Fertile Ground

Test your garden soil to be sure it’s giving plants what they need.

By Jan Wiese-Fales | Photos By Jan Wiese-Fales

A complete listing of U.S. Cooperative Extension Service offices with soil labs is available here.

The University of Guelph in Ontario tests Canadian soils.

Plants obtain nutrients essential to their growth and development directly from the soil, but not all soils are equal. Nutrient levels, as well as pH, can vary from year to year and from one part of your garden to the next.

The best way to know what’s going on beneath the surface is to do a soil test for primary and secondary nutrients. The big three are N, P and K: Nitrogen (N) aids vegetative growth and development; phosphorus (P) fuels healthy root systems; and potassium (K) protects them from water loss and disease. Deficiencies in secondary nutrients such as calcium, magnesium and sulfur also affect plant vigor.

You can purchase commercial kits to do your own tests, or go through a professional soil-testing lab to get an analysis of nutrient levels, as well as pH levels—the acid/alkaline balance in soil that aids in nutrient uptake. Soil labs, such as those hosted by many university Extension programs, also can perform the tests and offer recommendations for the amendments your soil may need.

Collecting Accurate Soil Samples

Each area sampled should share a fairly uniform soil type and history. Because they have varying needs and use nutrients at different rates, your vegetable garden, flower beds and fruit plots should be sampled separately.

1. Using a clean garden trowel, take six to 10 samples consisting of 6-inch-deep vertical trowel or shovel scoops of soil taken from random spots in the plot you want to evaluate. Avoid sampling from recently fertilized areas.

2. Thoroughly mix the samples together in a clean container, removing
any debris. 

3. Measure out 2 cups and, if the soil is wet, air-dry it before placing in a resealable plastic bag to take or ship to your soil-testing facility. 

4. Repeat the procedure for each plot you want to test, label each with its intended use, and submit your samples immediately.

Amending Soil 

If your soil tests low in nutrients or has unhealthy pH levels, you can add organic or synthetic fertilizers and other amendments that match what your soil needs. 

Synthetic fertilizers generally show the fastest results, while slower-acting organic fertilizers, such as manure and compost, are broken down by soil microbes and provide nutrients as they decompose over a longer period. Additionally, residual humus, which contains a variety of beneficial micronutrients, improves air and water circulation and retention. Commercial fertilizers and your compost can be added at any time, but well-cured manure should be applied in the fall.

Compost supports living organisms in soil and also can help moderate pH. A pH of between 6.0 and 7.0 is an ideal range for most plants. Exceptions include such things as blueberries, azaleas and rhododendrons that require a more acidic pH of 4.5 to 5.5.

In addition, amending soils with dolomitic limestone (lime) and wood ash raises pH and increases calcium and magnesium. Applying elemental sulfur lowers pH.