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Soil Biosolarization: Sustainable Weed Killer

Adapted from TeachEngineering activity contributed by the GK-12 program, College of Engineering, University of California Davis. 


In this three-part activity, students in grades 5 to 7 act as agricultural engineers, learning about and testing the effectiveness of a sustainable pest-control technique called soil biosolarization that uses organic waste rather than of toxic compounds to help eliminate weeds. Teams prepare seed-starter pots using a source of microorganisms (soil or compost) and “organic waste” (such as oatmeal, a source of carbon for the microorganisms). They then plant “weed seeds” in the pots, counting any sprouts and assessing the efficacy of the technique to kill weeds.

Grade level: 5-7

Time: 170 minutes (An initial 90-minute session, one 30-minute session a day later, and one 50-minute session a week after the second session.)

Engineering Connection

Engineers apply science and math to create products and processes designed for the betterment of humankind and the environment. Microbial engineers use microorganisms to transform waste into something useful. Waste management engineers are responsible for reducing landfill and incinerator waste as well as transforming the waste into something useful. Agricultural engineers create ways that farmers can make and use compost to help plants grow better, less expensively, and without harming farm workers or the environment. The principles of soil biosolarization span each of these engineering specialties as organic waste is transformed to increase crop production and protect crops from pests.

Learning Objectives

After this activity, students should be able to:

  • Describe the importance of organic waste to composting.
  • Explain the importance of sustainable pest control techniques.
  • Conduct a scientific experiment to test the effectiveness of a soil biosolarization pest control method as a means of reducing the impact of humans on the environment.
  • Examine experimental results to assess how well the soil biosolarization system worked.

Academic Standards

Next Generation Science Standards

  • Obtain and combine information about ways individual communities use science ideas to protect the Earth’s resources and environment. (Grade 5)
  • Apply scientific principles to design a method for monitoring and minimizing a human impact on the environment. (Grades 6 – 8)
  • NGSS correlation with the California Education and the Environment Initiative (Grades 3 – 5)

International Technology and Engineering Educators Association: Technology

  • Students will develop an understanding of the relationships among technologies and the connections between technology and other fields of study. (Grades K – 12)
  • Students will develop abilities to assess the impact of products and systems. (Grades K – 12)

Pre-requiste Knowledge

Students should be:

  • Familiar with the concepts covered in the associated lesson, A Daily Dose of Sun Keeps the Pests Away: How Soil Solarization Works. Familiarity with the greenhouse effect is helpful, but not necessary.
  • Familiar with the scientific method and able to explain that experimental controls provide a means of comparing treated samples to non-treated samples in order to assess the effectiveness of a treatment.
  • Able to calculate averages and percentages to assess soil biosolarization efficacy.


Each group needs:

  • 6 pots or cups with drainage holes, such as seed-starting plastic pots
  • Container or tray to catch draining water from the seed starting pots
  • 60 seeds, such as lettuce or other plant that sprouts within a week
  • 1 graduated container, to measure the volume of the seed starting pots
  • Bucket for mixing soil and “organic waste,” big enough to hold enough soil and organic waste to fill 3 of the seed-starting pots
  • Thermometer
  • Soil Biolsolarization Activity Handout [PDF]
  • Pre-Activity Quiz and Post-Activity Quiz, one each per student

To share with the entire class:

  • Potting soil or compost, enough for each group to fill its 6 seed-starting pots
  • “Organic waste,” such as a solid food source that is easy to mix with soil, like oatmeal, flour, or cornstarch
  • Transparent plastic wrap
  • Water



Soil solarization is a sustainable, nonchemical pest-control method that eliminates soil-borne pests via the high temperatures produced when solar radiation reaches soil covered with a transparent plastic tarp. The process usually takes four to six weeks and is performed during the hottest period of the year. The plastic sheets trap the sun’s heat in the soil, taking advantage of the greenhouse effect. The process can kill a wide range of soil-borne pests, such as weeds, nematodes, and insects. In some cases, this heating is not enough to kill the soil-borne pests. The addition of organic waste soil can boost the soil microbial activity by adding two new effects to the process: 1) the metabolic energy of microbes degrading organic matter slightly increases the temperature during the process and 2) during the degradation of the organic matter, volatile fatty acids made by the microbes can reach levels that are toxic to soil-borne pathogens. This method is known as soil biosolarization.


Image © 2016 Jesús D. Fernández Bayo

Before the Activity

Gather materials and make copies of the Soil Biosolarization Activity Handout, Pre-Activity Quiz and Post-Activity Quiz, one each per student.

Check the weather and consider conducting the activity outside if weather permits.

Administer the Pre-Activity Quiz, giving students enough time to answer the seven questions. Review their responses to determine which concepts need to be reinforced during the activity.

