The Effect of Acid Rain on Marigold Plants (page 2)

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Author: Ryan T.


The purpose of this experiment was to find out how acid rain affects the growth and health of plants. I became interested in this experiment when I saw a movie about toxins in the environment at school. I researched online about environmental toxins and became fascinated with acid rain. I chose to test on marigold plants after researching for fast-growing, flowering plants. The information from this experiment will help people understand the importance of curbing air pollution from coal-burning factories, power stations, transportation and furnaces.

Materials and Equipment

  • Distilled water, bottled (3 gallons)
  • 32 fl. Oz/473ml of white vinegar
  • pH test strips with small increments of at least .2 of change between 2.0 and 7.0 or a scientific pH meter with .2 increments of pH change. (I purchased pH strips at Ward's Natural Science at , catalog #15V3152 and #15V3153.)
  • Marigold plants - 29 (1 flat), 10 - 20 cms in height (small with the same potting soil, the same size and the same producer.)
  • Ruler with 30 centimeter measurements
  • Color digital camera to take pictures of plants at different intervals.
  • Potting Soil, 16 quart Master Gardener All-Purpose organic potting soil
  • Plastic cups, 8 oz. (1 cup or 250ml)
  • 500ml or 2 cup measuring cup
  • 6 large, 48 oz. (6 cup/1500ml) reusable plastic containers with lids
  • Medicine dropper measuring up to 1tsp/5ml with .1ml increments
  • Clean metal teaspoon for stirring (5ml)
  • Lab notebook with lined paper to document results
  • Permanent marker
  • Paper towels
  • Index cards to write the plant numbers on to identify them in the pictures
  • Pencil or pen to record results
  • Clock indicating hour and minute.
  • Computer (with a word processor and spreadsheet program) and printer to write report and print out results
  • Location that gets consistent sunlight and room for all plants
  • House thermometer indicating degrees in 1 degree increments


My research on acidic watering solutions and their effect on the growth of marigold plants can be related on a larger scale to acid rain and its impact on our environment. Studying acid rain is very important because acid rain can be very harmful to the environment, buildings, monuments and humans.

Soil acidity affects the way plants grow, beginning with their roots. The acidity inhibits the growth of roots and the absorption of necessary minerals through the root cell walls. This can drastically weaken plants, causing them to grow very slowly or turn yellowish in color. Some plants may have difficulty absorbing nutrients at all in highly acidic soils. If acid levels remain high, harvests will be smaller, seeds will not grow well and the plants may eventually wither and die.

In addition to the damage to forests and crops, acid rain also causes damage to buildings, monuments and stonework by causing them to weaken, wear down, rust faster and deteriorate. 

Acid rain raises the acidity in lakes and streams causing fish to grow more slowly, eggs to not hatch, fish to die and the animals that eat the fish die or move away as their food supply goes away. 

Acid rain is also harmful to humans. The tiny particles created by acid rain can cause respiratory diseases like asthma or chronic bronchitis or make existing conditions worse.

Terms and Concepts for Background Research

What is pH?

Acidic and basic are two opposites that we use to describe chemical compounds. Acidity is measured by using a pH scale. A pH scale runs from 0 (the most acidic) to 14 (the most basic) and measures how acidic an object is. A substance that is neither acidic nor basic is called neutral, and has a pH of 7. Normal water is about a pH of 6 to 7. Normally clean rain has a pH value of 5.6, which is slightly acidic. However, when rain combines with sulfur dioxide or nitrogen oxides produced from factories and automobiles, the rain becomes much more acidic. Typical acid rain has a pH value of 4.0. For optimum growth, most plants need a soil that has a pH range from 5.8 to 7.0. A decrease of pH from 5.0 to 4.0 shows a 10-fold change in the number of hydrogen ions in the solution. You can measure acidity with litmus paper (which only measures whether it is acidic or basic), pH paper (which measures in increments typically between 3 and 10), or a scientific pH meter which has various increments depending on the manufacturer.

My research indicated that to increase the level of acid in the water, I should add vinegar. Using pH measuring strips to monitor the results of adding the vinegar to distilled water, I was able to replicate many levels of acidity in water.

What is acid rain?

All rainwater is slightly acidic, a pH of 5.6, due to dust and pollen particles it can pick up when the droplets form. However, when rain contains pollutants, the rainwater can become very acidic. Acid rain is any type of precipitation (rain, sleet, snow, fog, gasses, morning dew and dust) that has been made acidic by pollutants in the air.

As pollutants rise and move in the air, they can combine with moisture in the clouds where SO2 and NOx  react with water and oxygen. This forms sulfuric acid and nitric acid in the atmosphere. Sunlight increases the speed of these reactions. Rain, snow, fog, sleet and other forms of precipitation then mix with the sulfuric and nitric acids and fall to the ground as acid rain. This type of acid rain is called wet deposition.

Pollutants can be carried by the wind for very long distances. Half of the acidity in the atmosphere comes from acid pollutants falling directly on plants, trees, buildings, equipment, lakes and any exposed surface. This is called dry deposition. If these gasses and particles are washed from trees and other surfaces by rain, the runoff water contains acid from acid rain and dry deposition, making the water even more acidic than from the rain alone. This combination of wet and dry deposition is called acid deposition.

