The “Dirt” on Soil

In honor of Earth Day, a look at the earth beneath our feet

Carly Anderson
Prime Movers Lab


Key Takeaways

  • Soil is NOT dirt! Soil is a super cool complex system of minerals, carbon-based (organic) matter, living organisms, air, and water.
  • It takes hundreds to thousands of years for soil to be created through natural processes. We’re losing it at a much faster rate.
  • Adding additional nutrients (aka fertilizer) to soil is currently necessary to get the crop yields needed to feed the world’s population at current levels/diets. Corn is particularly nutrient hungry — roughly 30% of US corn farmers’ spending on crops is for fertilizer.
  • Recent agriculture startups have been successful by reimagining soil management, harnessing the microbiome, and targeting reduced fertilizer needs in what has traditionally been a slow to change industry. These include vertical farming companies like Upward Farms (a Prime Movers Lab portfolio company), microbiome-based solutions for traditional farms like Indigo Agriculture, Pivot Bio, AgBiome, and many more on the horizon. (← that was a soil pun!)

The English word EARTH has several meanings: (1) the planet we live on. (2) The substance of the land surface; soil. (3) In hunting, to drive (a fox) to its underground lair (this was a new one for me!). In this post, let’s start to explore #2: earth, as in soil.

Soil doesn’t get a lot of airtime, but it is pretty darn critical. Of soil’s many important roles, I’ll highlight just two here — feeding the world, and helping moderate carbon. Over the past year we’ve noticed a surge in startups in both areas. Will one of them hit the pay dirt? (How many “dirty” jokes can I use in this blog?)

This is a relatively new area to me, and I’m grateful to Dr. Ilse Renner at Upward Farms, Dr. Rebecca Sanders-Demott at the USGS, and Dr. Dan Tomso at AgBiome for helpful conversations and sharing their perspectives. If this area is interesting to you too, please reach out! With that, I invite you to take a minute to appreciate the earth beneath our feet in preparation for Earth Day.

Let’s start with some basics…

What is dirt or soil? To a soil scientist, dirt and soil are two different things! Dirt is just the residue left on your hands and knees after you garden or play ultimate frisbee… it’s not alive. But SOIL is a complex living ecosystem. There are 5 basic ingredients in soil: minerals, organic matter (including dead and decaying stuff), living organisms, air, and water.

The different layers of soil: The first 5–10 inches are TOPSOIL, rich in organic matter. Below that are the 3 soil horizons: the surface horizon, the subsoil, and the substratum. (NRCS Guide to identifying them under CC)

The minerals in soil generally come from the parent rock, e.g. sandstone, limestone or bedrock underneath the soil. Glaciers, rivers, and volcanic eruptions spread minerals across the earth’s surface. Over many years of weathering, the hard rocks turn into clays, silt, and sand. Weathering also helps release key elements for plants in a form they can use: calcium, potassium, magnesium, sodium, and iron (see Soil Minerals and Plant Nutrition).

Soil organic matter — which comes from dead plants, leaves, animals, anything made of carbon — acts as a reservoir of nutrients for plants. It helps hold the soil together, prevents it from compacting and creates space for oxygen and CO2 from air to move around. Organic matter also helps the soil around the roots hold water, and buffers the soil so that minerals aren’t washed away. Many of the microbes living in soil depend on organic matter for carbon, nutrients and energy.

The microbes (or micro-organisms) in soil help decompose organic matter, cycle nutrients, and fertilize the soil by fixing nitrogen. Nitrogen fixing bacteria are a VERY important group of microbes that take nitrogen out of the air and convert it into a form of nitrogen that plants can use. [1] The levels of nitrogen, carbon (organics) and microbes in soil are very interconnected. [2]

The things that live in the soil, including microbes (like nitrogen fixing bacteria), plant roots, insects, worms, etc— still need air, so soil aeration is also important. Typically 30–60% of the volume of soil is empty space that is filled with air and/or water.

Water is the final critical component of soil, and the one we usually the think of FIRST when our houseplants are dying. Water and air move through the same pore space within soil, so the amount of air in the soil goes down as the amount of water increases and vice versa. How long the water remains in the soil depends on its organic matter content (more organic matter = spongy soil) and its structure. Soil is made of different sized rock particles: clay, silt and sand. Clay is the finest, and traps the most water; sand dries out relatively quickly. The relative amounts of clay, silt and sand in the soil reflect how the soil was formed and how long ago — these processes of breaking rocks into smaller and smaller pieces take hundreds to thousands of years.

The color of the different dirt/rock layers depends on not just the mineral content, but the presence of water when each layer was formed. Geology is just cool. (By Sofia AtmatzidouEulgem under CC)

Wait, it takes HOW LONG to create soil? It takes over 500 years to weather enough rock to develop soil. The time needed for soil to form depends on geography, topology (for example, next to a mountain, a river?), heat and cold cycles, the depth below the surface (horizon), and the organisms in the area. (Soil Science Society of America) If everything is perfect, it takes 100 years to make an inch of topsoil (the first 5–10 inches). Soil is technically a renewable resource, but with current farming practices we lose soil from erosion, salination (too many salts concentrating) and nutrient depletion at a much faster rate.

Why does this matter? Our ability to grow enough food for the world’s population depends on getting nutrients to plants via soil. This is becoming increasingly challenging as the world’s population increases and global diets change. The soil in a region limits how much food and what kind of crops can be grown. On the flip side, finding more sustainable, cost effective and less resource intensive ways to maintain soil health and nutrient content is both an opportunity to transform billions of lives and a massive economic opportunity.

