- When you're thinking about your favorite fruits and veggies, you might consider the freshness, appearance or maybe the cost.

But there's another important part of the story that's easy to overlook: soil.

I'm Sheril Kirschenbaum and on this episode of Serving Up Science, we're digging into the nitty gritty details of the dirt beneath our feet.

Okay, so what do you envision when you hear healthy soil?

Maybe you picture a rich brown color or a cakey consistency, but actually there are all types of varieties of useful soils like clay, silt, loam, and peat.

Even course, sandy soils can be what agricultural scientists call "productive."

Different crops flourish best in different soils, which is why, for example, some regions are targeted for wine while others for coffee beans.

- "Tea's no substitute for coffee!"

- No matter where you live, soil is the key to sustaining life on earth.

What productive soils do have in common, generally, is that they're dense and can hold a lot of water, but are also well draining.

And critically, they're full of nutrients, minerals, and microorganisms...tiny critters, that are necessary for plants to produce the fruits and vegetables we love.

The best soils are literally teeming with life.

Some that's visible to the naked eye like earthworms and spiders, but there are also millions of things we depend on that we just cannot see.

Soil is home to algae, fungi, protozoa, all sorts of insects, nematodes, yeasts, mites, and more.

One gram of soil alone contains up to several billion bacteria.

Yes, there are different types of microbes, good and bad, but when soil is well managed, the good microbes literally come to the table.

Altogether, they make up an ecosystem that does extremely important jobs for us and the planet like nutrient cycling, water filtration, and pest control.

And according to the US Department of Agriculture, just a teaspoonful of rich, healthy soil contains more microbes than there are people on earth.

The millions of organisms that live in the soil contribute to the carbon, nitrogen, and phosphorus cycles.

They keep those greens we love to eat growing strong and make plants more resistant to disease.

It also helps air and water flow well through the soil to reach the root of the plants.

And soil helps us deal with climate change.

After the ocean, it's the second largest carbon sink, meaning it stores and releases carbon in ways that regulate the greenhouse effect.

It literally sustains us, but unfortunately, it's been degraded all around the world.

A third of the planet's soil is moderately to highly degraded, which means the soil has lost some of its ability to support animals and plants because of the physical, chemical, or biological changes.

The structure of great soil keeps our crops healthy.

The microorganisms living in it are endlessly processing this medium into rich, dark, stable humus, which is decomposed organic matter that has released most of its nutrients.

- [Crew] It's pronounced hummus?

- I thought it might be humus, but I wasn't- - It feels weird.

- It's "hue-miss."

- Humus?

- Humus.

- Humus.

- [Crew] You were right.

- Humus.

Every time I say anything off, like, Rob immediately turns and looks at Nicole and I'm always like, "darn it, what did I do?"

Let's pause for a little experiment to demonstrate why the structure of soil is so important for our food system.

Take it away, Sheril.

Thanks, Sheril.

Now what we have here are two samples from the same farm.

Sample one has a lot of structure.

There was crop cover on the top and roots, as you can still see, are growing through this piece of soil underground.

This was some of the crop cover that's been on top of it.

Sample two comes from an often tilled part of the farm.

This is where corn was grown.

And every season, it's turned over and over and over, so it doesn't have the same structure or lasting roots.

So what I'm going to do now is just pour some water over each sample that's sitting on mesh wire and we'll see if we notice a difference.

First in sample one, the untilled soil.

So some of the soil is moving through the mesh, but not too much.

It's mostly still holding onto its structure.

We get a little bit of muddy color in the water, but not too much.

Sample two, as I said, repeatedly tilled.

We'll see if there's any difference in what happens.

Ooh, it is going down pretty quickly.

That does not look like something I would be excited to drink.

Visually, you can see there's quite a difference.

Sample two almost looks like chocolate milk, but not milk you'd wanna drink.

The soil just went almost right through the mesh 'cause very little is holding it together.

While in sample one, all those roots, all that biology that's alive is giving the soil a nice shape regardless of sitting in the water.

So this is just an example of what happens when it rains a lot.

You might get lots of erosion if you haven't been taking care of your soil or the soil could stay together fairly well and continue supporting your crops with lots of nutrients and minerals to spare.

So with that, back to you Sheril.

Thanks, Sheril.

I like your hat.

While our demonstration focused on the effects of intensive farming, there are many other ways soil gets degraded.

Deforestation, overgrazing, forest fires, and construction work have all taken a toll on soil quality in recent decades.

These activities disturb top soil, which leaves the system vulnerable to erosion and runoff and that damages the complex systems underneath.

And unfortunately, it's not like we can simply replace it.

An inch of top soil can take hundreds to thousands of years to form and centuries before it's fertile.

For these reasons and more, soil erosion leads to a loss in soil productivity, which is considered one of the world's biggest threats to our future food security, with developing nations anticipated to be hit hardest.

And that's where we come in.

Because when more of us recognize how important soil really is to our lives and the global food system, we can work together to change the way we care for and protect it.

Sample two almost looks like chocolate milk, but not milk you'd wanna drink.

So how can we promote healthy soil conditions?

One way is to host a variety of different plants that naturally support good soil structure, maintain crop cover year round, and avoid continually turning over soil.

Even simply leaving vegetation on soil after a harvest can allow important nutrients to return.

Think about this the next time you're at the market, ask how that- Well, but see, the grocery store people wouldn't know how the fruit was grown.

This is getting confusing.

What should I say?

Think about this the next time you are at the market.

Learn about how farmers take care of the soil in your area and you might even find yourself thinking a bit differently about the soil in your own backyard.

(Sheril singing) Okay.

(gentle music)