Wednesday, May 31, 2023

Gardening: Topsoils and Subsoils

Bare-RootsLucky Texans push a spade into the homestead soil then exclaim, “Ah, it’s brown.  I have topsoil.”

The State of Texas possesses 1,411 natural soil types, and the one thing they all have in common–unless they lay under a forest–is the absence of true topsoil.  Whaaa?

True topsoil is a dark brown 2-6 inch layer of partially decomposed plant and animal matter in the process of being chewed, digested, and excreted by a hungry population of worms, bugs, bacteria, fungi, and one-celled critters.  The operative phrase here is partially decomposed.  Commercial “landscape mix” is a topsoil put together by someone other than Mother Nature.

The layer of earth directly under natural topsoil is completely decomposed brown dirt scientists call “the humus layer.”  Think Halloween because humus is the skeletal leftovers of dead plants and animals.  Spaces between the little bones hold air and water and the occasional soil Trick-or-Treater.  Water has dragged the humus layer’s chemical candy, like screaming and kicking Persephone, down into the subsoil Underworld scientist types call its “storage horizon” or layer.

The problem with the storage layer is its location.  A gardener with a spade can dig down 25 inches or MORE before finding the stuff.  Most vegetables’ feeder roots grow about 6 inches long, so you see the problem.  Tomatoes, for example, send out long roots, but do they ever go very deep?  No.  Taproots take in some nutrients from the storage layer, but their main job is to anchor the plant.

How can an area the size of Texas have no topsoil?  In the eastern part of the state the answer is King Cotton.  The old plantation routine was to eat up the topsoil then pull up stakes and move west until you finally hit the brick wall called desert.  There’s no topsoil in the desert because there isn’t enough water to sustain the teeming life forms that die and will their bodies and excretions to gardeners.  What’s across the Rio Grande?  If you guessed more desert you’re right.

All the popular talk about composting is about creating topsoil in sufficient quantity to cover the garden 2-6 inches deep with the stuff, then adding water and waiting for local soil animals to show up and start reproducing, which they will do exponentially as long as the soil cupboard is full.  A full cupboard?  The Bard once said, “Aye, there’s the rub!”

Six inches of damp compost makes about ¼ inch of topsoil after about a year.  That means adding more compost on a regular basis.  Free yard waste, chipped wood, kitchen vegetable scraps, farm stall mucks, leaves, twigs, etc. start to look real attractive.  The good news is that topsoil formation increases exponentially as soil animals reproduce exponentially.  When you notice topsoil has “compacted” to leave your garden surface a couple of inches lower than it was a year ago, in fact inches of your compost have decomposed to a thin layer.  This is Mother Nature’s way of saying it’s time to add organic matter in 6 inch deep increments.

At this point gardeners bump into the philosophical question “So, am I taking care of my plants or my soil?”


Contact the writer at [email protected]

Noelle Hood
Noelle Hood
Noelle is our Resident Green Thumb and a Texas Agri-Life Master Gardener.

Related Articles

Popular This Week