Student s Name Class Date LAB-AIDS Kit #320 - Simulating Threshold Effects of Soil ph on Hydrangea Plants Student Worksheet and Guide Soil ph, the acidity or alkalinity (basicity) of a soil is a very important factor in plant growth. There are numerous situations where plant-growers want to adjust soil ph to achieve a desired result, such as increased yield. However, if soil ph becomes too low (acidic) or too high (alkaline or basic) plant health can deteriorate and plants can die. Hydrangea flowers can have a variety of colors, depending in part on how much aluminum is absorbed by the hydrangea s roots. As soil ph increases and becomes more alkaline, the amount of available absorbable aluminum decreases. A bluer color results when more aluminum is absorbed while a pinker color occurs when less aluminum is absorbed. The amount of aluminum available to the plant, and thus the flower color, can often be influenced by the ph of the soil surrounding the plant s roots as shown in the table below. Soil ph Available Aluminum Flower Color < 5.5 more blue flowers 5.5-6.0 blue and/or pink and/or purple flowers 6.0-6.5 less pink flowers Hydrangea growers often pay close attention to soil ph. Soil ph can be fairly easily modified by using an acidic or an alkaline soil amendment. To produce pink hydrangea flowers, hydrangea growers can use alkaline agricultural lime (mostly calcium carbonate CaCO 3 ) to increase soil ph. However, if soil ph goes above 6.5 hydrangeas may experience health issues due to iron deficiency. In this activity, you will be simulating a test to determine the positive effect threshold and negative effect threshold amounts for lime. If you add too little lime - any amount below the positive effect threshold - the color will remain bluish; adding too much lime - any amount above the negative effect threshold - can damage, or even kill, the hydrangea. Challenge: What is the optimum amount of agricultural lime for producing healthy, pink hydrangea flowers? Materials For each group of students 1 dropper bottle of agricultural lime For each team of students 1 Chemplate and stir stick 1 red/blue pencil access to dropper bottles of: plant mass soil aluminum plant nutrients access to water For each student safety eyewear 320-WSP
Procedure 1. Consult Table 1 on the next page and find the columns for plant mass, soil aluminum, and plant nutrients. What do you notice about the quantity each plant gets of these substances and why do you think the experiment is set up this way? 2. As directed by your teacher, take your Chemplate to the materials supply station and a. Fill the oval shaped well 2/3 full with water. b. Add the listed quantity of plant mass, soil aluminum, and plant nutrients to Wells 1-9. c. Return to your table with your Chemplate. 3. Find the row for Plant #1 and add the quantity of agricultural lime listed to Well 1 of your Chemplate. 4. Stir the mixture and then rinse the stir stick in the water in your oval-shaped well. 5. Describe the contents of Well 1 in the Description of Liquid Mixture column for Plant #1. 6. Repeat Steps 3-5 for Plants #2-9 using Well 2 for Plant #2, Well 3 for Plant #3, etc. 7. Use the Key below to determine Plant Health & Flower Color and then either write and/or draw each flower s reaction in the appropriate spots of Table 1. Key to Determining Hydrangea Response Description of Liquid Mixture Orange liquid, with or without orange specks Plant is healthy. Flowers are blue. Lightly colored liquid with dark specks Plant is healthy. Flowers are pink. Blue to dark blue liquid with dark specks Plant wilts, yellows, and then dies. Plant Health & Flower Color 8. Use the information in the introductory paragraph to help you estimate the soil ph for each of the plants. Record this in the appropriate spots of Table 1. 2
Table 1: Effects of Agricultural Lime on Hydrangea Plants Plant Mass Number of drops of Soil Aluminum Plant Nutrients Agricultural Lime Description of Liquid Mixture Plant Health & Flower Color Soil ph #1 4 1 2 1 #2 4 1 2 2 H y d r a n g e a P l a n t #3 4 1 2 3 #4 4 1 2 4 #5 4 1 2 5 #6 4 1 2 6 #7 4 1 2 7 #8 4 1 2 8 #9 4 1 2 0 3
Analysis 1. Explain how you came up with your values for soil ph. 2. Based on your results, describe the effect of agricultural lime on soil ph, hydrangea flower color, and hydrangea plant health. 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. 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. 5. What is the positive effect threshold amount of agricultural lime? Explain how you know this. 4
6. What is the negative effect threshold amount of agricultural lime? Explain how you know this. 7. According to your data, which hydrangea plant(s) could have purple flowers? Explain your reasoning. 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. 9. Describe the relationship between a. soil ph and available absorbable aluminum? b. soil ph and available absorbable aluminum? c. soil ph and available absorbable aluminum? d. soil ph and available absorbable aluminum? 5
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. 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. 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? 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. 6