By Jim Fisher

We know that the soil in our yards, fields and forests is host to myriad animals. Most of us first think of worms, of course, and indeed, worms are some of the larger creatures down there, lest we forget the burrowing rodents (and owls!) and other larger mammals who use burrows as shelter and protection from the elements and predators. But for the smaller creatures, the soil provides not only shelter but food. Everything they need: food, air and water, is there. Consider the variety of “soil organisms”: besides worms, there are nematodes, beetles, centipedes, springtails, protozoa, fungi mycorrhizae, bacteria, and even archaea-extremely small single-cell organisms without cell walls that date back to the earliest life on our planet.

But how can all this life be happening in what appears, to us, to be solid ground? Well, it turns out that soil is not so solid after all. All soil has porosity, defined as the space between mineral particles (and solid organic matter) filled with either air or water or both. It turns out that pore space contains and controls most of the functions of soil.


“Functions of soil”? What is that? Well, soil serves as shelter, food, a growing medium, a carbon sink, a water reservoir, a water filter for pathogens and harmful chemicals, among other things. Soil’s role in food production and climate mitigation is fundamental.

Getting back to porosity: it is not just the total amount of pore space that is important, but the size distribution of the pores and their interconnectedness that determines the function and behavior of soil. For more information on the subject, check out,  Victorian Resource Online.

Before going further, it might be helpful to further attempt to understand the scale of soil porosity and the life within it. You know the tagline of a famous TV show: “Space. The Final Frontier.”? That show was about the vastness of Outer Space, the Unknown. We here are also concerned with vast spaces-in the dirt beneath our feet. And everything is relative, isn’t it? Consider that a typical, well textured loamy soil with lots of biologic activity going on, could have between 10 and 50 percent of its pores in the range of 30 to 60 µm (microns), where a micron equals 1 millionth of a meter (a meter is approximately 39.37 inches) or 1 thousandth of a millimeter. So that is pretty small to us, living in our macro world. But to a bacterial cell with a diameter of 2 to 5 microns, that 30-60 microns pore is quite roomy. It is no wonder that within this vastness of inner space, there is room galore for biologic activity.
We have begun to set the stage for a further discussion of soil functions and biologic activity and will continue that discussion in future newsletters where we will get into, among other things, the water table-what it is and how it affects microbial life and the life of plants, which is to say the life we depend on.

Following are some interesting soil pore numbers (mostly from Victorian Resources Online).

Pore Diameter (µm) Nature of the Pore
20,000 A 20 millimeter (mm) crack
4,000 A 4 mm earthworm channel
300 Diameter of a grass root
60-30 The smallest pore that will be air filled at field capacity
100-2500 Nematode length
10 A fungal hypha
2 The size of a bacterial cell and the largest size clay particle.  The smallest pore from which a plant can readily obtain water.
0.2 The smallest pore that will give up water via suction by a plant root (permanent wilting point)
.003 The largest pore filled with water in an “air dry” soil
.0003 Diameter of a water molecule