My first organic farm mentor used to refer to topsoil as “skin of the Earth.” And each time this fine layer is broken by plow, nature’s response to repair by covering its wound with a green bandage of plants. This moment of repair becomes an opportunity. A chance for farmers to blend their own mixture of plants into a beneficial poultice.

This is where “cover crops” come in. Both preventative and restorative, cover crops are specific species of plants proven to restore soil fertility. On our farm, I make long-term rotational plantings of cover crops such as alfalfa, rye, clover, oats, buckwheat, vetch and sorghum key components of our fertility program.

But how do cover crops work? Why do they help? And why is topsoil so important anyway?

The answers to these questions begin with species diversity. There are over 800 billion microbes per pound of soil. The largest concentration of these organisms is microscopic and lives underground. That means the soil is simply teeming with activity underfoot. Nematodes, protozoa, earthworms, and mycorrhizal fungi bring life to our herbs and vibrancy to our fertility inputs. These tiny lifeforms live directly below the surface. In fact, 70% of beneficial bacteria reside in the top two inches of our soils.

That fact is worth repeating. 70% of our farm’s beneficial bacteria live in the top 2 inches of our soil. So yes, as Farm Manager, I care deeply about this 2 inch layer of topsoil.

Undisturbed cover crops create a sanctuary for these organisms, protecting them from wind and water erosion especially during our winter months. Without these microscopic organisms, soil nutrients are unavailable to the herbs we grow. Here’s how it works. Prior to plants consuming soil nutrients, these microbial creatures eat at the table first. After dinner, key crop nutrients, such as nitrogen, phosphorous and potassium, are converted into a microbially-digested form that the plants can use more efficiently for healthy growth.

Planting cover crop species also serves as our primary strategy against patterns of crop disease and pest damage. For example, fungal diseases are often crop specific and tend to spend the winter in the decaying autumn leaves, only to re-emerge again next spring. Cover crops let us combat these diseases without using harmful chemicals. A field rotation of cover crops planted after a long-term perennial planting will naturally break the cycle of disease without having to relying on pest and disease control products.

When our cover crops hit their peak growth, we till them back into the soil. This incorporates copious amounts of biomass back into topsoil. We call it “green manure.” In comparison to row crops, a thick cover of grass and clover provide an increased amount of organic matter over a short period of time.

And our legumes like clover, alfalfa and vetch provide us with an on-farm source of nitrogen. The legumes have the ability to absorb nitrogen from the atmosphere and store it in nodules attached to their root system. These nodules will convert atmospheric nitrogen into a usable form for the next herb crop.

Ultimately, our plant poultice mixture of oats, sudangrass or ryegrass with a leguminous clover, vetch, or peas helps prevent water runoff and soil erosion throughout each season, reduces dependence on off-farm fertilizers and pest controls, and enhances the overall organic health of our soil.

Our herbs can only grow as fertile as our soil. So, keep it green!

Article by Matt Dybala, the Farm Manager at Herb Pharm in Josephine County, Oregon.