Last week I talked about getting to know your yard in terms of sunlight and shade, dry and wet areas, so you can take those conditions into account when you pick your plants for your new microprairie.
This week, let’s talk about what’s going on under the ground. A soil test can tell you a number of things about your soil that can be very helpful when it comes time to pick plants for your particular site.
It’s also good to know what’s down there so you can address any problems. When a new house is built, the good soil that was there is often dug out for the basement and landscaping and hauled away. Once the house is up, the area is back-filled, oftentimes with lousy soil from someplace else or even with building scraps and waste. The soil in your yard could be very different from the natural local soil.
What is a soil test?
A soil test is an easy and important tool for helping your microprairie or any garden you plant thrive. It will tell you a number of things about the makeup of your soil, like whether it’s clay or sand, acid or alkaline, and what kind of nutrients it might be lacking—or have too much of.
How do I do a soil test?
The good news is that there should be plenty of places that can test your soil for you. There are some home soil tests you can buy that will tell you the pH of your soil. You can also do a home soil composition test (link here) to tell the clay and sand content. These are fine, but they’re pretty general estimates.
If you want more specific information, a soil test analyzed at a lab is the way to go, and it doesn’t cost all that much. Most states have at least one university that has a soil testing lab. Just google “soil test” and your state, and a service near you should pop up. Here’s the web site for the University of Minnesota’s soil lab.
If you can’t find a soil lab near you, contact one in a neighboring state and see if you can send your sample there.
How do I collect a soil sample?
The soil testing web site should give you instructions on collecting your soil sample, but it’s pretty straightforward. If you want your whole yard tested, just go around with a trowel and dig four to six small holes about six inches deep. Make sure those holes are fairly evenly distributed around the area you’re testing, like this.
You want to get the topsoil and a little below the topsoil but not roots, grass or weeds. Collect a few tablespoons of soil from each hole until you have however much your testing lab of choice says to collect, usually one to two cups of dirt. Mixe it up good, then put it all in a clean container or plastic baggie, fill out your forms, write out your check, package everything up according to the directions, and send it off to the lab.
For a fairly small fee, you’ll get back a basic analysis of your soil, like this.
How to read your test results.
The results you get back should tell you four main things:
- The amount of nitrogen, phosphorus and potassium present in your soil.
- The makeup of your soil—whether it’s clay, sand, loam or a combination of the three.
- The pH—whether it’s acidic, alkaline and how much.
- The organic content. This indicates how much yummy carbon is stored in your soil for the microbes to eat and provide nutrients to your plants.
Nitrogen, phosphorus and potassium, or N – P – K. These are the big three macronutrients and the numbers homeowners are told they need to be most concerned with. The N-P-K tells you the amount of available nitrogen, phosphorus and potassium present in their soil. When you look at a bag of lawn fertilizer, you’ll see three numbers, like 25-10-10. These numbers mean that of the 20 or so pounds of fertilizer in the bag, 25% of it is nitrogen, 10% is phosphorus, and 10% is potassium.
Each macronutrient serves a purpose. Nitrogen is important for general growth and healthy green leaves. Phosphorus promotes healthy roots, flowers and fruit, including vegetables, while potassium promotes strong stems and roots. You can see on my soil analysis where the proper range for phosphorus and potassium would be and that they are both way off the end of the scale in my yard. I have too much of both. We’ll get at why this is a little farther down, but for now, just notice how the recommendations for both is to not add any more.
The lab doesn’t include a number for nitrogen, just a recommendation of 1 pound a year. I’ll talk more about nitrogen and how to calculate how much lawn fertilizer to use in the future, but honestly, these NPK numbers aren’t that important to our microprairie. We’re building a tiny prairie to help restore some habitat for pollinators and sequester some carbon in the soil. For that, we need to know what’s on the rest of the analysis.
The pH. Remember litmus tests from high school science class? Dip the litmus paper in a solution and if it turns pink, the solution is acidic. If the paper turns blue, the solution is alkaline. For example, lemon juice is acidic, baking soda is alkaline.
Depending on where you are in the country, your soil can be acidic, alkaline or neutral. The happy medium for most plants is just below neutral, at about 6.5. A lower number indicates acid soil, while a higher number indicates alkaline. My yard is 7.3, or on the alkaline side.
The pH is important because plants can have a definite preference. Blueberry bushes, for instance, which grow wild in northern Minnesota, prefer acidic soil. I could never get one to survive in my yard as a kid, and I know now it’s because the soil there is alkaline. If I had tried growing it in a container or pampered it with a soil acidifier, it might have survived.
Knowing where your soil is on the pH scale will save you a lot of heartache by helping you avoid falling in love with plants that might be unable to tolerate your soil.
Soil makeup. This is the big basic. Soil comes in three types: clay, sand and loam, and you can have predominantly one, a combination of two or all three. Where I live, I have what I would call sandy clay, but leaning pretty heavily toward clay.
It’s important to know your soil type, because like pH, plants can have a definite preference.
Clay soil is composed of tiny flat particles that fit tightly together. They hold water and nutrients well, but the tight fit means the soil is heavy and solid and little air can get in. Roots need to get oxygen even underground, so too heavy of a soil can be a problem. For instance some of the plants native to my area have strong roots that can break up and drill down through the clay to reach water.
Sandy soil, on the other hand, is made up of large particles that don’t fit well together. They have a lot of space between them, therefore plenty of oxygen. But sandy soil doesn’t hold water well—it flows right through those big spaces, and the nutrients flow right through with the water. Plants in sandy soil need to be watered and fed more often, unless your plant is adapted to sandy soil.
The third type, loam, is the Baby Bear of the soil. It’s just right. It holds water, it holds nutrients, and there’s plenty of air. And the reason loam works so well is organic matter.
Organic content is the part of your soil that is made up of organic matter, the particles that are or were once living things. These particles are important because it’s this part of the soil, not the non-living mineral part, that gives your soil life, and that’s because organic matter is made up of yummy carbon, what soil microbes eat, process and poop out to provide nutrients for your plants.
It’s also what can fix soil that’s high in clay or high in sand. Loam is full of organic matter that gets in between the soil particles, pushing clay particles apart so roots get more air and holding sand particles together to hold more water and nutrients. Unlike slick, sticky clay soil or loose, grainy sandy soil, loam soil is fluffy and dark and smells like a forest after a light summer rain.
Your soil test will show what percentage of your soil is made up of organic matter. The higher the percentage, the better: 1%-2% is okay, 3%, 4% or higher is better. I’ll talk more soon about how to add organic matter to your soil.
How do I use these results to choose plants?
The biggest and best trick for making your garden work is to put the right plant in the right place. This is where all your factors of sun/shade, dry/wet, clay/sand/loam, NPK, and organic matter come together.
You may have already been checking out some plant nursery web sites and catalogs. They’ll all give you at least the plant’s sun needs. Good native plant sites will also tell you the soil preference and even the pH preference if the plant has one.
Here’s a trick for choosing plants suited for your soil: Use the web site’s filters to filter for condition. For my northern microprairie, I like the web site from Prairie Nursery in Westfield, Wisconsin (not a sponsor). I can set the filters for sunny, dry, clay soil, and it gives me a selection of plants that prefer those conditions.
Prairie Nursery doesn’t include filters for pH, but it’s sometimes mentioned in the plant description, and other web sites do include pH. I think pH is important enough to at least ask when you’re buying a plant, especially if your site leans heavily one way or another.
The good news is that when you know your soil conditions, you can choose plants that are already adapted to those conditions, and they’re more likely to be host plants and food sources to your local pollinators. It’s like bringing them home.
Have you had a soil test done? Any questions?