Science project

What is Mulch?


  • Moisture meter
  • Cardboard or newspaper
  • Manure
  • Straw
  • Notebook and pencil


  1. Find an ideal area with dimensions of about 1’ x 1’. The soil in the area should be dry, but you’ll need to be able to move the moisture meter into the soil, so choose an area that’s soft enough for you to do this.
  2. Create a hypothesis, your best guess about what is going to happen. Will the soil be the same after adding a layer of rotting material, or will it be more or less damp?
  3. Choose a day when it is warm and dry. It should be dry for at least three days before taking the soil moisture levels. If possible, look up the relative humidity levels so that you can understand how moist the air might be. Place the moisture meter in the middle of the area, and leave it for a few minutes. Read the moisture level, and note it in your notebook.
  4. Choose two more days with similar weather and humidity levels and take readings on those days as well.
  5. Now, create a layer of mulch in your small experimental area. Add cardboard or newspaper to the ground, and cover this with at least three inches of manure. Add three inches of straw to the ground. If you wish, you can cover this with some soil as well.
  6. Wait for a week, and then look for another day to take the moisture levels. Push the moisture meter through the mulch as far as it can go. You may need to push down the straw to compact it. What are the moisture levels underneath the mulch? Is the soil more moist than it was when you took the moisture reading in bare ground? Why or why not?


The soil under the mulch will likely increase in moisture.


In many places, nature loves to mulch. In a grassy area, dead grasses collect on the top of the soil. In a forest, leaves and needles fall onto the top of the soil and decompose.

Other places have less natural mulch. In a warm desert, it can be quite hot on top of the soil or sand. When soil sits in the sun without protection, it collects heat. All of this heat causes water to evaporate into the air.

In your garden, you have natural mulch. If you have deciduous trees—trees whose leaves fall onto the ground in the autumn—then you have natural fall mulch. As plants die, they fall onto the ground, and if you don’t remove them, your soil will build up naturally.

When you add mulch to a garden bed, you’re doing the same thing that nature likes to do, but instead of adding mulch gradually, you’re adding the mulch all at once. You help the soil retain even more moisture because the mulch itself is full of damp materials such as manure and straw.

If it rains or if dew falls, it runs into the pockets in the mulch instead of running off the soil. This allows water to move more slowly into the soil. The mulch also traps some of this moisture, acting as a wet blanket for the soil.

Is mulch all wet? Yes, and that’s one of the reasons that it’s good for your garden.

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