With the Students—Session 1: Experiment Setup

1. Present the Introduction/Motivation content to the class, highlighting the following main points:

    • The importance of organic waste and the role of microorganisms in transforming organic waste into compost
    • The terms “pest” and “pesticide”
    • The environmental and health problems associated with the use of pesticides
    • The importance of developing and using less harmful pest control methods
    • The (hypothetical) student role in the activity—acting as an agricultural engineer testing a method designed to eliminate weeds from soil
  1. Pass out the handout. Explain that the handout contains activity experiment instructions as well as questions and a data table for students to complete as their teams work through the activity. The activity is divided into three stages: experiment setup plus data gathering one day and one week later.
  2. Divide the class into engineering teams of four students each.
  3. Tell students that each member of the team is an agricultural engineer and that—while all team members are expected to participate in all components of the activity—each team member will be responsible for a specific task. For each engineering team, assign student roles. Reader: reads instructions. Writer: fills out the worksheet. Speaker: presents and explains results to the class. Organizer: leads the experimental setup.
  4. Tell students which materials they can use to prepare the “agricultural soil mix” and why they are important: Soil and/or compost provide the environment where the pests and microorganisms live.Organic waste (the oatmeal, flour, cornstarch or other food ingredient of your choice) is a source of easily degradable organic carbon to feed the microbes.
  5. Explain that each group will have two treatments: 1) control treatment, which is only soil, and 2) experimental treatment (soil and organic waste).
  6. Guide students to measure the volume of their seed-starting pots using a graduated container and potting soil. Prompt them to use this information to estimate the amount of organic waste they need to add (5% of the seed-starter pot volume) and record their findings on the handout.
  7. Have students fill three of their pots with soil only (the controls). Somehow (tape, sticks) identify these pots as the control pots for each team.
  8. To prepare the three treated soils, direct students to fill their mixer buckets with three times the amount of soil and organic waste estimated in step 7; then, close the mixer bucket and shake it to mix the soil and organic waste.
  9. After mixing, tell students to divide the mixture evenly and transfer it into the three treatment pots.
  10. Once each group has its six pots ready, direct them to plant 10 seeds in each pot. Explain that the seeds represent the weeds they are trying to eliminate.
  11. Have students water each of the six pots until water flows out of the bottom. (It is helpful to place a container or tray under the pots to minimize the mess.) Then cover each pot with plastic wrap.
  12. Place the pots in a sunny spot in the school and leave them for solarization for at least one day (no more than one week is recommended).With the Students—Session 2: Data Collection (after at least 1 day)
    1. Have students remove the plastic film from their pots, smell the control and treatment pots, and describe their smell observations in Table 1 on the handout.
    2. Hand out thermometers and guide students to measure and record soil temperatures.
    3. Have students calculate the mean temperature per treatment.
    4. Direct students to water the pots again. If possible, keep the pots in a humid place and/or cover them with a transparent box.
    5. Until the next session, have students keep the soil in the pots moist by watering every 2-3 days. This is especially important if the pots are not covered.

    With the Students—Session 3: Data Collection and Analysis (final session; 1 week after Session 2)

    1. One week later, have students count the number of plants in each pot and record their findings.
    2. Direct students to calculate the mean percentage of seed inactivation per treatment.
    3. Have each team present its results to the class and post the data on the classroom board for all to see.
    4. Engage the class in a discussion of the results and in determining conclusions. Expect the results to show more plants in the control pots than in the treated soils. Expect a higher percentage of seed inactivation in the soil amendment to be related to the smells perceived during session 2. This bad smell is attributed to the acids formed during the degradation of the organic matter and their accumulation to a toxic level due to the plastic preventing them from escaping.
    5. As a class, review the activity learning objectives.
    6. Administer the Post-Activity Quiz.

Safety Issues

  • Since students handle soil and compost,  advise them to use gloves or to wash their hands after the activity.
  • Verify that no students have allergies to the selected food waste.

Troubleshooting Tips

  • Prior to conducting the activity, plant some seeds in the substrate that the class will be using in order to confirm that the seeds are viable and will grow during the experiment.
  • If no plants emerge after one week, wait a bit longer to make sure they are watered sufficiently and in a humid place. If no plants emerge and the final session cannot be delayed any further, give students hypothetical counts for the number of emerged plants per pot; make these values show a higher number of plants (weeds) grown in the control pots than the treated pots.
  • Plant seeds in several extra pots, so that all teams can participate if a pot fails to sprout or accidentally spills.

Activity Scaling

  • For lower grades (3-4), skip or simplify the mathematical calculations. Also consider providing students with specific values of organic waste and soil, or calculate them as a class.
  • For higher grades (7-9), remove the equations and designs that clarify the calculations outlined on the student handout and have students independently determine the necessary calculations. Also consider diversifying the types and quantities of organic wastes added to the treatment pots.
  • For large classes in which more than four groups can be formed, consider having each group add a different amount of organic waste to the soil in the treatment pots. Then, as a class, compare results and discuss which amount was the most effective at eliminating “weeds.”
Andalusian Stories: Biosolarization – A new technique for strawberry cultivation. [YouTube 1:40]
Advances in Biosolarization Technology to Improve Soil Health and Organic Control of Soilborne Pests. Stapleton, et al. Proceedings of the Organic Agriculture Research Symposium, January 20, 2016.References

Gamliel, A., Stapleton, J.J. 1997. Improvement of Soil Solarization with Volatile Compounds Generated from Organic Amendments. Phytoparasitica, 25, S31-S38.

Katan, J., Greenberger, A., Alon, H., Grinstein, A. 1976. Solar Heating by Polyethylene Mulching for Control of Diseases Caused by Soil-Borne Pathogens. Phytopathology, 66(5), 683-688.


Jesús D. Fernández Bayo


© 2016 by Regents of the University of Colorado; original © 2016 University of California Davis

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