies. Use your downloaded pdf of Transparency: Observed Threshold Amounts for Agricultural Lime to prepare a transparency or a butcher paper poster for compiling class data. Because all student teams will need to share the bottles of plant mass, soil aluminum, and plant nutrients, set up one or more distribution areas where students can access these bottles. 1
Teaching Suggestions Safety Note: Use caution when handling chemical solutions. Always wear safety goggles and rinse any area of the skin that comes into contact with a chemical solution, particularly the eyes. GETTING STARTED Read aloud (or write on board) the fictitious ad headline Boost Your Grades with One Glass a Day. Ask students how they think that these claims were arrived at and then hold a brief discussion of the importance, and in some cases requirement, of rigorous testing before a product can be legally sold. You might want to ask if anyone has heard the expression, The dose makes the poison and then hold a brief class discussion to introduce, or clarify, the concept that any substance can be harmful, even those we need to survive, if we get too much of it. You may also want to point out that all medicines, but not all health supplements, must go through years of testing before being approved for widespread use. If desired, introduce the field of toxicology - the study of substances that are harmful to living organisms. Explain that one of the tasks of toxicology is to determine the boundary between safe and unsafe levels of exposure and that this is usually accomplished through large-scale testing using some combination of computer and chemical simulations, and testing on living organisms. Point out that two very different types of effects need to be thoroughly studied: the immediate, or acute, effects of a single exposure and the long term, or chronic, effects of exposure over an extended period of time. Next ask students if they know of any agricultural products that make claims of improved performance. Follow this up with a question or discussion of whether agricultural products undergo testing before widespread use. Explain that in this activity they will do an experiment to simulate the effects of using agricultural lime to adjust the ph of soil used for growing hydrangea plants. Note: The solution labeled agricultural lime is actually a 0.2% solution of NaOH. DOING THE ACTIVITY Distribute the Student Worksheet and Guides and ask students to read the introduction and Challenge. Answer any questions and emphasize that this is a simulation that uses chemicals with known properties and interactions to represent the various experimental factors in order to quickly create the effects of actual testing. Split the class into teams of two (or three) and, for the purpose of materials sharing, group two teams together at the same or adjacent tables. Distribute the equipment and explain that: Each student should wear safety eyewear at all times. Each student should use their own Student Worksheet and Guide to record their own answers Each team of students will get a Chemplate to use to do the activity. Each team of students will prepare their plants in their Chemplate at a distribution area, which contains bottles of plant mass, soil aluminum, and plant nutrients that are shared by all the teams in the class. Each group of two teams will share a bottle of agricultural lime at their table(s). Point out the location of the distribution area(s) and review the proper use of the dropper bottles. 2
Emphasize that to get the most consistent results the students should take the following steps: uncap only one bottle at a time to prevent caps from different bottles getting mixed up hold the dropper bottles vertically to achieve uniform drop size avoid touching dropper tips to any other liquid to prevent contamination Have student groups begin the Procedure and assist groups as needed. As groups finish, make sure they clean their equipment thoroughly. Sample results are shown below. Student estimates of soil ph may be much less precise than those shown below and many will likely only use the ranges given on the Student Worksheet and Guide: <5.5, 5.5-6.0, >6.0 for healthy plants and >6.5 for unhealthy plants. Plant # Drops of Lime Description of Liquid Mixture Plant Health and Flower Color Soil ph 1 1 orange liquid with orangish specks healthy, blue flowers ~4.9-5.4* 2 2 orange liquid with orangish specks healthy, blue flowers ~5.3-5.8* 3 3 yellowish or greenish liquid with dark specks healthy, pink flowers ~5.7-6.2* 4 4 greenish blue liquid with blue-green specks healthy, pink flowers ~6.1-6.6* 5 5 blue to dark blue liquid with dark specks health issues ~6.5-7.0* 6 6 dark blue liquid with dark specks health issues ~6.9-7.4* 7 7 dark blue liquid with dark specks health issues ~7.3-7.8* 8 8 dark blue liquid with dark specks health issues ~7.7-8.2* 9 0 orange liquid healthy, blue flowers ~4.5-5.0* * Estimates based on assumption that the transition from blue flowers to pink flowers occurs at ph 5.5-6.0, plant health starts deteriorating at ph=6.5, and that each drop of lime produces the same ph increase. Display Transparency: Observed Threshold Amounts for Agricultural Lime (or butcher paper version) and use it to collect and display each team s results. When all teams have reported in, determine the class average for the positive and negative threshold amount. Keep the information on this transparency displayed for future reference. The positive threshold - the smallest dose that causes pink flowers - is typically 3 drops. The negative threshold - the smallest dose that causes a decline in health - is typically 5 drops. FOLLOWING THROUGH Have students respond to the Analysis questions and when they have completed them, discuss them as a class. Make clear to students that soil ph can be increased or decreased, depending on the soil amendment used, to create the desired soil ph. Revisit the saying The dose makes the poison. and emphasize, that if too much ( the dose ) of any substance enters your body it can be harmful ( the poison ). If desired, and time permitting, discuss/debate the question of how to determine what amount is safe. If you feel it is appropriate, explain the concept of the lethal dose, or LD. In most toxicology studies, as the dose level increases, so does the percentage of test subjects that die. The LD for a substance is usually given as LDX, where X indicates the percentage of test subjects that died when given that dose. LD50, the dose 3
