Why won't my orange tree produce fruit? Why won't my hydrangea flower? Why do my tomatoes develop blossom end rot? Why is my corn weak and spindly? Why are the leaves on my witch hazel yellowing in midsummer? Why do the leaves on my apple tree look scorched? Is it safe to eat from my veggie garden? The answers to these and many other gardening dilemmas are rooted in the soil.
Soil is the foundation of terrestrial life. Keeping it healthy and productive is one of the best things we can do for our garden plants. Soil testing is the best way to determine if the nutrient levels are adequate and if you have a problem with toxins. When the science of soil testing was first developed, it was extremely controversial. Like any new technology, it was viewed with disdain by the old guard. Some university officials even decried it as mock science or voodoo. Today soil testing has become an essential tool for managing soil health. Autumn is the perfect time to have your soil professionally analyzed because it takes some time for soil amendments -- such as sulfur for lowering pH and lime for raising it -- to take effect.
For city gardeners especially, testing the soil is a very important practice. Urban soils are quite different from soils that have remained relatively undisturbed. The normal layers and stratifications are usually mixed up. Topsoil may be missing. Oftentimes construction debris or excavated subsoil constitutes the top layer. The natural process of nutrient cycling is often absent or reduced in city soils. Leaves and other plant debris are not allowed to decompose on-site and replenish the soil, so it's left depleted of necessary nutrients. Other characteristics of urban soil are compaction from construction equipment, contamination from construction materials and pollution, and elevated pH levels. All of these issues can be managed with good horticultural practices, and soil testing is the first step.
I recommend that gardeners contact a soil-testing laboratory before taking samples. Individual labs may require specific procedures for specific tests. Urban gardeners also need to know if the lab checks for toxins like lead, arsenic, benzene, cyanide, dioxin, and PCBs. Testing for these toxins will cost more, so review the history of your site so you know which ones may be a concern. Call your local Cooperative Extension Service or check the Internet for a soil-testing facility near you.
Once you have contacted a lab and learned the amount of sample needed and any specific recommendations for collection, you are ready to begin. Samples should be taken while the soil is above 50 degrees F and preferably on a day when the soil is not wet. You'll need a clean bucket or bag and a shovel, trowel, auger, or sampling probe.
1. Scrape back the mulch and dig a hole in the garden area. The depth of the hole depends on the plants you are growing there. Dig about 12 inches under trees and shrubs, 8 inches under veggies and flowers, and 4 inches under sod.
2. Take a thin slice down the side of the hole for your sample.
3. Ideally, repeat this process several times so that you get about eight thin-sliced samples per 100 square feet. This helps give an accurate assessment of the entire site.
4. Remove all roots, mulch, and plant debris from the samples.
5. Combine the samples in the bucket and mix them up.
6. Spread the soil over clean paper or fabric to air dry for a day or so.
7. Gather a half-pint of the soil (or whatever amount the laboratory requires) and place it in a sealed plastic bag.
8. On a label, write your name, address, description of sample (e.g., flower garden or turf grass), and the information you are requesting.
9. Put the plastic bag in a sturdy box or can to prevent the sample from opening or absorbing moisture.
10. Include any other paperwork provided by the lab.
11. Mail the sample, and within a few weeks you should have the results.
If the soil in your gardens is uniform from place to place (which it rarely is), then one sampling may be enough. However, if you have a large garden with radically different spaces (hillside vs. floodplain, acidic woodland vs. parkway sod, veggie garden vs. construction debris), you will need samplings from each distinct area.
Soil test results and recommendations are easy to interpret once you have some practice. Cooperative Extension Service agents will usually go over the results with homeowners. Following the recommendations this fall will pay big dividends in your gardens next year.