Science Lesson: Do Worm Castings Affect Plant Growth?

In this science lesson students will compare two identical sets of pea plants. The only difference is that one set will be watered with regular tap water and the other set will be watered with a tea made from worm castings. This science lesson plan assumes that the classroom has an ongoing worm composting or vermiculture project. This is where the worm castings will be obtained. Read the first article in this series, Our Worms Ate Our Garbage, to find out more on how to set up this project. All other materials can be purchased at a local garden or home center.

The materials required for this plant growth lesson plan are as follows. First, two identical plant trays with identical pots and growing medium are required. Using Jiffy tabs helps tremendously. These are available in sets that include a tray, all of the Jiffy pellets and a top to hold in humidity while the seeds germinate. Next, pea seeds are required. Bean seeds could also be used. Be sure that all of the seeds used are from the same source and are the same species of seed. For example if you purchase Burpee’s Sugar Daddy Peas from the local farm supply store, then all of the seeds have to be Burpee’s Sugar Daddy Peas from the same local farm supply store. You will need a supply of worm castings, measuring tape or ruler, black sharpie marker, and a science journal for every student.

To set up the pea plant experiment, prepare the Jiffy pellets according to the directions. Prepare one set with a worm casting tea and the other using tap water. Mark the plant sets A and B. Set A will be watered with only tap water and B will be watered with only worm casting tea. When the pots are ready for planting, plant a seed in every pot. Cover the trays and watch for signs of germination.

Making worm casting tea is easy. This can be achieved in one of two ways. If your worm composting project is using a store bought system that has a spigot, just use what comes out of the spigot, add tap water if there is not enough. If your system does not have a spigot, then save the worm castings when you change the worms from one bin to another. Soak the worm castings in water until it looks like weak tea. Then water one set of pea plants with this tea.

Once the pea plants have sprouted, mark each plant’s stem at the soil level with the black sharpie. Make the mark very dark and visible and have it go around the entire stem. This is how each plant will get an accurate measurement of growth.

The pea plants should not require much water if any, while they are under the plastic cover. As soon as most of the plants have sprouted and have their first set of true leaves, remove the cover. Water the pea plants as necessary, not allowing the pots to dry out completely. Measure the pea plants every few days with either a ruler or a measuring tape. The measurement should be taken from the soil level to the black mark. Students should record the results in their science journals. Note any plant fatalities in the journals.

The pea plants will grow quickly. They should be transplanted to either a garden or a large pot when they are 8 to 12 inches tall. At this point students should form their conclusions as to whether or not worm castings had an effect on plant growth.

Keeping a journal is important for this science lesson. Have the students monitor the growth of the plants as well as think about other questions. For example, have them explain why everything in the experiment is the same, except for the watering. Ask them to think about what would happen if light or other conditions were different? Would this impact the results? This is also a good time to dissect a pea seed and cover the parts of a seed and parts of a plant. Have the students draw and label each part in their science journal.

This pea plant growth lesson plan is a great way to introduce students to a controlled experiment. By using only one variable it is easy to come to a conclusion about the hypothesis. Keeping a science journal is a good habit for students to get into. This science lesson plan can be easily adapted to any grade level. The amount of writing required should be in line with the student’s age and ability.