Category: soil

A Better Way to Think About Soil

On By durhammastergardenersIn cover crops, Durham, how to improve soil, no-till gardening, plant fungi, plant-microbial relationship, polyculture, regenerative agriculture, rhizosphere, soil, soil health, soil microbes, soil test, Uncategorized

By Peter Gilmer, NC State Extension Master GardenerSM volunteer of Durham County

(Image credit: Pixabay)

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.

Planting beds flush with cover crops consisting of radish, crimson clover, and rye, with wood chips covering the walking paths. (Image credit: Peter Gilmer)

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

Take Me Out to a Tarped Field

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

Cover Crops in Raised Beds

Article Short Link https://wp.me/p2nIr1-4hE

Diabase Bedrock: The Foundation for Unique Native Wildflowers at Penny’s Bend

On By durhammastergardenersIn Natives, soil, Uncategorized

By Wendy Diaz EMGV

This article was first published in The Gardeners Buzz, a newsletter of the Extension Master Gardeners of Durham County in June, 2022.

On April 2, 2022 my husband and I joined the ‘Spring Botanizing at Penny’s Bend‘ tour organized by Duke Gardens Education Program for an easy hike at Penny’s Bend Nature Preserve[1]. Duke University, Department of Biology professors Paul Manos and Alec Motten guided us and answered our questions. The botany at Penny’s Bend is unique because of the geology of the site. The restoration of the rare Piedmont Savanna at Penny’s Bend is ongoing by the North Carolina Botanical Garden at the Preserve[2].

Water flows downhill, along a path of least resistance, and that is one reason rivers meander. Penny’s Bend is a local example of a pronounced sharp curve in the course of the Eno River. The reason for this abrupt change and subsequent re-correction of the river course is that it passes around the hard, erosion-resistant igneous rock called diabase. The easterly flowing Eno River encountered the diabase promontory exposed at the surface and abruptly changed course – flowing south and then for a short distance, north – along a diabase sill outcrop forming a pinched horseshoe shape. 

From left to right: Photo of trail head map at the entrance Kiosk at Penny’s Bend Nature Preserve, Durham North Carolina. Looking north from the top of diabase sill outcrop along Eno River. All Photos by Wendy Diaz, April 2022

A sill is the term for a geologic structure forming a sheet-like igneous body that is parallel to the bedding planes and solidified between the sedimentary rock layers of the host rock; whereas, a dyke is a sheet of igneous rock that cuts across bedding planes of the host rock. There isn’t a lot of exposed bedrock in Durham County but there are a few outcrops of the diabase rock, such as along the Eno River at Penny’s Bend and the old rock quarry walls of the bear enclosure at the Museum of Life and Science. I also noticed there are some transported diabase boulders (not outcrop) along the side of Brigg’s Avenue Community Garden.

Diabase bedrock wall near the bear enclosure (former quarry) at the Museum of Life and Science, Durham, North Carolina.

While the diabase rock may not be unusual, geologically speaking, the soil derived from it did stimulate the growth of a group of unique plants that flower in the spring and summer at Penny’s Bend. Dutchman’s Breeches (Dicentra cucullaria)[3], the endangered Smooth purple coneflower (Echniacea laevigata), Prairie blue wild indigo (Baptisia aberrans) and Hoary puccoon (Lithospermum canescens) are some of the unique plants that require neutral to alkaline soil conditions. While it was too early in the season to see the blooms of most of these unique wildflowers, we did see large patches of Dutchman’s Breeches along the north side of the rock face and the east bank of the Eno River.

Photo of Dutchman’s Breeches (Dicentra cucullaria).

Some spring ephemeral flowers that we saw on our walk were Spring Beauty (Claytonia virginica), which have pink stripes on the petals to guide pollinators to the nectar, Rue Anemone (Anemonella thalictroides) and Sessile Bellwort (Uvularia sessilifolia). A large colony of Painted Buckeye (Aesculus sylvatica) shrubs was also in bloom near the diabase cliff along the north end of the bend. There was also many Trout lily (Erythronium americanum ) seed pods and leaves along the hiking path. 

Yellow blossoms of the Painted Buckeye (Aesculus sylvatica).