which causes one-half the exposed organisms to die, is a commonly determined and reported dose for comparison. In the past, LD50 was the dose from which recommended doses were extrapolated but today, many researchers and drug manufacturers base their recommended dosages on a lower LD20 (or below). Sample Responses to Analysis Questions 1. Explain how you came up with your values for soil ph. Answers will vary. Estimates in the sample data table provided earlier in this Teacher s Guide are based on the assumption that the transition from blue flowers to pink flowers occurs at ph 5.5-6.0, plant health starts deteriorating at ph=6.5, and that each drop of lime produces the same (~0.5) ph increase. However, this method of deduction is rather quantitative and may not occur to some (perhaps most/all) students and student estimates of soil ph may be much less precise than those in the sample table. Many will likely only use the ranges given on the Student Worksheet and Guide: <5.5, 5.5-6.0, >6.0 for healthy plants of the appropriate color and >6.5 for unhealthy plants. 2. Based on your results, describe the effect of agricultural lime on soil ph, hydrangea flower color, and hydrangea plant health. As you add more agricultural lime the soil ph increases (becomes more alkaline). Below ph 5.5 hydrangea flowers are blue. Between ph 5.5 and 6.0, hydrangea flowers begin the transition from blue to pink and can be blue, pink, or even purple. When soil ph is greater than 6.0, flowers are pink but if ph rises above 6.5, the plant health can deteriorate due to iron deficiency. 3. Look at the quantity of each substance used for each plant. Explain the purpose of Plant #9 and why, overall, you think this is or is not a good experimental design. Answers will vary but students should realize that Plant #9 is a control and that holding the plant mass, plant nutrients, and soil aluminum amounts constant while varying only the lime is good experimental design. 4. If each drop of Plant Mass simulates 0.5 kg of hydrangea plant, what is the mass of the hydrangea plants you are using? Show your work. 4 drops x 0.5 kg/drop = 2 kg 5. What is the positive effect threshold amount of agricultural lime and how did you determine this? The positive effect threshold - the lowest dose that causes pink flowers - is typically 3 drops. 6. What is the negative effect threshold amount of agricultural lime and how did you determine this? The negative effect threshold - the lowest dose that causes a decline in health - is typically 5 drops. 7. According to your data, which hydrangea plant(s) could have purple flowers? Explain your reasoning. Plants #2 and #3 are likely candidates for purple flowers because these are the transition from blue to pink flowers and soil ph for these plants is most likely to be between 5.5 and 6.0. 8. Based on this experiment, are you confident that you can recommend an optimal amount of agricultural lime that will produce healthy pink-flowered plants? Explain why or why not. Answers will vary but students should conclude that for this size plant in this type soil, the optimal dose would be 3-4 drops. They should also realize that the optimum dose will be different depending upon the soil s initial ( natural ) ph and, most likely, other soil-, nutrient- and waterrelated factors. 4
9. Describe the relationship between a. soil ph and available absorbable aluminum? as soil ph increases, available aluminum decreases (inverse relationship). b. available absorbable aluminum and hydrangea flower color? as available aluminum decreases, hydrangea flower color becomes less blue and more pink. c. soil ph and hydrangea flower color? as soil ph increases, hydrangea flower color becomes less blue and more pink. d. soil ph and hydrangea plant health? as soil ph becomes greater than 6.5, hydrangea plant health deteriorates. 10. Do you think this experiment is a reasonable model for an actual test for the effect of agricultural lime on hydrangea plants? Explain why or why not? Answers will vary. Emphasize that the correlation between increased dose and observed response is an accurate representation of what would occur with real plants. If students do not mention that the chemical simulation is likely to show less variability than would be observed with real plants, point this out. 11. When certain air pollutants mix with moisture in the clouds, acid rain can occur. Acid rain has caused serious environmental problems in some environments. What effect might acid rain have on your crop of pink hydrangeas? Use your data from this activity to explain why this could occur. Acid rain could decrease the ph of the soil causing the flower color to change from pink to purple and then blue. The change in the color could be seen in weeks or months depending on the amount of rainfall and the original soil ph. Growers that are planning to reach their market with pink flowers should monitor the soil situation and be concerned of acid rain because carelessness or a lack of regular testing can lead to blue flowers instead of pink. 12. Leaching refers to the loss of water-soluble plant nutrients from the soil due to rain and/or irrigation. What effect would leaching have on your crop of pink hydrangeas over time? As lime or aluminum is washed away, the ph of the soil would change, altering the color of the flowers. Therefore a proper and planned out fertilizing program (after regular soil testing) should be in place especially if growers wish to sell pink flowers when it is time for market. GOING FURTHER Imagine that you are an extension officer tasked with educating the public on growing hydrangeas. Create an informational flyer that you can use to educate the public on the effects of agricultural lime on hydrangea growth. Informational flyers should highlight the effects of ph and available aluminum on the color of hydrangea flowers and indicate that soil amendments can be used to raise (or lower) the ph of the soil to achieve the desired flower color.
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