Soil testing 101: How to prepare a soil sample

The great thing about keeping this blog is that it holds me accountable. I am part of the county’s Extension Master Gardener program, and I think I can speak on behalf of the program when I say that if we could teach gardeners one thing, it is to test their soil.

This is something I tend to teach much more often than I practice, not because I don’t see the value in it—I certainly do—but because other stuff happens. Soil testing is not sexy. It’s like eating my spinach. I know it’s good for me; I know I should do it; but it’s just not the first (or second, or tenth) thing I want to do right now. I’ll do it, eventually. Sometime.

Further compounding my procrastination is that depending on the time of year, I may have to wait a long while to get my test results. For example, now is a bad time for a homeowner to submit soil samples in my state because the state’s commercial farmers are submitting their soil samples, and they get priority. So my results will probably come back in about two months. Sometimes turnaround can be as short as a week. But the best time to do it, I say, is when you can get around to it. Better sometime than never.

And so, on Monday I began sampling my soil. It’s a free service and, despite what you may gather from my grousing, the task is not terribly onerous. Here’s what you need to do:

1. Assemble your materials.

You’ll need some plastic bags (I use bags from frozen vegetables; why waste a good ziplock?), a non-galvanized digging tool (a bulb planter is my tool of choice), and a plastic bucket.

02 necessary tools2. Take your core samples.

Using the bulb plunger (or a plain old trowel), dig down to a depth of 4-8 inches, depending on how deeply you can dig. Take several samples from various points in the bed about which you are concerned, because soil pH can vary within the bed (who knows if someone spilled a bag of lime last summer?). Mix the samples in the plastic bucket. Then dump the soil into the bag, and label the bag clearly. You’ll need roughly a quart of soil.

This time, I sampled six beds: the pink-purple-yellow bed; the site near this bed that I hope may become an herb lawn; the under-construction white garden; a new bed that I’m calling the quince bed, because I have a quince shrub starting there; the rose bed/”hot border” (hot because it’s predominantly red/yellow/orange); and a prospective new front yard garden that I hope will be home to these millions of seedlings sitting about in milk jugs, waiting to happen.

3. Once you have all your sacks of soil prepared, assemble your sample boxes.

An array of sample boxes.

An array of sample boxes.

This is perhaps my least favorite part of soil sampling. The NC State School of Design has an excellent reputation; I think the Ag department should talk to them about redesigning these boxes.

The boxes are free, and you can pick them up at your local extension office.

12 box folded

Fold your box and begin to fill it with soil, up to the level indicated. It’s important to fill it up. If there isn’t enough soil, the lab cannot analyze the sample, and you’ll have wasted your time and theirs.

By the way, you should begin by reading all the instructions before you fold the box. This is why:

11 instrux on box

Poor design, Exhibit A.

Once you’ve filled the box up, tamp it down. You’ll be amazed at how much the soil will settle, particularly if you’ve mixed your sample well. Then top it off, and tamp again.

Personally, I fill the box to brimming. I know the instructions on the box state in big letters, “DO NOT OVERFILL.” But my extension agent told me once it’s better to send too much than too little; the lab will throw away what they don’t use. So Michelle, if I’ve been telling people incorrectly for the past seven years, you can give the lab techs my name to use in vain.

Finally, do your best to seal up the box. The problem I have with North Carolina’s boxes is that the tabs don’t meet well. It’s difficult to put the box together without tearing or crushing the box, even if it’s not full of soil. Good luck.

18 miserable box top

Almost there!

Please don’t seal the box with tape. The master gardener on duty who accepts your soil sample will check to make sure you have provided enough soil.

4. Fill out the remainder of the information on the side. Make up a code that will remind you of where you’ve taken the sample.

coded boxes

My coded boxes. My codes are, for example, PINKB for “pink bed” (pink-purple-yellow), PINKL (for pink bed lawn), WHITE (obvious), QUINC (for quince bed), and so on.

19 single box

The finished product.

5. The final step is to fill out the paperwork. In North Carolina, you can download this form and proceed (if you’re elsewhere in the US, search using the terms “[state abbreviation] extension soil sample form” and you should be able to find it.

And you’re done! Mail the sample boxes and paperwork or drop them off with your extension agent and wait for the report. The report can be difficult to interpret. When mine comes back, I’ll walk you through its interpretation.


2 thoughts on “Soil testing 101: How to prepare a soil sample

  1. Pingback: Christmas for Mother’s Day | MissingHenryMitchell

  2. Pingback: Azalea chlorosis | MissingHenryMitchell

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