Thanks to a beginning farmer loan through the USDA, in July 2021, Hayden and I purchased an 80 acre patch of soil and trees near our home. Elated would be an understatement for how we felt at that time!
We both come from traditional farming families in the typical Midwestern sense. Corn, soybeans, a little bit of wheat, and some animals, too. Pigs for Hayden’s family, cattle (and sheep early on) for mine.
Despite our backgrounds, somehow I don’t think either of us figured we would be rowcropping one day. Hayden was interested in fitness and pursuing a mechanical engineering degree, and I was studying agriculture with the hopes of becoming an agronomist for a seed company or starting a specialty crop farm.
We accomplished one goal–starting a cut flower farm (let’s be honest… it was my dream, not Hayden’s–ha!). 2022 was year 4 for our flower farm. We have grown steadily each year, either in quantity of stems or in infrastructure.
And now Hayden will be home full-time on the family hog and grain farm come Spring 2023. But we’ve had our first corn and soybean crop in the ground for 2022 thanks to the help of the family. We are eager to see yield results and hash out our future plans on the property.
But the first thing we really wanted to do when we bought our 80 acres was soil sample our field. And we wanted to do it ourselves so we could get a feel for our own soil in the hopes that, through various land management practices, we can better our soil for years to come.
Because of our own interest in soil health and fertility, in this article I’d like to share what soil sampling is, how you can implement soil sampling in your own backyard, and why soil sampling could be beneficial to you whether you have a small garden or a large farm.
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What is soil sampling?
Soil sampling is the process of extracting soil from a specific area. Later, the samples are usually tested in a soil laboratory for nutrient levels, nutrient availability, pH, and more.
Typically, people sample soil by using a soil probe. This is a metal instrument that is cylindrical in shape with two handles at the top and one side of the cylinder open.
The bottom is open so that when the user pushes the probe into the soil, the soil will push up through the tube. The soil is then extracted from the side of the tube and placed into a bucket or bag to be labeled and sent to a soil lab.
For most gardening and farming purposes, it is standard to sample the top 6 inches of soil. This is typically where the topsoil resides and where most of the biological and nutrient activity occurs. For perennial crops, like grapes, samples are usually pulled from the top 8 inches of soil.
You can also sample deeper into the soil. 12 inches is typically the max for smaller probes and for most people’s strength. If you need to sample deeper, you may use something called a soil auger. Or there are hydraulic powered probes set up as attachments on vehicles like a truck that can take much larger and deeper core samples.
Why You Should Test Your Soil
There’s been a big push in the last several years for farmers to reduce nutrient runoff into waterways, streams, rivers, and, ultimately, oceans. One way of reducing excess nutrient application is by sampling your soil to see what it may or may not need, instead of making blanket applications of fertilizer without knowing what your plants and soil are actually lacking. Blanket applications of fertilizer can result in over-application of nutrients.
For example, a garden that has had heavy applications of manure over time may not need any further phosphorus or potassium added to the soil due to high applications of these nutrients in previous years from the manure.
Another reason you may opt for a soil test is because you’re having trouble with a particular plant growing in your soil and you’re not sure why. It could be the pH instead of a nutrient deficiency. Certain plants, like blueberries, require a specific pH range to thrive in. No matter the nutrient levels, if the pH isn’t correct for the plant, certain nutrients are unavailable for uptake.
Blueberries like acidic soils (lower pH) and will not thrive in basic or alkaline soils (higher pH). Therefore, the pH of the soil will need to be adjusted wherever you want to grow blueberries.
You may also want to see how your soil changes over time using practices like cover cropping, adding compost, or applying manure. You may see physical changes in the soil over time, but at a nutrient level you’ll never know if something is out of whack at the chemical level of your soil until your plants tell you. And by then it may be too late.
What can soil tests tell you?
As mentioned before, soil tests can tell you mainly about chemical properties of your soil. A typical soil test should tell you:
- Nutrient levels, usually in ppm (parts per million) or lbs per acre
- Phosphorus (P)
- Potassium (K)
- Calcium (Ca)
- Magnesium (Mg)
- Percentage of organic matter (%OM)
- Cation Exchange Capacity (CEC; basically, the soil’s ability to hold onto positively charged nutrients)
Some soil labs will offer testing of more variables than others. If you’re interested in a specific type of test, make sure to ask the soil lab up front whether they conduct that test or not.
