Aspiring Soil Engineers Design a Perfect Seed Starting Mix

Geotechnical engineering is a specialty area within the environmental sciences that deals with geology and soil.  These professional experts analyze the substrate of a site to determine the viability for construction or development based on soil properties. I imagine similar skills are used to make the bagged “soils” we purchase at our favorite garden store or nursery.

In the NGSS-designed  “soil engineering” sequence described below, second grade students make observations of Earth materials to determine the best combination for making a seed-starting mix. Starting seeds is a popular garden activity, but involving students in engineering a medium for planting is even more fun!

miracle-gro-potting-soil-74978500-64_1000
Miracle-Gro is just one of may bagged seed starting products.

This lesson is derived from the ongoing debate I have as a gardener about whether to buy pre-made seed starting mix or make my own. I can typically find 8-quart bags of Miracle-Gro Seed Starting Potting Mix for under $5.00, but when starting a lot of seeds with kids, I need more than 8-quarts. A similar sized bag of perlite or peat moss (common ingredients found in seed starting mixtures) are about the same price, so it is much more cost effective to buy the materials in bulk. The main reason I prefer to make my own seed starting mix is because doing so provides a much richer and more authentic educational experience.

As part of a larger unit on plant growth, a guiding question for this investigation could be, “What kind of soil does a seed need to germinate?” Analyzing the Earth materials commonly found in seed starting mix exposes students to physical science standards not typically taught in the garden. Consider the following performance expectation for second grade:

NGSS 2-PS1-2. Analyze data obtained from testing different materials to determine which materials have the properties that are best suited for an intended purpose.

In the embedded lesson plan, students first observe six different Earth materials: three from once-living things (peat moss, worm castings, and coconut coir) and three from rock or mineral sources (perlite, vermiculite, and sand). After recording observations about the particle size, shape, color, smell, and texture of each, students devise a way to test how each material interacts with water. Then, they are presented with the  following:

Design Challenge Guidelines


  1. Include at least two, but not more than four of the materials in the mix.
  2. Use no more than six scoops in all and only up to three scoops of any one material.
  3. The mix should hold enough water to germinate a seed.
  4. The mix should drain well enough that the seed doesn’t rot.

After students collaboratively design their mixture, they should test their mixture by planting some seeds in it and making daily observations of germination.

I recently led a group of educators through this activity. Egg cartons make a low-impact resource for organizing the different Earth materials and then planting the seeds. For a more complete version of this lesson, check out Grade 2 5E NGSS Lesson_Soil Engineering.

This lesson also partially addresses NGSS Engineering Performance Expectations for K-2:

  • K-2-ETS1-1. Ask questions, make observations, and gather information about a situation people want to change to define a simple problem that can be solved through the development of a new or improved object or tool.
  • K-2-ETS1-2. Develop a simple sketch, drawing, or physical model to illustrate how the shape of an object helps it function as needed to solve a given problem.
  • K-2-ETS1-3. Analyze data from tests of two objects designed to solve the same problem to compare the strengths and weaknesses of how each performs.

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