How to take a soil sample is pretty straightforward. The first thing you must know to collect a proper soil sample is the depth the sub-samples should be taken.
The following table gives you recommended sampling depths for common landscape areas.
|Recommended Sampling Depths|
|Established lawns||2-4 inches|
|Vegetable and flower gardens||6-8 inches or tillage depth|
|Trees and shrubs||6 inches|
Sample depths are measured from the soil surface downward. For lawns, sample to a depth of 4 inches, excluding any turf thatch. For vegetable and flower gardens, sample to the depth that you plan to incorporate lime or fertilizer, usually 6 to 8 inches. In mulched beds of trees and shrubs, remove any mulch or surface debris, then sample to a depth of 6 inches.
When sampling soil around established trees and shrubs, take sub-samples from an area near the trunk to the outer edges of the branches (the drip line). For a particular landscape area, it is best to use the same sampling depth from year to year so soil test values can be more accurately compared.
How to Take Soil Sub-Samples
We learned earlier that an accurate soil sample must be taken from uniform soil areas. Within that area a soil sample must be made up of multiple sub-samples. These sub-samples are mixed together to make up the composite soil sample for that area. This section describes how to take the soil sub-sample.
|STEP 1: Open a hole with a shovel, spade or trowel from the surface to the proper depth for your landscape area. Set that soil aside. (If you are using a soil probe, insert it into the soil to the proper depth and remove the plug from the ground).|
|STEP 2: With your shovel or trowel remove a 1 inch thick slice from the smooth side of the open hole.|
|STEP 3: With the slice of soil on the blade of the shovel, remove the sides of the slice with a trowel, knife or your hands to create a ribbon of soil 2 inches wide and 1 inch thick of the proper depth. Place the ribbon (or plug if using a soil probe) into a container.|
|STEP 4: Remove any surface mat of grass or litter and any rocks. Place the soil in a clean bucket or container. Remember that a clean, plastic container is best.|
|STEP 5: Continue to take additional soil sub-samples from the uniform landscape area. By mixing these sub-samples together, you create the composite sample that will be sent to the Soil Testing Laboratory. The next section will explain how many sub-samples you should take for the composite sample.|
Making the Composite Sample
Take 10 or more sub-samples from different locations within each uniform sampling area to make a composite sample. Take the sub-samples in a random manner, such as with a zigzag pattern to minimize the variability that may be present in your sampling area. This allows you to obtain a reasonably representative soil sample.
In the figure at the right, composite sample #1 contains 12 sub-samples. Sample 2 should contain at least 10 sub-samples as well. You should avoid the eroded area when making composite samples #1 and #2. If it is large enough in size, a third composite sample could be collected from this area.
The larger the area, the more sub-samples that are needed. The more sub-samples you take, the more representative your sample will be of your landscape area. When you realize that your 1/2 pound composite sample could represent up to 2 million pounds of soil, you can understand why proper sampling is so important.
When you have taken sufficient sub-samples from a uniform area, thoroughly mix the sub-sample slices or cores, breaking up clumps and removing all foreign matter such as roots, stalks, rocks, etc. Now you are ready to prepare the sample for the Soil Testing Laboratory.
Before we go on let’s check what you have learned about how to take the soil sub-sample!