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UGA Cooperative Extension

DeKalb County Extension Office:
Agriculture & Natural Resources
Soil Testing

Soil Test

A soil test is the fertility gauge for lawns, shrubs, trees, vegetables, flowers or for whatever you are trying to grow. Your results will tell you what nutrients are available in your soil and at what quantities (low, medium or high). Macronutrients are those that are required in larger amounts such as: nitrogen (N), phosphorous (P), potassium (K), calcium (Ca), magnesium (Mg), zinc (Zn) and manganese (Mn). Nitrogen, phosphorous and potassium are the three primary ingredients of all fertilizers. The soil test also measures the acidity or alkalinity of your soil and reports it on a pH scale. A pH of 7 is neutral, anything below 7 is acid and anything above 7 is alkaline or basic. The preferred pH for most plants in our area would be between 5.5 and 6.5, which is slightly acidic.

If your soil is getting too acidic, plant growth nutrients are tied up and other soil elements (such as aluminum) can express toxicity and damage plant growth. Therefore, a soil test reporting too low a pH (too acid) would recommend the addition of lime to bring that pH back up to a more desirable level.

NOTE: Adding lime to your soil does not instantly return it to a desired pH level. To be effective, lime should be thoroughly incorporated (mixed) into the soil and not just applied to the soil surface.

How to Collect a Soil Sample?

Remember: Soil test results are only as good and as accurate as the quality of the sample the lab receives. When collecting, make sure you sample to an adequate depth so that the soil is representative of the area. Example: a lawn would ideally have roots 4-6 inches down into the soil. Therefore, a good lawn soil sample should go as deep as 4-6 inches and be collected randomly over the soil area.

Steps to Gathering Sample

Take a random plug...
or core from at least 8-12 places scattered around your site, especially, if you are doing a large area like a lawn. Fewer samples are required for other areas like small flowerbeds.

Tools to use…
any tool that will help you collect a thin, vertical slice or hunk of soil that goes about 4-6 inches deep is fine. A garden spade or trowel often works fine.

Size of the area to be sampled…
This depends upon the uniformity of your soil (is it all alike?) and whether or not you have made previous changes to these soil areas by adding compost, lime, fertilizer, etc. If you have treated the soil areas differently by adding different materials (amendments) to them, they should be sampled separately and be brought in to our office labeled as separate samples, such as, perennial flowerbed, lawn-fescue, etc.

Collecting the sample…
Take your 8-12 random plugs of a uniform soil or uniform planting bed and mix them together in a bucket or other clean container (no outside contaminants). Put some of this mixed soil into our UGA brown, labeled soil bags. Fill the bag up to the dark black fill line so that we have sufficient soil to run the requested tests. If you do not have our UGA brown bag, you can collect your sample in a plastic bag or container. The soil can be transferred to a UGA brown bag at the office. You will need to collect approximately one cup of mixed soil.

Information Card…
The information card comes attached to the soil test kit and is required for our records. It supplies your name, address, phone number and the type of crops you are planning to grow. You have to specify a category of plant: vegetable, fruit, lawn, tree, shrub, annuals, perennials, etc. For lawns/pastures specify the type of grass grown or being planted: lawn-zoysia, lawn-fescue, lawn-Bermuda, lawn-centipede. The information card is provided at the Extension office.

Fee…
A routine or basic test (nutrients and pH) is still very reasonable at $8 per soil sample, however, you should call your extension office before bringing a large number of samples. For other tests, you should call for availability and current cost. Checks are payable to: DeKalb Cooperative Extension.

When to Soil Test?

The best time is before you start a new project – new lawn, new garden, etc. Samples should be taken about 4-6 weeks before your results are needed so that you have time to do your soil preparation. For a spring planting, take samples in October thru January. For fall plantings, collect samples June-August.

Once you have adjusted your pH level as recommended by your test results, your soil should not have to be corrected again or tested again for another 2-3 years. Fertility and acidity levels do not normally change that quickly unless you go overboard with soil amendments.

Precaution: Do not automatically add lime to your soil every year or you will eventually get the pH too high (over 8) and again plant growth will suffer because important growth nutrients will be tied up and become unavailable for plant growth. Proper pH is one of the keys to good plant growth!

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University of Georgia (UGA) College of Agricultural and Environmental Sciences (CAES)