Day #4: Soils AKA Dirt

Dirt – it’s not just for kids anymore.

The 2016 class of Green Spring Gardens master gardener trainees got into it on Thursday.

If we thought botany was technical, it had nothing over the Soils and Nutrient Management session.

There is, in fact, more to soil than … dirt.

In addition to the solid matter, there’s water and air, too. And not just a little.

“Perfect” soil contains 25% water and 25% air in addition to the … dirt, or solid matter, as we say in pedology, the study of soil in the environment. (Look how much you’ve learned already, and you haven’t even reached the Lessons Learned section.)

That solid matter starts as rock. But with one thing and another, over tens of thousands of years it gets worn down to sand, silt and clay. It takes about 500 years to make an inch of soil.

Those three elements create the soil’s texture. Sometimes it’s smooth, other times gritty or sticky.You’ve noticed the texture of soil; that humus-y stuff from the nursery feels a lot better that the stuff in your back yard.

How do we tell what is sand, silt and clay? This is cool. We look at the diameter of the particles. Well, not us, but somebody with a really good measuring stick.

  • Sand is 0.05 mm to 2 mm in diameter. That’s a fraction of a centimeter, which is a fraction of an inch. So, pretty small.
  • Silt particles are smaller still, between 0.002 and 0.05 mm.
  • Finally, a particle of clay is less than 0.002 mm, or 2 microns.

The last 5% of soil is organic matter, which is either dead – think crinkly brown autumn leaves – or alive. Live organic matter requires that you think beyond the mammalian, air-sucking variety of life. The following are all live organic matter:

  • Roots
  • Worms
  • Beetles
  • Microbes
  • Fungi

Lessons Learned

  • Soil is important because it keeps water and nutrients flowing to your plants.
  • You cannot change the composition of your soil. If you’ve got clay, you’ve got clay.
  • But … you can improve any soil by adding organic matter. (Think compost and humus.) It won’t change clay into silt, but it will improve other soil properties, which we don’t have time to go into.
  • Adding organic material to your soil increases the amount of water available for plants.
  • Virginia clay soil is only 20-40% clay. Disappointing, I know, when it turns to concrete in July and August.
  • Some dirt is soil; some dirt is just dirty.


Thou shalt not keep the soil bare.

Cover it with something, anything is better than nothing.  Bare soil erodes.


Read Native Plants for Wildlife Habitat and Landscape Conservation plus Terrestrial Invasive Plants of the Potomac River Watershed.

Today’s Photo

Brugmansia, or Angel’s Trumpet. Like Datura, part of the Nightshade family. Sweet dreams.


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