SCV Outdoor Report: Hole-y Dirt
By Wendy Langhans
I learned a lot by playing in a sandbox when I was a child. I particularly remember pouring buckets of water over the imaginary landscape. It was fascinating to watch the sand change color as it soaked up the water, much like our kitchen sponge turned bright red as it soaked up my spilled cherry Kool-Aid. I discovered that dirt is hole-y.
Notice the subtle color changes between wet and dry soil.
But as I got older, I learned more. Holes are not simply the two-dimensional circles we see at the surface; rather, they are a constantly changing, three-dimensional network of twisting and interconnected passageways. But to keep our discussion brief, we can classify holes into three types, based on their diameter.
- Micropores are 5-30 microns in diameter. (By comparison, the eye of a needle is about 749 microns wide.) They serve as water reservoirs in the soil, because they are small enough to hold water by capillary attraction against the pull of gravity. This is where you will find bacteria and other microorganisms.
- Mesopores are 35-70 microns in diameter. They allow water to move slowly. This is where you will find fungi and root hairs.
- Macropores are greater than 75 microns in diameter. Water drains through these pores by gravity. These pores also allow oxygen to enter the soil. This is where you will find invertebrate animals such as worms and larger plant roots.
Soil is a "mineral/organic framework" in which air and water are distributed through a matrix of holes. In simpler words, soil is made up of stuff, empty space and the surface area where they meet. An English researcher once calculated that the surface area "lining the tiny pores and passageways in a couple of tablespoons of soil added up to a quarter of a million square feet - the area occupied by a city block". (James Nardi, "Life in the Soil"). And just like any city, these passageways are teeming with life.
Who knew that hole-y dirt could be the portal to such a complex and interesting world? Or as Alice said after she went down the rabbit hole, "curiouser and curiouser."
Upcoming Outdoor Events:
Saturday, October 17, 8-10 AM. Bird walk in Towsley Canyon. All year round, the habitats of Towsley Canyon attract a wealth of birdlife. Beginners are welcome. Bring your binoculars. 2 hours, easy walk. For a map, go here.
You can listen to stories like this every Friday morning at 7:10 a.m. on "The Hike Report", brought to you by your hometown radio station KHTS (AM1220) and by the Mountains Recreation and Conservation Authority.