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Know Your Soil, Know Your Plants: The Secret to Interpreting Soil Test Results


Getting your soil tested can unlock a new understanding of your growing conditions.

“The soil is the great connector of our lives, the source and destination of all.” - Wendell Berry, The Unsettling of America, 1977

If you learn a little bit about the nature of the soil around you, you will gain tremendous insight into why your immediate environment looks the way it does. You will understand better why certain plants grow well while others struggle, and you will be able to make more informed choices about how to influence your landscape. I'm going to share with you a couple of secrets to becoming a wiser gardener and land steward, starting with the soil beneath your feet.

Soil Basics

The soil is the foundation of terrestrial life on Earth. It is made up of mineral components (rock, sand, silt, and clay), organic matter (decaying and composted vegetation), and living organisms (bacteria, fungi, and many other life forms). Water, nutrients, and air comprise additional essential elements. Together, this wonderful mixture provides the growing medium for plants.

If you have ever dug a hole, you know that sometimes it's like shoveling brown sugar (OK, that's rare), and sometimes it's a real pain in the neck (seems like most of the time!). That's because soils have a wide range of characteristics in different geographical areas and at different times of the year. Dry, clay soil appears akin to solid cement, while sandy soil makes you feel like a kid on a beach. And rocks, well, they bring their own set of challenges.

Soil has three foundational mineral components: sand, silt, and clay. Sand particles are the largest, while clay particles are the smallest. The mixture of the three components in a given area will determine the texture of the soil, and therefore its behavior. Very sandy soils are dry because water drains quickly through the coarse particles, while very clayey soils cause ponding because they drain so slowly. Most soils contain some combination of all three particle sizes. The presence and percentage of organic matter provides fertility through various soil nutrients that plants draw up through their roots. Other factors influencing soil behavior include slope, depth to bedrock, the presence of gravel, climate, and land use history.

Soil is King

I remember a seminal moment in my thinking about the environment. I was leafing through a book called The Nature of New Hampshire: Natural Communities of the Granite State, by Daniel Sperduto (2011, University of New Hampshire Press), and came across a section on geology and soils. Sperduto explained clearly how soil types give rise to certain ecological communities in any given place. Thus, if you look at a soil map, you can largely predict what plants will grow there, whether it will be a desert, prairie, or forest, and what animals will find habitat there. 

In short, soils give rise to ecology. 

It's a little bit more complicated than that, of course, but this nugget of knowledge glowed like a treasure from the pages of the book. It seems so obvious, and yet I had never heard it put in such deterministic terms before. Why hadn't we learned about this in elementary school? I wish it had been part of the curriculum. 

Essentially, over geological time, the Earth has weathered, mountain ranges have worn down, new rock has formed, and slowly, very slowly, mineral particles have developed over the crust of the planet to become hospitable enough for plants to take root. Millions of years of evolution later, plants have lived and decayed, lived and decayed, until a layer of rich, fertile topsoil formed. Ice ages and glacial movement have scraped and shifted soils around; areas once underwater or in the outwash plains behind retreating glaciers now possess some of the richest soils. Meanwhile, plants and organisms in various regions evolved together with the slowly changing soils and climatic conditions. 

What we see around us today is the result of this constantly evolving relationship between the chemistry of the soil and the biology of the organisms inhabiting it.  The ecology of a place depends upon the soil beneath it. Scientists have identified ecoregions to describe this geographic particularity. Maps of North America's ecoregions can be found on the US Environmental Protection Agency's site, at various levels of classification. 

How does this help me?

As a gardener or caretaker of your property, you can use knowledge about your soil to inform your decisions when selecting new plants or caring for what you already have. By understanding your soils and where you are in an ecoregion, you unlock key insights into how your habitat might function best. For instance, if you live along the Eastern seaboard, your environment is some form of the Eastern or Northern Temporate Forest ecoregion. But you might be located in a Piedmont or mixed forest region, or in the Southeast Coastal Plain, which have significant differences in their ecology.

How to learn more about your soil

There are several ways to quickly gain insight into your soil and growing conditions:

Start by checking out the ecoregion maps noted above. What region do you live in? Look up the name of your ecoregion to learn more about the natural communities that evolved in your area. The soils of the United States have been thoroughly mapped by the National Cooperative Soil Survey and its antecedents over the past 125 years or so. Check out the USDA's Web Soil Survey. Click the green button at the top of the page that says "Start WSS" to open a new tab and explore soil characteristics around your home. It takes a bit of playing around to find the information, but it's easily accessible once you get the hang of navigation. You can learn about the soil types and the suitability of each type for various human purposes.The best and most specific way to understand your soil is to conduct soil tests. These are available either free or at reasonable cost through your County Extension office or nearby university. A soil test will give you the site-specific information you need to understand growing conditions and make management decisions.

Conducting your soil test(s)

First, decide how many soil tests you need. You can order as many as you like, and then sample individual planting areas. For instance, you might want to know more about your vegetable garden, your foundation beds around the house, a shrubbery area, the back meadow, or a woodland shade garden. Each area will have its own characteristics. 

Order your testing materials and read the instructions for sampling your soil. You'll need to dig a number of small slices of earth from each bed or area and mix them together for each sample. Make sure to go deep enough (6-8 inches, typically), and pull out plant debris and rocks. Let the soil dry out as much as you can before sending it in.

The lab will test your soil and send you results via mail or email. The report will contain information about the mineral and nutrient characteristics of the soil samples. It will also provide recommendations for soil amendments based upon your stated purpose for each testing area.

And now for the secret

A lot of people will receive their soil test results and immediately look to the nutrient recommendations. You might read that your soil has a low pH and needs so many pounds of lime added to it to bring it up to optimal levels. You might be advised to add certain amounts of Nitrogen (N), Phosphorus (P), Potassium (K), Calcium, Magnesium, or other plant nutrients.

But here is the key: Take or leave the soil test  fertilizer recommendations. The important part is understanding your soil as it exists.

I am not saying that you should ignore the nutrient recommendations, or that they have no value. However, if you learn to foster the plant communities that naturally grow from your soil, no matter how droughty, lean, wet, etc., you will achieve greater success as a gardener while also supporting the ecological functions of your site.

Use your soil test results to form the search terms when you look for plants to add to your landscape. It is likely to be a waste of time and money to try to grow a plant that requires constant moisture and rich soils if you are in a sandy, droughty area, even if the plant grows in your hardiness zone. Goodness knows I've learned this over and over. Once you stand over a plant watering it for the thirtieth time, you realize that perhaps this isn't the right plant in the right place, and perhaps there is a better alternative that naturally thrives in your conditions.

A caveat: soil test results may well include recommendations for adding organic matter. Because we humans have mostly cleared land, disturbed ground, let soil erode, and variously used land without giving back to it, our soils have suffered. Rich topsoil takes a long time to form, but is easy to destroy. Adding organic matter back to the soil is almost always a good idea, so don't shy away from it. At the same time, you don't necessarily have to dump tons and tons of compost everywhere. Too much added fertility can also disturb the ecology. Use it strategically in the root zones around what you plant, or search for plants that have low nutrient needs in the first place. Nitrogen fixing plants can help to build the soil quality as well.

Knowing more about your soil will give you greater understanding of the nature of your immediate environment. Take this knowledge and apply it to your gardening and landscaping plans so that you are working more with nature than against it. Good luck!

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