The next bit of info is important, but if you’re just wanting to learn how to take a soil sample, you can skip down.
Testing for Nitrogen
Nitrogen is taken up by plants mainly in the form of ammonium (NH⁴⁺) or nitrate (NO₃⁻). Most of the time soil labs do not test for nitrogen because it is easily lost from within the sample itself as a gas. When we tested for nitrogen for our soil fertility research projects in college, we would typically freeze the samples immediately so that little to no nitrogen was lost from the samples before testing could occur.
There are other methods that test for nitrogen in plant-unavailable forms, but these are not standard to most labs either. Basically, testing for nitrogen can be complex.
It’s a great idea to talk to your soil lab if you’re interested in the nitrogen content of your soil.
Testing for Organic Matter
Percentage of organic matter is essentially the measurement of carbon in the soil. High organic matter is typically associated with higher fertility and higher water holding capacity, which is why growers may want to increase their organic matter percentage over time.
Organic matter percentage does not necessarily indicate plant available nutrients (basically, nutrients immediately available for uptake), but over time organic matter breaks down and releases nutrients like nitrogen, sulfur, boron, etc.
The percentage of organic matter (%OM) gives you an idea of what nutrients could potentially be released over time.
Organic matter is measured usually by methods like loss on ignition (LOI), combustion, or by chemical tests, like the Walkley-Black test. Oregon State University has a great article that breaks down these tests and more.
The most common test performed by labs is LOI, which basically means the sample is dried and weighed, then heated in a muffle furnace which burns off all of the organic matter at a high temperature, and then it is weighed again after. The loss of weight is essentially the %OM.
However, some labs will determine organic matter by none of the means mentioned above. Instead, they’ll use something called Munsell color charts. These charts were developed so that people could identify %OM by comparing the soil color to the color chips on the chart. Each color chip indicates an estimated %OM. The general idea behind the Munsell color charts is that darker soils typically have higher organic matter content.
While the Munsell color charts were developed through research, and in some cases they can give a pretty fair assessment of %OM, for me it opens up too much room for human error. The soil has to be at the right moisture and then we’re relying on the human eye and the lighting.
While there’s debate on whether any of the methods like LOI, combustion, or Walkley-Black are completely accurate, I would advise asking your soil lab which method they use to determine %OM if it’s an important variable you want to study.
Unfortunately, we learned this the hard way when we took our samples to a local lab and didn’t ask beforehand what they use to test for organic matter. They used the color chart method. By the time we found out, our samples had been thrown out.
It wasn’t the end of the world, but %OM is something we were really wanting to study over time as we implement cover cropping and change our field from corn/soybeans to pasture. So, a truer estimate of percent organic matter would have been useful. We’ve learned for the future!
How to Take a Soil Sample
Now that you know the basics of what a soil test can tell you, let’s get into the meat and potatoes of this article: how to take your own soil samples.
First off, you will need to acquire a soil probe if you’re serious about soil sampling. We sampled our entire 80 acre field using my AMS soil probe I bought in college. I wanted to have one on hand because I was using it so much for my thesis project on cover crops and their effect on soil fertility.
You can find more cost-effective probes that will get the job done as well. And if you’re only sampling a small area, a hand trowel might work just fine for you. Find one with measurements on it so you know the depth you’re extracting from. I can also see where a Hori Hori soil knife could be used to sample soil, too. I love mine.
Steps for Collecting a Soil Sample with a Probe
If you’re using a soil probe, here are the general steps involved with sampling:
- Remove any surface debris like plant material or rocks.
- Position your feet in a wide stance and grip the handles of the probe firmly with the open side of the probe facing away from you. Keep the probe perpendicular to the ground.
- Push the probe firmly into the soil to the desired depth. Some people will mark the length on the probe with permanent marker or tape at the 6 inch, 8 inch, and/or 12 inch marks so they know when to stop pushing.
- Slowly pull the probe out of the soil. Don’t rock it back and forth or jerk it around.
- Use your finger or a screwdriver to pull the soil out of the probe. You may need to use a screwdriver to get soil out of the end of the probe if you have particularly clayey soil.
If you’ve got a big area to sample and you just don’t have the time or manpower to do it yourself, you can also hire someone to do it for you. There are many consultants that will sample your field in much quicker time with a probe on the back of an ATV.
What to Consider When Soil Sampling
There are a few things you’ll want to consider when soil sampling, like time of year, quantity of samples, and labeling.
