Soil Testing

12 October 2005

Virginia Cooperative Extension, Charlottesville/Albemarle County Office
460 Stagecoach Road, Charlottesville, Virginia 22902
phone: 434.872.4580 fax: 434.872.4578

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a soil test box and form

A soil test can provide information on the proper amount of lime and fertilizer to apply to your lawn, garden and other areas of your landscape. When gardeners apply only as much lime and fertilizer as is necessary and at the appropriate time, nutrient runoff into surface or ground water is minimized, money is saved, and plant health is optimized. Fall is an excellent time to test your soil because many problems can be corrected during the cool part of the year. Soil testing can also be used to diagnose common nutrient deficiencies or toxicities for plants that are growing poorly.

The reliability of the soil test, however, can be no better than the sample you submit. For results you can depend on, it is vitally important that you take samples correctly to accurately represent the soil in your landscape.

How to Take a Representative Soil Sample

The first thing you must know to collect a proper soil sample is the depth the sub-samples should be taken.

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

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. Here are five steps to guide you through the process.

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).
With your shovel or trowel remove a 1 inch thick slice from the smooth side of the open hole.
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.
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.
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.

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.

The number of sub-samples that are needed for a meaningful test is based on the size of the area to be tested primarily. 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.

Submitting the Sample to the Lab

Obtain free Soil Sample boxes and Soil Sample Information Sheets from your local Virginia Cooperative Extension office. Use permanent ink or pencil to fill out forms and label boxes.

Fill the sample box completely with loose soil and label it with your name and sample identifier. Give the sample an identifier of up to five letters and/or numbers. Choose a unique identifier that will help you remember the area it corresponds to, such as FYARD, BYARD, ROSE2, or GARDN. Be sure to keep a record of the areas sampled with their corresponding identifier. This is particularly helpful if you are taking multiple samples.

Complete the Soil Sample Information Sheet

The Virginia Cooperative Extension Soil Sample Information Sheet is an important part of the sample process.

To get the most value from your soil test, take the time to fill in the blanks on the information sheet as completely and accurately as possible. Be sure to list the correct plant code for each sample you submit. Also, check to make sure that the sample identifier you put on the Sheet corresponds to the identifier on the sample box and in your records. Costs for the various soil tests that are offered are listed on the information sheet. If you need assistance with the Information Sheet, contact your local Extension office.

Mail the completed Information Sheet with the sample box and payment in a sturdy shipping box to the Virginia Tech Soil Testing Lab, 145 Smyth Hall (0465), Blacksburg, VA 24061.