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153-The Science Behind Great Soil

| Care, Podcast

I don’t know if you know this about me, but soil is my favorite topic. I love all things gardening, and I’ve even hurled myself into a large heap of finished compost. Still, soil is king in my book, and the more we learn about the science behind great soil, soil health, and all of the amazing processes going on under the surface, the more enamored I become.

My guest today is Dr. Jake Mowrer, and this guy spends much of his day researching and exploring the many aspects of soil we have yet to fully understand. Jake earned a Ph.D. in soil fertility and soil chemistry, which ultimately landed him at the Texas A&M University Department of Soil and Crop Sciences.

Although Jake is a native Georgian, he says the Lone Star State is the ideal home base for soil scientists. That’s because there are 12 scientifically-designated soil classifications, and the landscape of Texas includes nine of them – a broader soil diversity than any other state in the U.S.

 

Jake Mowrer Texas A&M

Within the boundaries of the state of Texas are 9 out of 12 soil classifications – a unique boon of diversity for Jake and his fellow soil scientists. (photo: Courtesy Jake Mowrer)

 

Soil or Dirt

As gardeners, we sometimes like to say that we play in the dirt. Well, dirt and soil are two very different things – or are they? Oftentimes, the difference in terms is explained by the presence – or lack – of organic matter. Well, Jake has a slightly different take on the subject. It’s his opinion that dirt is a colloquialism not a categorization. He believes it’s not what you call it that really matters. What’s important is how you treat it.

Dirt can collect under your fingernails or in the corners of your garden shed. Soil, on the other hand, is something to be treated with respect – something to be cared for. Call it what you will, but when you invest care and attention to the soil, you will always reap the rewards.

Jake has some other interesting opinions when it comes to soil. Take the soil food web, for example. The organisms of the soil food web are generally considered to be part of the soil itself. It refers to the presence and activity of billions of fungi, bacteria, protozoa, earthworms, and other creatures which help to process nutrients which feed plants.

When Jake talks about soil, he is referring to only the inanimate elements, like minerals and organic matter. Organic matter is comprised of material which was, at one time, alive but is in some state of decay. In Jake’s opinion, the inanimate elements of soil are a habitat for the living creatures of the soil food web.

Should that impact how you care for the soil in your garden? It might cause you to view things a little differently. However, what’s most important is that you have an understanding of the relationship between the inanimate elements and the living organisms – including plant roots.

Soil Health

So, how should we respect and care for our garden soil? Well first, let’s take a deeper look at what constitutes healthy soil. Soil health is a set of recommended conditions & goals. If you had to describe a nice glass of wine, you would probably say it has good flavor, body and fragrance (nose). For soil, it’s the tilthe, texture, and structure that matter.

  • Tilthe refers to the physical properties of the soil which allow for good root proliferation (growth and development).
  • Texture is the physical quality of soil that you can feel with your fingers.
  • Structure refers to the way in which all the particles within the soil are bound together.

Maybe you’ve heard the phrase “loamy soil”? It’s what many gardeners aspire to in their landscape beds.

Loamy soil is a result of a good tilthe, texture and structure due to a fairly even distribution of sand, silt, and clay particles. It also contains water particles, gases, and organic matter. Soil at optimal health is comprised of 25% gases, 25% water molecules, 45% minerals, and 5% organic materials. That’s not always precise, but it gives you the general idea.

 

Fava bean root system

Plant roots take up nutrients and moisture from soil, but they also exude chelates, proteins, acids, and other substances into the soil to interact with surrounding particles and soil-dwelling life. (photo: Courtesy of Dr. Jake Mowrer)

 

That make-up provides soil roots with the oxygen and water they need to be healthy and an environment that allows them to expand easily. It also creates the right habitat for the organisms of the soil food web which interact with all of those materials and the roots to cycle the nutrients, water, and gases (like carbon dioxide and oxygen) for healthy plants.

Notice that I said it creates the right habitat and environment. Some gardeners think that, if some organic matter is good, more must be better. The fact is, every component of soil plays an important role. Jake has observed that the roots of his plants are more robust when he adds organic matter like compost to his soil. However, he still aims to keep it to 5% overall; because he knows the minerals, water and gases are just as important.

Now, odds are good that the native soil where you live isn’t loamy. Most of us are dealing with soil that is mostly sand or clay. There are physical measurements to quantify that for us too. According to Jake, any mineral particle under .002 millimeters in size is considered a clay particle. Silt particles are a little larger at .002-.05 millimeters. Particles which are .05-2.0 millimeters large are sand. Anything larger than that – well, that’s just a rock.

It’s those tiny particles which cause clay to feel like talc and to bind together like glue. On the other end of the spectrum, sand particles feel gritty and coarse, and they don’t bind together well – if at all.

Why does that matter? Well, when at extreme levels, the tightly bound clay particles don’t allow much room for the gas and water molecules that are needed for a healthy soil environment. The loose lack of structure in sand means that it’s unable to hold on to water particles or nutrients.

