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Here’s the “Dirt” on Soil Testing

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For those who are just itching to get out in the dirt, now’s the perfect time for taking a soil test. Testing soil takes the guesswork out of fertilization.  It not only eliminates the waste of money spent on unnecessary fertilizers, but also eliminates over-usage of fertilizers.  Soil fertility fluctuates throughout the growing season each year. The quantity and availability of mineral nutrients are altered by the addition of fertilizers, manure, compost, mulch and lime or sulfur, in addition to leaching.  Furthermore,  large quantities of mineral nutrients are removed from soils as a result of growth, development and the harvesting of crops. A soil test will determine the current fertility status.

Most turf grasses, flowers, ornamental shrubs, vegetables, and fruits grow best in slightly acid soils which represent a pH of 5.5 to 7. Plants such as rhododendrons, azaleas, mountain laurel, and blueberries require a more acidic soil.  A soil test is the only precise way to determine whether the soil is acidic, neutral or alkaline.

Most soil nutrients are readily available when the pH is at 6.5.  When the pH rises above this value, nutrient elements become less available, if the pH drops below 6.5, manganese can reach a toxicity level for some sensitive plants.

How do I take a soil sample to insure accurate test results?

A good soil sample consists of a composite sample, which contains 10 to 15 cores or slices of soil with each one taken at the same depth and volume at each site.  Samples should be collected at random in a zigzag pattern over the area and mixed together in a clean plastic bucket.

Separate samples should be taken from lawns, gardens, flowerbeds or shrub borders.  Remove the top debris, residue or turf from the soil surface before taking the sample.  Samples should be taken 6 to 8 inches deep.  For the lawn, lift the sod and sample three inches deep.

Where do I send my sample?

WSU has an excellent publication listing soil testing labs, which is available from the publications office (  Search for EB 1578E, titled “Analytical Laboratories and Consultants Serving Ag in the PNW”.  It is downloadable for free!  No matter which lab you choose, be sure to call the lab first for instructions and appropriate forms.

How reliable are the Soil Test Kits found in gardening stores?

Garden store soil testing kits are relatively inexpensive compared to the tests done by more sophisticated soil testing laboratories.  They can give you a very general indication of your soil pH and fertility status if that is all you want. 

If our soil test indicates that we need to apply fertilizer, what is the best time to make the application?

As a general rule, the optimum time to apply fertilizer is in the spring, just as plants are ready to begin their new season of growth.  The best time to fertilize vegetable gardens and flower beds is at the time the soil is being tilled and prepared for planting.  The desired amount of fertilizer can be broadcast evenly over the garden space and tilled into the top 6 to 8 inches of soil.  This distributes the fertilizer evenly thought the root zone of the plants.

Spreading the desired amount of fertilizer over the root zone and incorporating the fertilizer by shallow cultivation and or irrigating the area fertilize trees, shrubs and perennial plants.

For lawns, the fertilizer application is usually split into four smaller applications that coincide with the Easter, Memorial, Labor Day and Thanksgiving Day holidays.



Editor’s Note: This article first appeared on 2/7/13. If you would like to respond to this story go to
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