The everyday magic of soil

A framework for agricultural soils

You have been incredibly gracious. Here I am, writing a newsletter called “Topsoil,” and after nearly three years, I have yet to be called out for not covering the newsletter’s namesake: soil.

Today we are changing that.

I had an opportunity to interview my friend, Dr. Andrew McGowan, about all things soil. Andrew earned a PhD in Agronomy at Kansas State University, where he was a soil researcher. He has applied his expertise at several companies, including The Climate Corporation, Pattern Ag, Mineral, Vayda, and currently at Terradot. Throughout his career, he has helped bring farmers several soil-based innovations, from models and hardware that rely on soil data to tailored management practices based on soil.

Soil is vast, complex, dynamic, and highly diverse. These features are one big reason why agricultural innovations that interact with the soil (spoiler: most innovations) are difficult to apply universally. Today, Andrew will share basic understanding of soil science and how it impacts innovation in agriculture.

Let’s dig in!

Soil Searching is the new Birding

After forming over hundreds of millions of years, soil quietly, relentlessly provides functions that humans rely on every single day. It supports plant life by providing a growing medium, nutrients, and water. It is home to nearly 60% of the entire planet’s living species, many of which are yet to be discovered. It keeps carbon locked away. It stores and filters water. It allows the dead to live on again through nutrient cycling.

As Andrew describes, “There’s something kind of magical about discovering this whole new world that’s been around you the whole time and you just weren’t aware of it. It’s literally around us, everywhere. And yet, you can’t really work with soils without adopting a systems framework because of how complex and interconnected the pieces of the soil system are.” (Side note: any quotes are from Andrew unless otherwise noted).

“When I know I am going to be visiting a farm in a new place, I like to look at the US Soil Survey maps and read about the soil types and hopefully I’ll have a shovel with me and I can dig a little bit and check them out. And part of it is kind of like birding, ‘I haven’t seen this type of soil before! I want to see it, feel and understand what it’s about and how it might be impacting what’s going on in the area.’”

The fantastical diversity of soil is also what makes it a natural limit to scale for many innovations and practices in agriculture. “The most common misconception that people have about soil is that they understand soil. Pick your favorite topic in agriculture where there is a soil component, and I guarantee there are some issues that came up from oversimplifying the soil system and making some assumptions that turned out to not be true. Moving from one geography to another, things can change. Things can even change if you’re looking at the first foot of soil versus a few feet further down.”

To get a sense of what Andrew is talking about, take a look at these maps of the US and Brazil, color-coded for Soil Orders or Classes (the broadest soil taxonomy for each, respectively). Both are vibrant patchworks:

Source: USDA Natural Resources Conservation Service The Twelve Orders of Soil Taxonomy

Source: Embrapa

Andrew teases apart some of the complexity by looking at soil through three lenses: soil chemistry, soil physics, and soil biology. These three interwoven pillars help explain why soil in one field can differ from anywhere else in the world, and how to evaluate the ways that farming practices impact soil health.

Soil chemistry

One thing that struck me about my conversation with Andrew was the vast time scale he thinks about when considering soil formation. To understand the chemical makeup of a particular soil, we must go back tens to hundreds of thousands of years. “The minerals, rock fraction, and even the clays in the soil change over time, slowly degrading. We call that weathering. Elements are released as part of that process that impact the overall characteristics of that soil.”

Andrew gave a great example comparing how soils from two of the most productive agricultural zones in the world formed: the United States Midwest and Brazil. “Having just come back from Brazil, I’m thinking about that a lot. The soils in the area we were in are considered much older. The material from which that soil formed sat in place for much, much longer and under much hotter, more moist conditions that accelerated chemical transformation. The chemical properties of those soils are completely different from what you would find in the Midwest, where soil scientists consider the soils relatively young. Those Midwestern soils were deposited from glacier events and windblown sediments and haven’t been around for nearly as long.”

As a result, soils in many parts of Brazil have a deep red tinge and US Midwestern soils often have a rich, dark brown or black hue.

Left: An example of the Mollisol soil order found in the US Midwest. (USDA). Right: An example of the Oxisol soil class found in Brazil. (Embrapa).

Much of the applied agricultural research on soil chemistry is specific to soil fertility: how soil can provide the nutrients that a crop needs to grow. Each of the major crop nutrients (nitrogen, phosphorus, potassium), as well as micronutrients, can move and cycle through the soil. Agronomists apply this type of soil science to provide recommendations to farmers as they consider what nutrition their crop needs throughout the year.

Soil physics

Andrew says, “When I am trying to understand a soil in a new area, soil texture is one of the first things that I look at. It impacts so many aspects, like how water moves through the soil, how a plant would grow in the soil, and more.”

Texture describes the distribution of the particle sizes in soil. We usually break it down into:

• Clay, which is the finest particle in the soil

• Silt is medium-sized particles

• Sand is the coarsest – picture beach sand

Relative size of soil particles. There is roughly a 1000:25:1 ratio between sand:silt:clay. Data from: University of Nebraska, Lincoln. Illustrated by me.

When you ask a farmer what kind of soil a field has, they will often reply with phrases that describe the texture, “heavy clay” or “light” (meaning it is sandier). The relative proportion of sand, silt, and clay determines soil texture, shown in the texture triangle below.

