Science project

Plants and Soil Acidity

Difficulty of the Project



$15 for soil test kit

Safety Issues

Walking in the forest, must wear appropriate shoes and outdoor gear, working with chemicals.

Approximate Time Required to Complete the Project

4 hours


To determine whether the types of plants growing in a Pacific Northwest rainforest environment are influenced by the soil acidity.

Materials and Equipment/Ingredients

  • Soil pH test kit, found at your local hardware store or garden center
  • Plant identification book, found at the library.


Temperate rainforest soil is composed of the needles from coniferous trees, and these needles decompose and lead to increased soil acidity. When forests are logged or experience a disaster like a forest fire, the first trees that come back are often deciduous trees. This can lead to an entirely different ecology in the same place. Is this because of the reduction in needle-bearing trees, or are there other factors involved that strongly influence what plants grow where in the rainforest?

Terms, Concepts and Questions for Background Research

Temperate rainforest, soil pH, soil acidity

  • Is the pH of temperate rainforest soil the main determinant of what plants can grow in a particular location?
  • What other factors might influence plant growth?

Experimental Procedure

  1. Take two soil samples from each site. One should be a site with very few coniferous trees and one from a site with many coniferous trees. Dig down under the top layer of detritus to reach the soil’s surface.
  2. Test the pH of each soil sample. Make a note of the pH.
  3. Find a shady place in the coniferous site and draw and count the plants that are growing there. Identify them with the plants book.
  4. Find a shady place in the deciduous site and draw and count the plants that are growing there. Identify them with the plants book.
  5. Look up each plant to see whether it prefers a very acidic, mildly acidic, neutral or basic soil pH.
  6. Draw two bar charts, one for each site. Show the number of plants that prefer each type of soil, one per bar.
  7. Compare the soil pH on each site with the number of plants that prefer that soil, to see if there is a correlation between the soil pH and the plant type.


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