A Brief Geologic History of North Carolina

In Durham County, we live among the rolling hills of one of three physiographic regions, known as the central Piedmont, which is located between the Blue Ridge Mountains to the west and the relatively flat Coastal Plain to the east. Each region can be divided into geologic belts or ‘terranes’ that have similar rock types and geologic histories[4]. Furthest west, the oldest Blue Ridge Belt is a mountainous region with rocks dating all the way back to 1.5 billion to 1 billion years. These rounded mountains were thought to have peaks as high as 20,000 feet before millions of years of erosion took their toll. The youngest belt is the Coastal Plain-a wedge of marine sedimentary rocks which thicken closer to the coast and formed from sediments eroded from the mountains to the west between 65 million to just a few million years ago[5]. The bedrock which underlies the Piedmont consists of several different rock types formed over several hundred million years of geological history. Piedmont bedrock age ranges from the metamorphic Inner Piedmont Belt of 500 to 750 million years old to the sedimentary rocks of the Late Triassic Basins formed between about 235 million and 200 million years old. Most of the southern half of Durham County is underlain by one of three Triassic Basins, named the Deep River Basin which extends from Granville to Union County. Younger still, are the diabase dykes and sills intruded into the Late Triassic sedimentary rocks during the Early Jurassic Age[6] around 200 million and 175 million years ago when the supercontinent Pangaea was breaking apart forming what would eventually become the Atlantic Ocean. (This is illustrated in the YouTube video by California Academy of the Sciences[7]). Yes, the Jurassic Period, known as the peak age of the dinosaurs. When the continental plates diverged, the earth’s crust thinned when it was stretched, then the crust over the mantle became less dense and the reduced weight promoted the uppermost mantle to partially melt producing basaltic magma which intruded into the Triassic sediments.

About 220 million years ago when the Deep River Basin (rift valley) began to form, the continental tectonic plates pulled apart, forming cracks in the earth’s crust, roughly perpendicular to the direction of the moving continents. This basin was bordered by faults. Over millions of years it gradually filled up with eroded sediments from the much higher Blue Ridge mountains and the Piedmont Plateau to the west of the basin. These sediments solidified and formed mudstones, siltstones and shale. Other cracks continued to widen until they filled with ocean water. The pulled apart edge of early North Carolina (minus the coastal plain which was deposited after the pulling apart of the continents that formed the supercontinent of Pangea) roughly fits with the coastline of Africa from Mauritania and Morocco. At this point in geologic history the North American continent was closer to the equator.

Rock Mineralogy

Diabase is a dark grey to black, medium-grained, mafic igneous rock composed of Iron and Magnesium-rich minerals such as pyroxene. The exposed rock of the diabase sill at Penny’s Bend weathered and formed a soil markedly different in chemistry and properties to the surrounding acidic soil which was derived from the Triassic sediments. 

Close up of unweathered medium-textured diabase rock. Rust-colored surface of weathered diabase rock in the background.

When the minerals of the diabase rock breakdown they form clay minerals that form ‘sweet’ or basic soil with high pH and richer in iron and magnesium that what many other native plants can tolerate2. The soils are also rich in calcium from the plagioclase feldspar mineral[8]. Medium-grain size in igneous rock indicates the basaltic magma intruded and cooled underground near the earth’s surface (microscopic grains would indicate the molten rock cooled quickly on the earth’s surface). The diabase rock is more resistant to erosion because it is igneous rock that solidified from molten rock and not from cemented together grains of the surrounding less-resistant sedimentary rock. 

Fractured and weathered boulders from the Diabase sill outcrop along the east bank of the south flowing part of the Eno River.

The old diabase bedrock beneath Durham formed millions of years ago, and is now the foundation of our current natural world and determines the native plants we enjoy today on our spring and summer walks through the Penny’s Bend Nature Preserve. If you have not yet visited this site, I recommend a short walk along the Eno River, especially in the spring time. You will be rewarded when you see the unusual flora and fauna – and don’t forget to take a closer look at the bedrock that is exposed along the river bank which makes this area so special.

Black snake resting on dead tree stump along the Pyne trail at Penny’s Bend Nature Preserve. The new snake skin scales reminds me of the texture of unweathered surface of diabase igneous rock

DRIVING DIRECTIONS to Penny’s Bend from Durham (3710 Snow Hill Road, Durham, NC 27712)

Head north on Roxboro Street.

About 1.5 miles north of I-85, turn right onto Old Oxford Road.

Stay straight for 3.2 miles until you cross the Eno River.

Turn left onto Snow Hill Road and on the left is a gravel parking area on the left side of the road.

References:

[1] https://www.enoriver.org/what-we-protect/parks/pennys-bend/

[2] A River Runs Around It: Restoring the Rare Flora of Penny’s Bend by Emily Oglesby, Conservation Gardener Magazine, Spring/Summer 2022, North Carolina Botanical Garden, The University of North Carolina.

[3] https://www.wildflower.org/plants/result.php?id_plant=DICU

[4]Generalized Geologic Map of North Carolina, N.C. Department of Environmental Quality https://deq.nc.gov/about/divisions/energy-mineral-land-resources/north-carolina-geological-survey/ncgs-publications

[5] Crossroads of the Natural World. Exploring North Carolina with Tom Earnhardt. University of North Carolina Press: Chapel Hill, 2013, 314 pages.

[6] Geologic Map of the Southwest Durham 7.5 Minute Quadrangle, Durham and Orange Counties, North Carolina by Charles W. Hoffman and Patricia E. Gallagher, North Carolina Geological Survey Open File Report 2001-XX

[7] https://www.youtube.com/watch?v=ADsjdu27WaM California Academy of the Sciences

[8] Exploring the Geology of the Carolinas, Kevin G. Stewart and Mary-Russell Roberson. The University of North Carolina Press, Chapel Hill, 2007, 298 pages.