By Peter Gilmer, NC State Extension Master GardenerSM volunteer of Durham County
Spring is coming, and the soil temperature is rising. Soon enough, it will be time to direct seed and to transplant the garden. Is my soil ready? What should I do to get it ready? Do I need a new soil test?
Many of us know that a soil test can be helpful, particularly when it comes to pH and lawn care. The usual soil test results recommend adding nitrogen, plus or minus phosphorus (P) and potassium (K). My results have always shown a reasonable pH, P and K values in the normal range, and the default recommendation to add nitrogen (N). As you may know, nitrogen is not actually measured. Is this enough to care for my soil?
Microbial Role in Soil Health
There is a way to build better soil, and it requires thinking about how soil works. These ideas come from the growing movement sometimes called regenerative agriculture. The basic ideas are simple, ideas that many of us sort of know. Soil function depends on microbial life, and the key to strong soil performance lays in promoting a healthy biome. Although there are other important players, we will focus on bacteria and fungi.
It turns out that these tiny organisms feed the plants by providing what the plant needs in the form that the plant requires for uptake. The essential nutrients, including nitrogen, phosphorus, and potassium (along with others) are already in the soil in nearly inexhaustible amounts, but are either remote from the plant’s reach or not in a plant-available form. The microbes are able to reach far beyond the root tips and can process the nutrients into plant-available form for uptake by the roots. There is no need for us to put the nutrients there when the microbiome is healthy. There are exceptions to this rule to be mentioned later. There is need to promote the microbial life central to soil health.
The Plant-Microbial Relationship
Before we get to the “how to” regarding microbial health, let’s review the role the plant is playing in getting its nutrient needs met. The plant is the energy source (food) for the bacteria and fungi in the soil. In other words, it is the plant that feeds the microbiome. This food is in the form of carbohydrates (sugars) produced by the plant via photosynthesis. It is estimated that 30% of the carbohydrates produced in the plant leaf are given to the microbiome. This ancient, essential relationship between soil, microbes, and plant has evolved over millions of years. Our job is to promote this functionality. It is not our job to provide nutrients directly. They are already there, in abundance.
How To Support Soil Microbes
Minimize Soil Disturbance
How do we support the microbes? The ideas are simple and basic for most gardeners. First, we need to minimize disturbance. This means less tilling. Why? Because tillage disrupts the microbial-plant dance that makes it all work. Soil aggregates, those tiny bundles of soil packaged with the help of microbes, are destroyed, and the ability of soil to hold onto water reduced drastically. The rhizosphere, the micro environment where fungi, bacteria, and root cellular membranes make critical exchanges, is disrupted physically in the process of tillage. So, step one, reduce disturbance.
The area of soil surrounding the plant root, the rhizosphere, is teeming with microbial activity. (Image credit: NC State Extension)
Of course, when planting anything, there will be disturbance. When I put a transplant into the ground, I dig (disturb) an appropriate-sized hole. What I do not do is turn over (till) the entire bed. Initially, tillage is useful in establishing a bed in an area that historically was lawn but is not used annually to clear the bed of weeds. Tillage disrupts the microbiome and doesn’t control the weed either.
Cover the Soil
Second, we should always cover the soil, either with living plants or with materials derived from dead plants (mulch). Exposed soil does not do well with direct sunlight, falling rain, and wind. In the garden, coverage is best done with cover crops in between planting seasons and wood chips or mulch where the paths are located. In a planting bed, coverage might consist of a perennial ground cover that not only covers the soil between plantings, but also puts another living root into the soil. Large areas of mulch in a planting bed might initially look good, but over time will get weedy, and the mulch cover does not provide a living root to feed the microbes.
In terms of cover crops, there is much that goes into the decision as to what cover crops to use. There are warm season covers (like buckseed) and cool season options (like crimson clover). There is the need to terminate the cover crop when the time is right to plant the main crop. This might involve mowing followed by a silage tarp. There are cover crops that winter kills. Terminating the crop before it goes to seed is critical. Polyculture, meaning multiple types sown at the same time, is better than a mass planting of a single crop. A full discussion of cover cropping goes beyond the scope of this article.