Take multiple samples.
First off, you’ll want to take multiple samples and mix them together to account for any random anomalies that may occur.
Let’s say that you want to sample your 40 by 40 foot garden and you only want to take one soil sample because you’re only mildly interested in pH, P, and K. So, you walk out and collect one sample from the middle of your garden.
But, little did you know, a cat came and pooped on that exact spot two days ago and buried it right there. Now, your sample data will be skewed because of the large amount of cat excrement in that one sample.
A better way to test your soil would be to take multiple samples from random spots in your garden. You could sample in all 4 corners plus the middle or you could sample from 7 spots randomly as you zigzag through the garden, and so on.
After collecting samples, you should mix, or homogenize, the sample. Mixing takes into account the inherent variability of the soil in your garden.
There are different methods of soil sampling that is beyond the scope of this article. If you’d like to learn more, check out this article here from Purdue University.
Label your soil samples.
Nothing is worse than receiving data and not knowing how to use it because you don’t have anything to attach it to.
When Hayden and I sampled our field, we printed out an aerial map of our field from Web Soil Survey (if anyone needs help with Web Soil Survey, let us know!). The map showed us the different soil types in our field, so we developed a plan and subdivided the field into 15 main sampling areas based on topography and soil type in the field. That way, in the future, we could go back to the same general areas and sample again to see changes over time.
Using the practice mentioned previously of collecting multiple samples from an area, we took 10 to 12 random samples in each numbered area or main sample area. Then, we mixed the soil together and labeled the bag with the corresponding sample number.
Collect Samples at the Same Time Each Year
For many farmers, the best time of year to soil sample is in the Fall after the crops have been harvested. Spring is often too wet and busy, and in Winter the ground may be frozen.
For the most part, it doesn’t matter when you sample your soil. I would advise not sampling when it’s super dry and hard because you’ll be doing a lot more work than if the soil is moderately moist.
The main thing to consider is to sample your soil at the same time each year. It doesn’t matter if you’re sampling every year or every 4 years. Just try to sample your soils in the same season as previous years so that as you compare your data over time you will have accurate readings.
The reason for this is that some nutrients, like potassium, can produce different readings in the Fall and Spring based mostly on moisture and weather patterns. So keep your sampling season consistent if possible if you’re wanting to look at changes in your soil over the long term.
What To Do Next After Soil Sampling
After you’re all done collecting soil samples, you’ll want to send them off to a soil lab as soon as you can.
Some soil facilities are consultants only, so if you send your samples to them or hire them to sample for you, they’ll send your samples on to the lab of their choice.
Other facilities have small labs, but they aren’t able to test for all variables.
Do your research before selecting a lab or consultant so that you’re able to receive all the information you need. You likely won’t get your soil samples back once they’re sent off and processed.
Reading Your Soil Test
After you’ve shipped your samples off and have received your results back, then it is time to read your soil test and determine what the heck it all means.
To be honest, this is a topic in itself. We’ll be putting a blog post up later on reading your soil test, so stay tuned! In the meantime, feel free to send us a message if you have questions.
If you’ve hired a consultant, then the consultant should sit down with you and go over what the whole test means. Most of the time the tests will indicate whether your results are high or low based on typical crop needs.
But sometimes, if you’re growing a specialty crop (basically, not corn, soybeans, or wheat), any suggested levels on a soil test results sheet may not be accurate for your situation. Knowing what crop(s) you want to grow and the basic needs of that particular crop is essential for moving forward.
Now you can sample your own soil!
The first step to growing healthy plants is having healthy soil. For the most part, your plants will tell you if there’s something lacking in the soil, but in some cases it may be too late in the season to fix it. That’s why soil sampling ahead of time is so important!
Soil sampling can help you determine nutrient levels in the soil and how capable your soil is of holding onto nutrients. Basically, soil sampling informs you about how “fertile” your soil really is. Then, you can make decisions about any amendments you’d like to add to your soil to either increase fertility or correct a specific deficiency.
An interesting note to consider about soil sampling in these modern times is that now some soil labs may even offer tests that look at your biological activity in the soil, too. Microbiological activity is another important aspect of soil health.
With only a few tools, you can be on your way to sampling in no time! And if you feel like this is too daunting of a task, you can always hire someone to sample for you.
Drop a comment below if you have an example of how soil sampling has helped you in the garden or on your farm!
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