• Most of us walk across soil every day without ever pondering its complexity. Yet, soil is an essential part of the ecosystem. Soils filter and drain our water; support and feed crops; cycle nutrients into, out of and back into food systems; and provide habitat above and below the surface for all manner of organisms. Ecologists, chemists, pharmaceutical scientists, farmers, and engineers have all taken serious interest in the sophistication of soil, and as such soil experiments are great classroom projects.
  
  

Soil pH

• The pH of soil greatly influences the biotic communities above and within the soil; those communities, in turn, affect the pH of the soil surrounding them. Factors such as soil composition, rainfall, and leaching also influence the pH of soils. Measure the pH of soils using a garden soil pH kit, using samples from different locations and taken from different depths. Make note of the location, vegetation, topography, and land use from which each sample was drawn to make note of how those factors may relate to soil pH. Soil-test kits also can be used to test for the presence of other substances in the soil, such as the essential plant nutrients nitrogen, phosphorous, and potassium.
  
  

Soil Drainage

• Determining how well a soil drains depends on the particles within the soil as well as the compaction of the soil. Examine soil samples visually and by texture to determine whether it is mostly clay (which drains poorly) or sand (which drains quickly), or contains a lot of organic matter (humus). Compact the samples equally by dropping a weight from a specified height the same number of times onto equivalent masses of soil samples. For example, drop a five gram weight 10 times on a 10 gram soil sample. This compaction is usually done in the the instrument that will be used for the drainage part of the test to prevent disturbing the sample once compacted. After the soil is compacted, slowly drip measured volumes of water through the soils and record the rate at which the water collects beneath the soil sample.
  
  

Earthworms and Soil

• Earthworms aerate soils and alter water drainage patterns. The effect of earthworms on the soil can be illustrated through a series of projects. To determine the effect of earthworms on soil, layer differently colored, moist soils on top of each other in a jar. Add an earthworm to the jar and cover the jar, especially the sides, with opaque material to keep the soil column dark. After a few weeks, remove the opaque barrier and observe the mixing that has occurred within the soil. To tweak this project a little, compare the drainage in the earthworm soil against the drainage of soil layered the same way, but without the worms.