What are the effects of acid rain?

Acid rain can be extremely harmful to forests, lakes and streams, and humans. 

In forests and crop fields, it can seep into the ground and dissolve nutrients such as magnesium and calcium that trees need to be healthy. In addition, acid rain causes aluminum to be released into the soil, which makes it difficult for trees to take up water. The pollution causes haze by scattering light back towards the sky. Haze reduces the amount of light available for plants to use in photosynthesis, causing problems with creating food for plants. Acid rain also wears away the protective waxy coating on leaves, causing damage to leaves and brown spots. The leaves are then unable to use photosynthesis to turn the energy from the sun into food, making the tree or plant unhealthy. The result is trees that grow more slowly, leaves or needles that turn brown and fall off, weakened trees unable to withstand cold, insects or disease, and death.

Acid rain also causes an increase in acidity and aluminum levels in lakes and streams which is deadly to aquatic wildlife. Acid rain flows to streams, lakes and marshes after falling on forests, fields, buildings and roads and then being washed into them. Acid rain can also fall directly into these water areas. Most lakes and streams have a pH between 6 and 8. As lakes and streams become more acidic, the numbers and types of fish and other plants and animals that live in the water decrease. They leave or die. At pH 5 most fish eggs cannot hatch. At lower pH levels, adult fish can die.

Acid rain damages buildings, statues, monuments and cars. Acid rain will eat away at stone, metal, paint or almost any material exposed to the weather for a long period of time, speeding up their deterioration and making them appear old and worn down, reducing their beauty. Marble and limestone are dissolved by acid rain as it reacts with the calcium carbonate, causing many buildings and monuments to be damaged. According to the Environmental Protection Agency, this repair can cost billions of dollars and historical monuments can never be replaced.

Finally, acid rain is harmful to humans. The tiny particles created by acid rain can cause respiratory diseases like asthma or chronic bronchitis or make existing conditions worse. This causes more time off work, makes people less healthy and sometimes leads to death.

Causes of Acid Rain:

Human activities are the main cause of acid rain. Over the last 30 years humans have released so many chemicals into the air that they have changed the mix of gases in the atmosphere. Factories and coal-burning electric power plants release the majority of sulfur dioxide and much of the nitrogen oxides. In addition, the exhaust from cars, trucks and buses and the gas from burning oil and natural gases release nitrogen oxide and sulfur dioxide into the atmosphere. There are also natural sources of acids such as volcanoes, geysers and hot springs. They contribute only a small portion of the acidic rainfall in the world. It is the large amounts of acids produced by human activities that cause the ecosystems to be off balance.

Acid rain is caused by a chemical reaction that begins when compounds like sulfur dioxide and nitrogen oxides are released into the air. Acid rain then mixes with non-acidic materials, such as air, soils, bedrock, lakes, and streams. As these materials are washed away by rain, the acidity increases and causes damage to crops, trees, lakes, rivers, and animals.

Why is it important to study its effect?

The pH of the water supplied to a plant influences its growth because it affects the availability of needed nutrients from the soil so they don't reach plant roots. Soil pH decreases the solubility of nutrients and minerals that are needed for plants to grow. Solubility is important because fourteen of the seventeen plant nutrients that are necessary for plant growth are derived from the soil. Without these nutrients, plants can get diseases including brassicas and club root.

Studying the effect that acid rain has on the growth and health of marigolds will help document the more global negative effects of acid rain so that solutions can be identified.

Research Question

How will the height, number of flowers, number of buds/sprouts and overall health of a marigold plant be affected by increasing the acidity of the daily watering solution to replicate acid rain concentrations?


If I increase the acidity of the watering solution, then I believe the rate of growth of the marigold plants will decrease. I believe that for each 1pH increase in the acidity, the plants will grow at a rate that is 25% less than normal, eventually resulting in the death of the plant with the highest acid rain amount. I also believe that the flowers will show measurable signs of unhealthiness including a decrease of 10% at each 1pH increase in the acidity in the number of flowers, a 10% decrease in the amount of sprouts, and plant death at the most acidic pH. I base my hypothesis on acid rain's effects on the maple trees in Vermont and New Hampshire, the red spruce trees in New York and the decline in spruce forests in the Appalachian Mountains. We can also look at the dead pine trees in Germany's Black Forest. By replicating the acidic conditions in these areas, the results should be similar. In the United States alone, according to the United States Geological Service, the average pH levels of rain in certain areas are as low as 4.3. Los Angeles fog has been measured at 2.0 at times. We can see the effects in these areas on trees, crops and other vegetation.

Independent Variable:  the acidity of the water provided to the plants daily.

Dependent Variable: the change in marigold plants including height, sprouts and flowering, and overall health as indicated by smell, color, and droopiness.