However, particles binding together can be a benefit when the overall makeup is more balanced. When clay particles bind together into what Jake calls super-structures (also known as a ped) about the size of pea gravel, their strength creates a resistance against soil compaction. Soil containing lots of peds has good structure.

Whatever the native makeup of our soil, our job in the garden is to understand what we’re starting with, and how to keep bringing it closer to that healthy, loamy state.

Treating Soil with Respect

One really easy way to begin to respect and care for the soil you have is to stop tilling. Now, you might have to bring in a tiller when first building a new bed. I get that. However, avoid it if you can, and leave the soil as undisturbed as possible season after season to improve tilthe, texture and structure.

Turning over soil introduces more oxygen into the materials. You might think that’s a good thing, but the truth is that higher levels of oxygen cause soil bacteria to become more active and multiply. As a result, they digest the soil’s organic matter at a much faster rate (and release more CO2 too). Lightly scratching the soil surface won’t increase oxygen levels significantly. I often do that in my garden beds when I amend with compost or other nutrients. It’s deep disturbance (8-10” or more) that you want to avoid.

Did you know that when a raindrop hits bare garden bed soil, it lands with the force of a hammer driving a nail? The impact of thousands of drops during a heavy rain can compact the soil particles at the surface, as they are struck over and over again. At the same time, the force can also dislodge surface particles, and that can lead to erosion.

 

mustard as a cover crop

Jake and his colleagues have found that mustard has been particularly beneficial as a cover crop on Texas cotton fields. It might be a good choice for your home garden too. (photo: by Beth Ann Luedeker)

 

The risk of this damage is just one of the reasons it’s so beneficial to protect soil with a layer of mulch. Whether you install a layer of natural mulch material (like shredded leaves), use a living mulch (like ground cover), or postpone removing the debris from a previous season’s crop – protecting the soil surface is key. This is actually known in the soil science world as “armoring the soil.”

Here’s something else you can do that’s easy – plant! Research has confirmed that the soil food web becomes more robust when plants are living in the soil.

Year-round Planting

Plant roots exude chelators, enzymes, proteins, acids, and other substances into their rhizosphere – the area near the root zone. The role of those substances is to benefit the plant – to protect it from soil-borne diseases or to extract nutrients from mineral particles, for example.

So, let’s say you have two identical garden beds. In one bed, you plant summer edible crops but leave the space empty through the winter months. In the other bed, you plant summer edibles, but once those crops are finished, you grow a cover crop through winter. The year-long presence of living, exudating plant roots in the second bed will make for a more robust and efficient soil food web under the surface in that space. The living roots keep the soil food web party hopping.

In fact, soil scientists are encouraging farmers to take this approach on a large-scale. By using cover crops, farmers can protect their soil from erosion during the off-season, and at the same time, the crop roots will continue to rev up the soil food web for richer soil ready for another growing season.

Let’s get back to those two identical beds for a minute. According to Jake, even if you kept both beds planted during the same time period, their soil food web still wouldn’t actually be the same.

The life and activity at work under your gardening feet is so complex that the soil ecology is different under every step you take on your property. The organic matter, the water, and light availability, the roots interacting and exudating, the microorganism population, and their activity – each of these elements contributes to profound differences in every few feet of soil (and at different depths) throughout your backyard.

I don’t know about you, but I can’t get enough of this stuff. The more I learn, the more in awe I am at how little we really understand about this vast network of life, death, and interaction taking place every moment within the dirt that covers our planet – a little bit of which is always under my fingernails.

 

Cover crop as a soil barrier

The soil scientists at Texas A&M are working with farmers to encourage the use of cover crops. This mature cover crop of mustard is being rolled over to act as a soil armor and weed barrier as it slowly decomposes. (photo: Courtesy of Jake Mowrer)

 

Microorganisms in Bagged Products

In the natural order of things, plants engage in a give-and-take relationship with soil. For example, plants generate sugars through photosynthesis and deliver most of those sugars to the rhizosphere. Soil fungi rely on plant sugars as a food source, while also feeding on soil particles to release nutrients bound in minerals and organic materials. The released nutrients can then be taken up by the plant roots.

The soil food web is happy. The plant is happy, and I’m happy because my garden is healthy and productive.

Now if you’re a container gardener, you might be wondering how all of this impacts potting soil. Well, soilless mix or seed-starting mix is typically sterile. That’s by design, because young seedlings are particularly vulnerable to some fungal diseases.

Potting soil isn’t sterile, but it isn’t really soil either. Soil is defined as a naturally-occurring body made up of mineral and organic materials at or near the surface of the earth that supports plant life. Potting soil is manufactured from a blend of ingredients to create a soil-like substance.

Potting soil may include microorganisms that hitched a ride on the organic materials in the mix, but those creatures are generally sensitive to temperature and moisture fluctuations. So, there’s a good chance any microbes in a bag of potting soil have died or that the population is relatively low. So if a bag of potting soil is touting that it includes mycorrhizae (mycorrhizal fungi), there’s no way for you to know if any of the fungi survived shipping.

