How to Take an Accurate Soil Sample

How to Take an Accurate Soil Sample

Reading Time: 5 minutes

By Kristi Cook – Do you want your garden to produce the best-tasting food in town? Do you want to maximize production to fill your farmers market stands? Maybe you simply wish for your first garden to be a success and produce enough fresh, flavorful food for your dinner table. Whatever your goals are for your garden, you need a soil test. Yet, this simple and inexpensive garden chore is the most commonly overlooked aspect of gardening. How do I know? Just take a listen around the garden center and you’ll hear countless folks complaining that their garden “just didn’t do well this year.” Yet when asked how their soil tested, they nearly always tell you they’ve never taken a soil test. So, take a few moments to discover the how and why behind soil testing so your next garden can be a chore of pleasure rather than a chore of frustration. 

Why Conduct a Soil Test? 

Just as your body requires certain nutrients to perform at its best, so does your garden. Basic needs for every garden include the proper pH, or acidity/alkalinity level, and sufficient levels of nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) to name just a few. And while it is true that nature is often able to provide what the soil needs in a more naturalized setting such as in a forest or a valley, a home garden is unique in that we as humans determine what we wish to grow in the garden. Most often, our choices are not in sync with what nature is capable of naturally supporting.  

For example, we may wish to grow blueberries, but our soil is too alkaline. Or we wish to grow cucumbers, but the soil is too acidic. To make the situation worse, when we plant these same crops in the same general area year after year, the soil is drained on a continual basis resulting in the dreaded summer when “the garden just didn’t do well.” This constant drain on the soil’s resources requires us to provide and replenish nutrients in order to meet our specific crops’ needs. Soil testing tells us exactly which nutrients are needed and to what level. 

But wait, can’t I just toss in some fertilizer and call it a day? Sadly, this is most often the case with home gardeners. The reason is simple — it’s fast and easy. But the end result is often a lackluster garden with pest and disease issues caused by a lack of proper nutrition. Just as people cannot fight off disease and pests easily when nutrition is lacking, neither can the garden. Nutrition is not just about robust growth and maximizing production. It is also a key factor in a plant’s ability to fight off the dangers it faces within its environment.

How to Conduct a Soil Test

Taking a soil test is simple, requiring little more than a spade, a bucket, and a way to transport the sample. First, determine which facility to use. Most labs offer free, or inexpensive, analysis of the potassium and phosphorus levels in addition to pH. Most, if not all, also routinely check for magnesium (Mg) and organic matter levels for the same nominal fee. More in-depth analyses may be obtained for an additional fee such as soil salinity, aluminum (Al), boron (B), copper (Cu), manganese (Mn), and zinc (Zn). Nitrogen (N) is most often not included unless requested due to the fact that nitrogen is not held in the soil very well and thus must be replenished each year anyway. 

Once you’ve selected a lab, gather the collection materials needed. Some require samples to be submitted in specialized containers supplied by the lab while others simply need a pint jar or Ziploc bag of the sample. Other equipment needed is a spade or soil probe, a bucket for collecting samples, a pen, and paper. 

Samples consist of multiple “cores” or “slices” of soil. For most family-sized gardens, five to 10 cores are usually sufficient while larger areas such as yards need 10 to 15 cores. If sampling areas are larger, consider up to 20 or more samples. If in doubt, contact your selected lab to determine their recommended number of cores to include in the composite sample. 

To obtain samples, first remove all plant residue, rocks, mulch, etc. from the soil surface where the sample will be taken. Then take the spade and cut a triangular wedge in the cleaned-up area. Take wedges from multiple areas in the garden using a zigzag pattern to ensure samples are taken from the entire area. Once the wedge is removed, take a sharp knife or a clean hand trowel and slice a section of soil from the edge of the hole, roughly two inches or so in width from the surface down to about six to eight inches of depth. Then further reduce the width of the soil sample to about one inch.

A wedge the size of the shovel is all that is needed to open the ground enough to take a good sample. Photo by Lance Brooks.

Place all samples from the same area into a bucket and mix thoroughly. You want the samples to be mixed as well as possible to create a composite of the entire area. Once mixed, allow the soil to dry thoroughly before packaging for transport. Most labs require two cups (or a pint) of soil to test, so check the sample size of the selected lab. Label the sample and make notes as to which sites were sampled so that you know which results go with which location.  

Once you receive the results, read them thoroughly and contact the lab if you have any questions regarding the instructions. Most include recommendations for amendments based on acreage and square footage to facilitate different sized operations. These recommendations also include how much nitrogen to add to your particular plot of land, so be sure to follow instructions closely so as to not add too much of any amendment. Some basic computations are occasionally required to tailor the recommendations to your garden size, but labs are very helpful in assisting with the conversions. (These results may be used with both conventional and organic practices.) 

A basic soil test is the most often overlooked aspect to growing a successful garden and is simple and inexpensive to do. Even better, tests need only be repeated every three to five years, so testing is not a laborious task to be added to an already busy season. Test in the fall when things slow down or test when the urge hits you, it’s all the same. Be sure to test so you never find yourself saying your garden “didn’t do well this year.”

For a listing of soil testing labs in the U.S. go to: https://gardeningproductsreview.com/state-by-state-list-soil-testing-labs-cooperative-extension-offices/ 

Originally published in the July/August 2021 issue of Countryside & Small Stock Journal and regularly vetted for accuracy.

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