Controlled Variables:
  • Sunlight for all plants - put in a garden window that allowed the same sunlight for all plants and observed at night to allow for the maximum exposure to sunlight per day.
  • Soil content - replant all plants immediately using the same amount of soil from the same package.
  • No wind - conducted inside
  • Shock from transport from store - didn't change solution until the 3rd day to allow the plants to adapt to their new surroundings and soil.
  • Plants - all purchased from the same store, the same flat of plants from the same grower. Plants were selected based upon consistency of health at onset.
  • Water - all plants were given the same amount of water at the same time each day.
  • Room Temperature - conducted indoors with temperature between 70 - 75 degrees Fahrenheit (21.11 - 23.89 degrees Celsius)
  • Daily Changes - amount of time between measurements was the same amount of time so that the change rates could be determined correctly.

Control Sample: Plants using water solution #1- distilled water

Experimental Procedure

I plan to observe marigold plants over a period of 10 days and test the effect acid watering solution has on the growth and appearance of the plants. I will then relate the results to the effect acid rain has on the environment.

  1. Prepare marigold plants: 
    • Rinse the plastic 8 oz. cups with water thoroughly. Do not use soap because it can coat the plastic container and may be harmful to the plants. Dry the cups with a clean paper towel.
    • Fill the 8 oz. cups with 6 oz. of Master Gardener All-Purpose organic potting soil. Put a 3 cm. hole in the middle of the soil for the marigold plant.
    • Gently pull out the marigold plant from the flat and tap the soil out of the roots. Put the plant inside the hole in the middle of the 8oz. plastic cup. 
    • Label each 48 oz. container with a number grouping and letter, using the permanent marker. 5 plants will be labeled for each pH level (1 - 5), 4 plants for 6. For the plants within each number, label them a through e. So, for each group you will have 5 plants in the sample, except group 6 which will have 4 plants in the group. (ie: 1a, 1b, 1c, 1d, 1e, 2a, 2b, 2c…)
  2. Prepare the vinegar solutions:
    • Using the measuring cup, add 500 mL (2 cups/16 oz) of distilled bottled water to each 48 ounce plastic container, numbered 1 - 6. Distilled water must be used because tap water may contain chemicals, like chlorine or chloramines, which could harm the plants.
    • Using the pH strips, test the pH of the water in container #1. When using pH strips, dip a clean pH strip in the solution while holding the end. Wait for 2 seconds and then read the result. Record the pH on a data table like the example one below after step 3. Do not add any vinegar to this container. Double check your result.
    • For containers 2-6, use the medicine dropper to add vinegar to the water, one container at a time. After adding each addition of vinegar, mix the water with a clean spoon and measure the pH with the test strips. The goal is to create five different solutions with increasing amounts of acidity, ranging from just below the pH of distilled water (using the measurements for container #1 as a guide) to a pH array in the acid rain range of 3 - 5 (the acid rain range according to Environment Canada is 1-5, the average acid rain pH according to the United States EPA is 4.3). Most normal rainfall falls into the pH range of 5 - 6. Container #2 should have the fewest drops of vinegar and thus be the least acidic (except for the control, distilled water, in #1). Container #6 should have the most drops of vinegar and thus be the most acidic. Record the number of milliliters required to achieve the pH level required. Double check your results using fresh pH strips.
Amount of Vinegar
0 drops
18ml (3.65 tsp)
36ml (7.3 tsps)
45ml (9.13 tsps)
54ml (11 tsps)
125ml (25 tsps)
  1. Observe and document your results:
    • At the same time each night within a 1 hour period (to give them a full day of sunshine), observe the plants and write down observations in the data table. (See forms used in log book)
    • Measure heights of each plant (consistently measuring from the top of the soil to the top of the highest stalk), number of flowers and sprouts (noting each), and make observations about each plant such as color changes, smell, dead flowers, droopiness, etc. Be consistent in the method of measuring and observing.
    • Mark down your observations in a table which lists the plant number, height, number of flowers/sprouts, and other observations. Observations include changes in the smell, color of flowers, wilting, dead flowers, etc. Double check.
    • After making your observations, place an index card with the day number next to each group of plants and take a picture of each group of plants each day from a distance that will show all plants in the picture. Continue this step for each day through day 10.
  2. Water Plants with various acidic solutions:
    • For the first two days, water each plant with 4 oz. (½ cup or 150ml) of distilled water since they were just replanted into dry soil. On the second day water each plant with the medicine dropper with 10 mls (2 tsps.) distilled water from container #1. This 2 day control watering will allow the plants to adjust to their replanting without affecting the results.
    • Beginning on Day 3, recheck the pH levels of the acidic watering solutions. Then water the plants using the medicine dropper with 10 ml (2 tsps) of the appropriate acidic (vinegar) watering solution. Plants labeled 1a - 1e get watered with watering solution 1, plants labeled 2a - 2e get watered with acidic solution 2, etc. Wash the dropper out with distilled water in between each move to the new acidic solution. For each day after, continue to water the plants with 10ml (2tsps) of the appropriate acidic solution.
  3. Place plants in an area inside where they receive the exact same amount of sunlight. The average temperature of the location should be between 70° - 75° Fahrenheit or 21.11 - 23.89 degrees Celsius. 
  4. Complete steps 3 - 5 through Day 10.
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