Acting as a living mulch, crimson clover is interplanted with cool weather vegetables. This cover crop provides weed suppression, erosion control, and maximizes the number of living roots in the soil–all while adding to the nitrogen pool through nitrogen fixation. (Image credit: NC State Extension)
Maintain Living Roots
Third, we should keep a living root in the soil as much of the time as possible. Remember, it is the plant through its roots that is feeding the microbiome and getting essential nutrients in exchange. The plant heals the soil. Remember when a field in fallow was considered good, a way to let the soil rest? All along the soil really needed living roots pumping fuel (carbon) to the microbiome. Planting something in between the main crop is an effective way of getting more roots in the soil. For example, you may have heard of marigolds being planted in between tomato plants.
Choose Diversity in Planting
Fourth, we should diversify whenever possible. Plants are not all the same, and they have unique relationships with their microbial partners. Monocultures are not nearly as healthy and resilient as polycultures.
Encourage Regenerative Animal Grazing
Finally, specifically in the world of farming, grazing animals can play a major role in restoring soil health. We cannot delve into this fascinating relationship in this article, but grazing animals (think bison) have played a major role for millions of years in keeping soil healthy. While this might not be applicable to the home gardener and not something I do myself, grazing is a powerful tool in managing and improving soil health.
To summarize, what should I, the home gardener, do to ensure healthy soil in my garden?
- Minimize tillage
- Keep soil covered
- Keep living roots in the soil
- Diversify plant life
I did mention that there are exceptions to these rules, and one that I should highlight is lawn maintenance. We have learned that lawns are poor supporters of the greater ecosystem. They are generally monocultures with superficial root systems, and do not develop healthy microbial life. Lawns are therefore more dependent on fertilizers and irrigation systems, all the more reason to reduce their size whenever possible.
Below you’ll find some of the best resources that I have encountered on the topic of soil health. I encourage you to dive deeper into this fascinating world.
- Brown, Gabe. “Dirt to Soil. One Family’s Journey into Regenerative Agriculture. “ Chelsea Greensboro Publishing. 2018. This is a fascinating story, well told, of going from frequent tillage and fertilizer to cover crops and soil carbon capture.
- Frost, Jesse. “The Living Soil Handbook.” Chelsea Green Publishing. 2021. This is a great how to reference book for vegetable farming.
- Lowenfels, Jeff and Lewis, Wayne. “Teaming with Microbes.” Timber Press. 2010. This is where my journey of discovery began. It was given to me by a fellow Master GardenerSM volunteer.
- Lowenfels, Jeff. “Teaming with Fungi.” Timber Press. 2017. I knew next to nothing about fungi, and this book introduced me to these critical microbes who are critical to many basic life functions on this planet.
- Lowenfels, Jeff. “Teaming with Bacteria.” Timber Press. 2022. If you want to know more about the rhizosphere, this is the book for you. What an incredible relationship between microbe and plant root.
- O’Hara, Bryan. “No-Till Intensive Vegetable Culture: Pesticide- free methods for restoring soil and growing nutrient-rich, high-yielding crops.” Chelsea Green Publishing. 2020. This guy is the master at growing vegetables and knowing what inputs (homemade compost teas) are needed and when.
- Still, Jon. “A Soil Owner’s Manual.” 2016. A 75-page, concise overview of soil management that you will likely read more than once.
Happy gardening this spring. Go microbes!
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Resources and Additional Online Information
Learn more about soil testing with NC Cooperative Extension’s handy online guide chock-full of resources.
https://durham.ces.ncsu.edu/2020/05/nows-the-perfect-time-to-test-your-soil
For a detailed look online about the benefits of no-fill farming, check out resources by Penn State University, North Carolina State University, and Cornell University.
https://extension.psu.edu/no-till-farming-and-healthy-soils-work-hand-in-hand
https://plants.ces.ncsu.edu/garden_detail/vegetable-no-till-garden
Planting cover crops is an excellent way to maintain soil health, suppress competition from unwanted plants like weeds, and stem erosion. See NC State Extension and Clemson Cooperative Extension’s online sites about how to use these plants in traditional agriculture and in raised beds.
https://growingsmallfarms.ces.ncsu.edu/growingsmallfarms-covcroplinks
https://caldwell.ces.ncsu.edu/2022/07/try-cover-cropping-a-guide-for-home-gardeners
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