How Can We Measure the Health of Our Soil?

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In this post I will teach you a few ways that you can measure the health of your soil. Some of those ways are measuring the soil directly.  We can also measure soil health indirectly by testing the plants growing in the soil.  Healthy soil will grow healthy plants.

Note:

  • When testing, write down your readings and observations in a notebook. Compare notes to previous test dates and years.
  • Try to test at the same spots in your garden, site or field.
  • Test between 10am and 2pm Learn why here.

These are the tools we use to measure soil health:

Penetrometer

A penetrometer measures how hard pack the soil is.  It is a tool that we push into the ground.  A gauge on the meter shows how hard we are pushing.

How to use a 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

What do the numbers mean?  Through observation we have learned what grows and does not grow in various levels of soil compaction.  Soil microbes cannot grow in soil that is tighter than 200psi.  The feeder roots of most plants and crops cannot push through 300psi.  If you cannot push through deeper than 1.5” before your gauge shows 200psi then your microbes have 1.5” of soil to live in.

Tillage can break up and mellow your soil but only in the short term.  The clay particles will settle together again in a few days or weeks.

You will notice in the Spring that soil is very mellow for quite a depth.  It’s wonderful.  Winter frosts have done this.  Keep testing your soil every few weeks and see if it stays mellow.

How To Use A Spade Instead of a Penetrometer

A penetrometer is expensive. Instead of buying one you can use a spade to look at your aerobic zone. You should be doing this anyways.

Directions:

  • Using a spade, dig straight down as deep as you can
  • Carefully raise out a spade full of soil. Try to keep the soil plug intact.
  • Observe the different soil types on your plug.
  • While you’re at it, smell your soil.

What do you see?

The aerobic zone should look loose and airy. It will be the top layer of soil anywhere from 1-4″ thick. This is where your beneficial microbes live. Plant roots can penetrate this soil This is where the fine hairs of roots are getting their nutrients from.

The anaerobic zone is below the aerobic zone. It usually is made up of smaller clumps that have flat sides as they come apart. Plant feeder roots, the ones that take up nutrients, cannot penetrate this soil It’s too hard.

Earthworms are generally found in the transition between the two zones. If you have earthworms in your first inch of soil, it’s an indication your aerobic zone is not very deep.

Testing Soil Electrical Conductivity

Without getting too technical, I’ll state that plants need electricity to live.  They get electricity from the air, fertilizers (mostly nitrogen) and from soil microbes.  These little guys make electricity.  An Electrical Conductivity (EC) Meter measures this electricity.

Directions for use:

  • Turn on your EC meter
  • Push testing probe into the soil no deeper than the depth of your plant’s roots (the aerobic zone which you measured with a penetrometer or spade). 
  • Wait a few seconds for the reading to stabilize
  • Read the meter display
Testing Soil EC / Conductivity

Sometimes the EC reading needs to be converted.  If my meter gives a reading of 0.26 I need to convert that to 260.  I ignore the decimal and add a zero for a reading of 260.

What do the numbers mean?

A reading below 200 means your plant is starved for energy.  Above 1200 and it’s roots are being electrocuted.  An ideal EC range is 300 to 800.  It’s common to start out at 300 in the Spring and see the EC climb to 800 throughout the growing season until harvest.  If you get a very high reading and you fertilized your soil, move over a foot and test again.  It’s possible you hit a fertilized band.

Using a Refractometer to Test Plant Health

We can easily test the sugar level of a plant.  The sugar level in the plant sap correlates to the level of minerals in the plant.  The more minerals, the healthier the plant. The mineral content of the plant sap correlates directly to the health of the soil.

Garlic Press
Refractometer

Steps:

  • Open the cover shield on the refractometer
  • Select a few leaves or bits of stem from a few plants to fill the garlic press
  • Squeeze out 2-3 drops of juice from the plant parts
  • Place the drops on the refractometer’s glass window
  • Gently close the cover
  • Look through the eyepiece

Take note of where in the index the white zone becomes blue.  This index number is the measurement we call “Brix.”

The image below shows a brix reading of 13.

What do the numbers mean?

If the brix is below 2, a plant has no nutrients to work with and is not producing sugar. It is completely vulnerable to any insect, disease or pathogen that comes along.

If the brix is 3-7, the plant has a fighting chance. It is producing some sugars and it’s immune system is somewhat operational.

If the brix is 8-12, this plant is no longer vulnerable. It is producing sugars and taking up minerals. Insects will not want to eat it. They will avoid this plant. In fact, the insects do not even see the plant! The plant is also able to protect itself from fungal infection.

A brix reading above 13 means that this plant is humming along, thriving, and flourishing. Its seed will be high quality. Fruit and vegetables will be full of flavor and very sweet!

Sometimes the line is blurry. This is OK. As brix increases the line sometimes gets blurry.

For fun, test the produce you buy from local farmers or the big box grocery store and see the quality of what you are buying!

courtesy Dr. Tom Dykstra

When Testing, Make Some Other Observations:

  • What is the general impression you get of plant health? Are the plants a deep green? Somewhat yellowed? Are there fungal infestations in the leaves? Are there bugs present? Are they eating your plant? Has the crop grown since the last time you looked at it?
  • What is the soil condition? Is it moist? Sodden? Dry? Is old straw matter present? Is it breaking down? How does it smell? How is it’s texture in your hand? Do you see earthworms? Where are they in the soil profile? Do you see their castings (poop) on the soil surface
  • What has the weather been in the past week? Windy? Calm? Hot? Cool?

How often should I test my soil and plants?

Soil compaction (aerobic zone) should be tested every month or two to give you an idea of how texture changes throughout the season. In regions where the ground freezes in Winter soil will always be very mellow in Spring. Frost has pushed the soil particles apart and everything is wonderful. As the season progresses soil usually hardens.

Soil EC should be tested weekly. The presence of microbes and fertilizer causes electricity in the soil to fluctuate and can affect your EC readings. Sometimes soil runs out of energy for the plant. If this is the case, the EC reading will show this. You can help the plants today by foliar feeding and save your crop. It’s been done.

Plant sap should also be tested with a refractometer every week. Take note of the weather when testing. If rain is imminent, brix readings are often 2 points lower. It is theorized the plant is storing resources in the roots just in case a storm lops off the top of the plant.


Whew! That was a lot. Now we know how to test the health of our soil and plants. We can see how hard our soil is with the penetrometer or spade. We can test the Electrical Conductivity with an EC meter. And we can test how healthy the plant is right now with the refractometer. These three tools will go a long way in showing you how your soil is doing. Have fun out there and watch your growth!


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