These fungi may provide some benefit to your containerized plants, and they certainly won’t cause any harm. Just be aware that you don’t have any way of knowing whether or not the fungi in the bag are still viable. Is it worth spending an extra few bucks on that product? Only you, as an informed consumer, can decide.

The same viability issues are true when it comes to bagged garden soil labeled as including mycorrhizae. Are they in there? Probably. Are they alive? Who knows. If they are alive, Jake says they are more likely to become a food source to the soil organisms already in residence in your native soil.

Unfortunately, our society loves the quick fix approach – like an instant dose of beneficial fungi. If the solution to our problem takes time, we aren’t always very interested. Instead, we browse store shelves or online for the bottled solution that will be the miracle cure. It’s one of many reasons that the fertilizer industry is booming.

Are your plants unhealthy? Just add a scoop of this or a few drops of that, and your plant will be vibrant and productive. Unfortunately, that approach can do more harm than good. Building healthy soil just takes time.

If you care for the soil as described above and by adding yearly or twice-yearly amendments of compost, you won’t have soil that is instantly healthy, but you will have soil that gets better and richer every season. It will feed your plants more efficiently than fertilizer, and it will reduce many common garden problems too – like weeds.

That said, fertilizer does play a role in a soil care regimen. There are some key differences between organic nutrients and synthetic nutrients, that I’ve covered in earlier podcasts. Whichever option you choose, what really matters most is application.

Application instructions aren’t just a rough guideline, they should be followed precisely. If you’re using organic ingredients, you can better customize the level of nutrients you add by better understanding the organic material.

 

Beneficial arbuscular mycorrhizae hyphae in a corn root

Beneficial mycorrhizal fungi can penetrate plant roots (like this section of a corn root system) to form hyphae. The hyphae allow the fungus and plant to exchange compounds symbiotically. (photo: Courtesy of Jake Mowrer)

 

Blood meal, for example, is high in nitrogen but provides minuscule levels of phosphorus or potassium. Bone meal is high in phosphorus with very little nitrogen or potassium. Kelp and seaweed are excellent sources of potassium with almost no nitrogen or phosphorus. Understanding these nutrient levels can equip you to calculate an appropriate blend.

Regardless of the source, overapplication of nutrients is a big problem in the health of our garden soil – but also in our environment.

Nutrient runoff – due to poor application practices – is responsible for algae blooms and dying fish populations all around the world. According to Jake, the Dead Zone in the Gulf of Mexico has been directly attributed to horticultural fertilizer runoff moving through the country via our waterways, eventually making their way into the waters of the Gulf.

Pay attention to the ratios on the packaging, and do the math to calculate an appropriate dosage whether you’re applying a natural, organic material or a chemical synthetic.

There is so much more that Jake and I could have covered, but we’ll save that for a future episode. For now, I hope you’ll be sure to click the Play icon in the green bar under the page title to listen in to our conversation. Jake provided plenty more interesting tidbits, and he brings a sense of humor to it all that I’m sure will bring a smile to your face.

Links & Resources

Episode 048: The Simple Science Behind Great Gardening, with Lee Reich

Episode 063: Garden Fertilizer Basics: What to Know Before You Grow

Episode 100: Understanding Cover Crops: The Basics and Beyond, with Jack Algiere

Episode 110: Why Mulch Matters in Every Garden: What You Need to Know

Episode 116: Understanding the Soil Food Web, with Dr. Elaine Ingham

joegardener Blog: The Numbers on Fertilizer Labels, What They Mean

joegardener Video: No-Till Gardening: If You Love Your Soil, Ditch the Tiller

joegardenerTV You Tube: How to Prepare Your Soil with Amendments in Fall

joegardener Online Academy  Three popular online courses on gardening fundamentals; managing pests, diseases & weeds; and seed starting!

Master Seed Starting – My newest online course teaching you how to master the art of starting your own plants from seed and seeding care! Registration closing soon, so don’t miss out!

joegardenerTV YouTube

joegardener Newsletter

joegardener Facebook

joegardener Facebook Group

joegardener Instagram

joegardener Twitter

Growing a Greener World® Episode 622: Commercial Composting & How to Make a Worm Bin

Noble Research Institute

Soil Health Institute

Texas A&M University Department of Soil and Crop Sciences

U.S. Department of Agriculture: Natural Resources Conservation Service Soil Health

U.S. Department of Agriculture: Natural Resources Conservation Service Soils Map

U.S. Department of Agriculture: Natural Resources Conservation Service Soil Texture Triangle

Milorganite® – Podcast episode sponsor and Brand Partner of joegardener.com

About Joe Lamp'l

Joe Lamp’l is the creator and “joe” behind joe gardener®. His lifetime passion and devotion to all things horticulture has led him to a long-time career as one of the country’s most recognized and trusted personalities in organic gardening and sustainability. That is most evident in his role as host and creator of Emmy Award-winning Growing a Greener World®, a national green-living lifestyle series on PBS currently broadcasting in its tenth season. When he’s not working in his large, raised bed vegetable garden, he’s likely planting or digging something up, or spending time with his family on their organic farm just north of Atlanta, GA.

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