HOW TO: Take a soil sample
“You can't tell anything without a soil sample.” This cliché has been repeated so often — sometimes as the punch line of a joke — people often overlook the truth in the statement. Of course, you can tell some things about soil without an analysis, but the chemical properties that make a soil good or bad are mostly difficult or impossible to discern by sight. So you must submit a soil sample to a reputable lab for proper chemical analysis if you wish to get a handle on what shape your soil's in.
Most laboratory soil analyses are fairly routine. If an error occurs in the process, it's not likely to have occurred in the lab. Rather, the sample itself is likely to be where the problem lies. And that's where you come in. The lab can only test what you give them, so you need to give them a sample that will tell you something meaningful. That means taking samples in the right places, at the right depths and with the right tools; all while keeping a meticulous record of where and how you sampled.
The right tools
If you are in the turf or horticulture trade, you ought to have a soil tube. They are fairly inexpensive, durable and allow you to sample soil quickly at controlled depths. Because soil tubes are also useful just for “poking around” in landscapes, it's a good idea to have one on hand. However, an alternative is to use a spade or shovel (as discussed in “Taking the sample,” below). Augers, trowels and powered samplers all may be suitable as well.
Soils vary from spot to spot, sometimes over just a short distance. A representative sample, therefore, should include soil from different points at the site. Further, the cores that you combine to form the sample you submit should be roughly the same size so that no single sample will skew the results.
Most labs request that your sample includes soil from at least a dozen individual points in the sampling area. You may end up using quite a few more than this, depending on the size of the area, however. It often is helpful to sample in a systematic fashion to ensure that you thoroughly cover the site.
The key to determining sampling units is to define a more or less homogeneous area. For example, don't mix turf samples with those from a shrub bed. On a golf course, sample tees, fairways and greens separately. At a residential site, keep front and back lawn samples separate. Use your judgment when defining the sampling area. It may be acceptable to submit one sample for a relatively large area (a fairway or athletic field, for instance) if the entire area is more or less uniform and maintained similarly.
If you're sampling an area because you're trying to diagnose a specific problem, be sure to sample only from the affected area. (However, the lab may ask you to sample from a healthy area for comparison.)
See the illustrations below. Each dot represents a sampling point. Each colored area represents one sample. That is, the soil from all sampling points within the area are combined for one sample.
- Be consistent
Soil samples should always be taken in a consistent manner. In particular, make sure each sample is the same size, and that each core or slice is uniform from the soil surface down to the sampling depth. A soil tube makes consistent sampling easier, but you still can also take good samples with a spade or trowel, though it will require a bit more care during sampling.
Also, in larger areas, sample systematically so that your sampling pattern resembles a grid or some other pattern that ensures more or less even spacing between sampling points.
- Use appropriate depth
Six inches is adequate in most cases, including tree, shrub and bedding areas, or open ground you're preparing for planting. Three to 4 inches is usually adequate for established turf, since most turf roots don't reach deeper than this.
- Remove debris
Be sure you remove surface debris (sticks, leaves, etc.) and rocks from the sample. Also remove turf mat and thatch.
- Collect and mix samples
Carry a pail or similar container with you as a receptacle for the samples. After you have completed sampling the area, thoroughly mix all the soil together. Labs usually only need about a pint, so put that amount into the sampling bag (often provided by the lab) and then discard the rest. For small areas, make sure you pull enough samples to provide an adequate sample.
- Record information
Be sure you record all the information requested by the lab, and label each sample bag clearly.
- Follow laboratory instructions
The preceding steps are general instructions. Labs may request that you follow specific steps that could differ. Be sure to read and follow any instructions the lab provides to you, including how to deliver the sample to the lab.
Timing is not critical, with a few obvious exceptions, such as wet or frozen soils that would make sampling difficult. Another situation where timing might be important is with salt damage (such as from deicers). Salt leaches through soil, so if you are testing to confirm salt damage, do so quickly before concentrations drop due to leaching. Further, be sure not to sample within a month after applying fertilizer or soil amendments, because these materials will skew the lab results.
Remember that if you are getting ready to install new turf, test the soil first so that you can make any amendments that the lab results might suggest before installing the turf.
Be sure to keep copies of all records relating to soil testing. Obviously, you need to match the results to the right locations, but it's also useful to compare current test results to those from past years to see if and how your soil might be changing.
“Garbage in, garbage out” is a saying applied to computers. The principle is the same with soil analyses — the analysis is only as meaningful as the sample. Ensure that you get your money's worth by delivering a proper sample to your laboratory.
Sample form courtesy of Spectrum Analytic Inc.
PO Box 639/1087 Jamison Road
Washington, C.H., Ohio 43160
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