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The Dirt on Soil Health

Written by Jane Summerfield

What is soil? 

What some of us might think of as simply dirt is actually a complex web of ecological interactions.

Soil is very much alive.

Biological, physical, and chemical interactions drive the system.

Good soil is roughly half solid and half empty. Substrate material (essentially broken down rock), minerals, and organic matter, make up the soil parts while air and water fill the voids.

Macroinvertebrates (like earthworms and beetles), fungi, bacteria, and all other types of life work together in these voids to thrive, often through symbiotic relationships.

One of the most important parts of soil is its 5% organic material–something called Soil Organic Matter (SOM).

Essentially carbon sequestered in the soil, SOM comes from decomposed organic materials, such as plant and animal waste.

SOM is a vital aspect of sustainable farming. It is a source of energy for plants, a reservoir of bioavailable nutrients, provides protection from metal toxicity, and facilitates soil structure and porosity.

It is also a potential sink for atmospheric carbon and global warming mitigation.

What is Soil Health?

Simply put, healthy interactions between these physical, biological, and chemical interactions make soil healthy.

When everything is working properly, SOM is built, nutrients are maintained, and the soil ecosystem flourishes.

Some very specific chemical and biological indicators contribute to healthy soil.

Here are just a few:

Soil pH

An indication of the acidity and alkalinity of the soil. The lower the pH, the more acidic the soil, the higher the pH, the more alkaline the soil.

If pH is too high or too low plants will not be able to get nutrients due to chemical reactions. The ideal range for soil pH is 6.4-7.

Cat-ion Exchange Capacity (CEC)

An indication of the amount of electrical charge in the soil. Believe it or not, plants need negatively charged soil, to be able to take up nutrients. The ideal range for CEC is 20 meg/100cm3.

Humic Matter

An indicator of the organic matter in the soil.  It is the portion of the total organic matter that is chemically active.

Typically, an ideal percentage for the total organic matter would be about 5%, and an ideal percentage for the humic matter would be about 1.5%.

Bulk Density

A measure of the weight of the soil divided by the volume, this is an indicator of how porous and drainable the soil is. Typically, the lower the bulk density the better.

Phosphorus Index

A measure of the phosphorous in the soil that is available to plants. Ideally, this would be a measure of 30 or above.


An indicator of biological activity. This is a fun and easy test to do at home. Dig up a 1ft x 1ft x 1 ft cube of soil and sift through it, pulling out the worms.

Good active soil can have up to 50 worms/ft3

What does healthy soil do?

The benefits of keeping soil healthy are huge! Here are just a few examples of what healthy soil can do.

Prevents erosion

Networks of roots, fungi, earthworms, and other biota work to keep the soil in place.

Builds coastlands and saves coastlines

Healthy soil on coastal lands provides filtration of erosion and runoff before it has the chance to potentially harm fisheries – a vital source of food for many people in the world.

Grows better crops

A healthy below-ground ecosystem can more easily transfer nutrients to plants above.

Filters runoff

During rain, root systems hold water, above-ground biomass slows down the movement of water and microbes break down fertilizers and other nutrients, allowing clean water to move through the watershed at a non-flooding pace.

Reduces the number of inputs

Biological and chemical interactions in a healthy soil managed properly can provide plants with much of the nutrients they need to grow, reducing fertilizers.

Healthy plants are much more resistant to pest pressure, reducing the number of herbicides, pesticides, and fungicides needed to protect crops.

Sequesters Carbon

As plants absorb CO2 from the atmosphere, a healthy soil builds soil organic matter, a stable form of carbon, locking it into the soil and working to reverse climate change.

How Can Healthy Soil Save our Planet?

There’s a long list of why soil is fast emerging as a new facet of the environmental movement: long-term sustainability of food, fiber, and fuel production, alleviation of world hunger, potable water, thriving coastlands, and fisheries, drought and flood protection, and carbon sequestration.

Recently, there has been a lot of hype about this last one.

More and more academics, farmers, and everyday people are getting interested in the idea that building healthy soil can mitigate climate change.

In a recent interview on Experience Life, author Kristin Ohlson gives a pretty succinct explanation:

Here’s how it works. Plants remove carbon dioxide from the air through photosynthesis and convert it into carbon-based sugars and protein to fuel their growth. But plants only use about 60 percent of this carbon-based food themselves.

The rest of it is sent down to their roots, where the microorganisms cluster like pigs at a trough. (Each tablespoon of healthy soil contains billions of microorganisms like the Purple Snout Mite seen here.)

The roots ooze out a carbon meal for the microorganisms. In turn, the microorganisms leave behind mineral nutrients the plants need, which they’ve liberated from sand and rock.

That carbon cycles through the soil, as the microorganisms both eat it and use it to build tiny structures for protection and to hold water. Finally, the carbon is “fixed” in the soil, semi-permanent, until hit by a disturbancelike plowing.”

Some experts believe that building healthy soil around the world could potentially stop climate change.

How can we support healthy soil?

Take the PLEDGE! Protect our soil—YOUR soil—one backyard, one neighborhood or village, and one local farm at a time.

Start composting your food and garden waste—an excellent way to raise awareness about soil and build up the soil in your own backyard (not to mention reducing the waste that you put into the landfill).

Feel like digging deeper?
Check out “Why Soil Health? Earth Day, Global Soil Week and the International Year of Soil.” by Damon Cory-Watson, our own in-house expert on soil health. Break down those rotten barriers and learn how to compost in just 7 easy steps, or discover how you can build a simple composting system in your backyard in less than an hour.

Here are just a few resources for further reading:

  • Soil Health via Natural Resource Conservation Services via The United States Department of Agriculture
  • Soil Health Literature via The United States Department of Agriculture 
  • Soil as Carbon Storehouse: New Weapon in Climate Fight? via Yale.edu

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