Soil Air – THE Foundation of Soil Health

In this article I will tell you what the soil aerobic zone is, why it matters, and how to check how deep it is.

What is the Soil Aerobic Zone and Why Does it Matter?

The top layer of soil where microbes and feeder roots grow is called the aerobic zone. Microbes need air just like you and I do.  They will flourish in soil with abundant air.  Good soil is 25% air by volume.  Hard-packed, crusted soil does not have much air.  Because air cannot get into the soil, there is little to no microbial life.  Microbes can only live where there is enough air!

Let us consider the implications:  Microbes are the bridge on which minerals are transferred from soil to plant. If plant roots and microbes have a shallow aerobic zone to live in then the plant is not accessing as much nutrition as it needs to be healthy.  If you have 6 inches of nice black topsoil but only the top 1 inch has enough air to support microbes, there is a lot of untapped potential.  Not only that, but if that top 1-inch layer of soil is dry, many of the microbes there will go dormant due to lack of water.

The aerobic zone also matters because the depth of the aerobic zone determines the water-holding capacity of the soil. A deep aerobic zone will take up more moisture during a heavy rain and hold more moisture through long stretches without rain. Rain that falls on soil with shallow aerobic zones will have a higher rate of run off and water loss. Microbes, like people, cannot survive without water. So, a deep moist aerobic zone is the ideal environment for them to thrive.

Are there microbes in my soil?

You can see if microbes are flourishing in your soil simply by looking at your plant’s roots.  Gently dig up a small garden plant, tap off the loose soil and look at the roots.  If the roots are covered in what looks like a ‘fur’ of soil your roots are well populated with microbes.  This layer of soil on the roots is called the rhizosphere. 

How deep is my aerobic zone?

There are a few good ways to find out how deep your aerobic zone is.

  1. Look at a root ball.  In your field or garden look for an old root ball from last year such as from a corn plant.  Pick it up and take a close look.  You will see fine roots and more coarse roots.  The fine roots are the ones picking up minerals from the microbes.  They grew in the aerobic zone.  The coarse roots are for taking up moisture.  A few years ago my corn looked like it grew on a tabletop.  The root balls had a flat bottom about 1-1/2” below the soil surface.  That was as deep as my microbes were feeding the roots. [insert photo of 2 root balls]
  2. Use a spade.  Using a garden spade dig straight down in the soil and lift out a plug.  Aerobic soil rich with microbes will have a mellow crumbly texture.  The anaerobic zone (soil with few microbes, below the aerobic zone) will come apart in clumps with flat or plated sides.  How deep is your crumbly top layer?

3. Use a soil penetrometer.  A penetrometer is a tool that is used to read the hardness of the soil. This is done by pushing it into the soil. Microbes cannot live in soil above 200 psi (pounds per square inch).  Plant feeder roots cannot push through soil above 300 psi.

How to use a soil penetrometer

  • Gently push the tip of the penetrometer straight down into the soil with consistent pressure.
  • When the gauge reads 200 psi stop pushing.
  • Take hold of the probe at the soil surface.
  • Pull the probe from the soil.
  • Note the distance from the tip of the probe to your hold where the soil surface was.  This is how deep your aerobic zone is.
  • Probe again this time stopping at 300psi.
  • Mark your probe again at the soil surface and remove the probe. This is how deep your feeder roots can penetrate your soil.

Try probing a few different locations:

  • A low area where puddles form
  • The middle of the field
  • 15’ from the field edge
  • In tire tracks
  • Your garden path

Dig up the soil at these different test spots and note their texture.  Note also how much plant residue there is in the different soil zones.

Bonus tip:  Look for earthworms.  Where do you see them?  In which zone?  Earthworms typically populate the transition zone between aerobic and anaerobic zones. If your worms are near the surface, that means you have a shallow aerobic zone (or water-logged soil).


Soil microbes need a deep aerobic zone so they have air to breath and a nice environment to live and move in.  We have shown you how you can look for signs of microbes in your soil and how to know if they have the aerobic zone that they need to thrive.


Q: How can I make the soil aerobic zone deeper?

A: Check out our article on Improving your Soil Aerobic zone with Calcium. link to calcium article.

Q: How can I increase the microbial population of the soil aerobic zone?

A: Read our article on adding microbes to your soil here.

Q: How does mulch affect the soil aerobic zone?

A: Good question! Many people use mulch for weed suppression, soil protection, and/or to maintain moisture levels in the soil. It’s important that the mulch you apply does not block the soil’s access to air. If you choose to use mulch, use something that is not too dense or thick.

Q: How does no-till affect the soil aerobic zone?

A: This question is related to the one above. The layer of dead plant material left on the soil after a growing season is essentially a layer of mulch. Farmers who use a no-till method will seed through this mulch each spring. Most no-till operations will have a layer of mulch build up to the point of suffocating the microbes in the aerobic zone. The microbes are also not getting the food they need in the form of dead plant matter (carbon). Learn more about the consequences of no-till farming methods here. ***LINK TO CARBON ARTICAL? or NO-TILL ARTICLE***


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