Soil texture triangle. Each intersection represents a different proportion of sand, silt, and clay, with the corresponding soil texture. The arrows on each edge indicate the direction to read that line. For example, a soil with 60% sand, 10% silt, and 30% clay is a “Sandy Clay Loam.” Source: USDA Natural Resources Conservation Service

After looking at a soil’s texture, next Andrew asks, “How dense is the soil, does it have a lot of air in it? Soil is a collection of these small particles stacked together.”

This has practical applications for farmers, and anyone designing tools for farmers. Picture a 35,000 pound sprayer rolling through the field. While the large tires of the sprayer distribute that weight, each pound presses down on those four points. Every time the sprayer runs over a patch of the field, the tiny air pockets between particles collapse, leaving denser soil behind. This is referred to as compaction. Compacted soils don’t absorb water as well. When roots meet tightly compacted soil, it’s like hitting a brick wall that they struggle to grow through.

Soil biology

“Plants have a huge impact on soil. Their roots grow in the soil and release exudates, really driving the biology.” Exudates are substances that contain sugars and other compounds that feed and influence microbes in the surrounding soil. The plants and microorganisms in the soil create a deeply alive, underground ecosystem.

One of the critical functions of soil is nutrient cycling. “Soil is where a lot of dead plants and animals end up, and the soil microorganisms break all of that down so that the nutrients can be available for other living organisms.” Soil microorganisms are critical to other aspects of soil health, including water holding capacity and providing nutrient support for plants.

“Soil biology is one of the most exciting areas in soil science because people are realizing how little we know about it. There have been a lot of advances that are allowing us to get a sense of how complex it is and how many different types of organisms are down there. We don’t know what a majority of them are, let alone how they interact with each other.”

Agricultural practices that improve soil

“When we degrade our soils, we’re making our jobs a lot harder.” Beyond making it more difficult to grow a healthy and productive crop, “soil can be one of our biggest buffers to have resilience to climate change: good water holding capacity, having organic matter that helps improve infiltration during extreme weather events, reducing flooding, having more water available during dry patches.”

Farmers in the U.S. and in Brazil are finding innovative ways to restore and build up soil health.

Andrew has worked with row crop farmers in the Mississippi Delta in the US who are now practicing cover cropping or no-till. Cover cropping, or growing a crop in the off-season that is not harvested, adds organic matter and nutrients back into the soil. You simply let the crop grow, die, and turn into fertilizer for the next crop.

No-till or low-till is what it sounds like: farming without tillage (or disturbing the soil mechanically), which protects against compaction and erosion. “A lot of the farmers were viewing these practices as inevitable. It was more like, ‘we better figure this out now because we know that this is the direction that things are going.’” As with any new practice, there is a lot to figure out, like what modifications are needed on planting equipment to cut through last year’s crop residue.

In many parts of Brazil, Andrew notes that farmers have adopted a range of soil-enriching practices: rotational diversity, intermittent pasture, no-till, terracing, intercropping, and cover cropping. Protecting soil from erosion is one motivating force. The major agricultural areas in Brazil get 40 to 100 inches of rain per year (compared to 25 to 40 inches of rain in the US Midwest), meaning that “massive gullies can form in a single season if the soil is left unprotected.” Unlike in the Midwest US where “we’ve been kind of spoiled in many ways with deep organic matter from when the land was tall grass prairies,” Brazil’s older soils are naturally low in organic matter, so “you see the benefits of those practices right away on soil quality and productivity and so the adoption has been higher.”

Soil research and technology

Many technologies in agriculture rely on quality soil data, like models to recommend how much nitrogen or water to apply to a crop, or carbon marketplaces that sell credits for carbon sequestration in soils.

“In the US we are lucky that we have the US Soil Survey maps (SSURGO) that are accessible in a number of ways.” After the Dust Bowl, the Soil Conservation Service was founded with public investment, “to protect our soils and understand them more.” This has been a long-term investment in soil research. “We need to keep making progress on addressing issues around soil quality and how we keep our croplands productive.”

There are also entrepreneurs who are working on bringing new soil sensor technologies to farmers. Creating reliable tools that can gather data across an entire farm is difficult. “Something might work in a lab, but then epically fail in the field environment.”

One example of an innovation succeeding in the field is soil compaction mapping. Historically, farmers and agronomists painstakingly pushed special equipment into the soil at several points throughout the field to determine if there was compaction, and how deep it was. Now, there are sensors that can be mounted on an ATV to provide a “new data layer that could be used directly for a management decision.”

“Anytime we get a new way of looking at soil, especially at scale and at a price that makes sense for farmers is a huge opportunity.”

Deeper reading on soil

While we just scratched the surface of the world of soil, there are many resources to learn more. A few of Andrew’s favorites are below:

• Nature and Properties of Soils: The classic introductory textbook

• Soil Health Institute: For resources and learning about soil health

• SoilWeb: Comprehensive data about the soil in your area (if in US), developed by USDA NRCS (with easy-to-use map developed in partnership with UC Davis).

• University Extension websites are a great resource, with the bonus that they usually focus on regional topics. One example is UMN on soil management and health.

• The Worst Hard Time: The Untold Story of Those Who Survived the Great American Dust Bowl by Timothy Egan

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Topsoil is handcrafted just for you by Ariel Patton. Complete sources can be found here. All views expressed and any errors in this newsletter are my own. A sincere thank you to Andrew McGowan for sharing his soil expertise and enthusiasm. I can’t look at soil the same way! Thanks to Mike Rigg, Tim Durham, and Steven Adler for thoughtful feedback on this edition.

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