As I suspected, someone has been adding soil to my garden. *The plot thickens…*

Hitting the Paydirt

Today’s crops were developed for specific purposes, and have very specific needs. The results have been incredibly impressive — yields for crops like corn and wheat have increased by 2–5x in the past 100 years. [3] Today we use about 50% of the Earth’s habitable land for agriculture. If our farms were still operating at 1900s efficiency levels, there wouldn’t be enough land to feed everybody. (Although yes, food spoilage and waste are also a significant part of the problem.)

“Liquid fertilizer application” by eutrophication&hypoxia under CC

Corn, wheat, soybeans etc. need a lot more nutrients than the soil is able to replenish in less than a year. As a result, our food system is VERY dependent on synthetic fertilizer. In the US, we use over 20 million tons of fertilizer each year. Nitrogen [4], phosphorus and potassium (together, called NPK) are the most common nutrients. Fertilizer sales in the US have been over $12B annually for the last 3 years. Crop nutrient expenses are equivalent to 32% of all crop inputs/services costs for growers to produce their corn crop each year, according to AgResource Co — a significant fraction of cost. The global fertilizer market is somewhere between $100 and $200 billion (estimates vary widely).

The combo of a large market, the potential to drive down a significant cost center, and increased attention to sustainability have created fertile ground for innovation. Our portfolio company Upward Farms has reimagined farming entirely, with revolutionary soil management practices that reduce fertilizer use by over 10x and eliminate pesticides.

Other highly successful AgTech startups like Indigo Agriculture, Pivot Bio, and a Gingko Bioworks/Bayer partnership are developing products that use microbes to help add nitrogen back to the soil. While we are still a long way from being able to replace synthetic fertilizer [5], it’s a step in the right direction. Our friends at AgBiome have created microbiome-based alternatives to pesticides (pesticide use also contributes to poor soil quality). Check out the Microbiome Webinar we hosted last fall, featuring Dan Tomso as a panelist!

A large new crop of soil and ag startups are developing additional tools. There are hundreds, including soil additives, soil health and water sensors, biobased nitrogen fertilizers that limit run-off, and hyperspectral imagery to estimate soil organic carbon. Many are focused on digitalization, which will undoubtedly increase efficiency. However, changing the underlying mass balance of carbon and nutrients moving out of soil is a much harder problem.

Looking ahead, a theme from Dr. Renner stuck with me: can we shift our perception of “yield” to include nutrients as well as dry mass or calories? If growers could get paid not just by weight but nutrient quality, perhaps this could promote way both better agricultural practices and better nutrition.

Food for Thought: Soil as a Carbon Sink

The key role that soil plays as a carbon “bank” is worth a whole separate discussion, so I’ll keep this brief. The best explanation I’ve seen of how soil affects carbon dioxide and climate is this wonderful 14-min Ted Talk by Dr. Asmeret Asefew Berhe. At a high level, the good news is that better farming practices and soil management could help store CO2 in the ground more permanently than planting trees. The bad news is that in other areas (like the arctic tundra), soils are releasing greenhouse gases that create even more challenges. Bill Gates and I agree on this one — we’re going to need both nature-based and technological solutions (e.g. carbon capture) if we don’t want to end up like Kevin Costner in WaterWorld.

Not Kevin Costner but you get the idea. By Queensland State Archives under CC.

For non-impact investors, it’s easy to write off soil-based CO2 solutions as too difficult to validate, too hard to defend (from an IP standpoint), and too challenging to realize revenues from (no 45Q or LCFS credits). Specifically, a lot of biochar companies have come and gone. These things are still worth doing, but may need to see some innovative financing strategies. I’d love for someone to change my mind here.

A parting thought and shout out— doing large-scale ecological or agricultural research is challenging for a host of reasons. It’s slow — it may take several growing seasons to see results. It is labor intensive. Controlling for varying soil conditions and microclimates is difficult, as is collecting representative samples. There are often weak if any ties between academia and industry, and navigating an idea or technology into the field can be a decades-long journey. Thank you to the researchers who are out in the dirt, finding ways to keep the earth in balance!

I went to the library to get a book about soil... *but they only have one copy, and it was already out on loam*


  1. Nitrogen fixing bacteria convert nitrogen in the air (N2) into ammonia (NH3). Other nitrifying bacteria convert ammonia into nitrates (NO3) or nitrites (NO2).
  2. Some microbes turn up or down their activities when there is a lot of organic matter around. For example, some nitrogen-fixing bacteria will keep the nitrogen it makes for itself when there is a lot of organic carbon present (both plants and microbes need N for metabolism) — bad for plants. However, certain kinds of fungi that live in decaying organic matter kill harmful nematodes, a benefit of having more carbon than nitrogen on hand.
  3. You can see the increases in yield for a range of crops and geographies through Our World in Data.
  4. If you’ve spent too much time with chemical engineers, you’ve likely heard the term “Haber Bosch” tossed around! The Haber Bosch process combines hydrogen (usually from fossil fuels) with nitrogen from the air to synthetically make ammonia. This process alone consumes about 1% of the world’s energy, generates about 1% of the world’s CO2 emissions, and is one of the biggest industrial uses of hydrogen. (C&EN)
  5. For example, 2019 data from large field studies of Pivot Bio’s product showed that it increased yields by an average of 6 bushels of corn per acre, compared to fields with synthetic fertilizer only. (The average corn yield in the US is 180 bushels per acre, so this is about a 3% increase.)
This avocado tree was the one plant I successfully kept alive during grad school. Interestingly, potting soil is not actually soil! It’s usually a mixture of coconut hulls, perlite (a ceramic), peat moss, vermiculite, and/or bark.

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