Extension Search / Soil Labs / Washington

Soil Testing in Washington

Washington’s land-grant university doesn’t run a soil lab for the public, so samples go to a private lab. Extension says which tests to ask for, so your numbers match the state’s fertilizer guidelines.

Private Labs

Ask the lab for these tests:

Basic test
Organic matter

WSU Extension: soil testing labs has more on sampling and reading results.

Test Methods

Ask the lab for:

Phosphorus: Olsen. Sodium bicarbonate. Made for alkaline, high-lime soils like most of the West.

For acid soils west of the Cascades, ask your county office which test to use.

Labs pull nutrients out of soil with different chemicals, and each one gives a different number for the same soil. 20 ppm on one test and 20 ppm on another don’t mean the same thing, so check which test your report used before you compare it to a chart.

Tests

Basic test. pH, phosphorus and potassium, usually with calcium and magnesium. Enough for most gardens and lawns.
Organic matter. How much organic matter the soil holds, which sets how much nitrogen it releases.

WSU Extension doesn't test soil. County offices keep lists of certified private labs for homeowners.
WSU recommends Olsen phosphorus for the mildly acid to alkaline soils of eastern Washington.

Sources: [1] [2]

Sample Tips

Take 10 or more small cores across the area and mix them in a clean bucket. One spot tells you about one spot.
Sample gardens and lawns about 6 inches deep, below any mulch or thatch.
Keep beds that are grown differently, like a lawn and a vegetable garden, as separate samples.
Wait a couple of months after fertilizing or liming, so the test reads the soil and not the last bag.
Use the same lab and the same tests each time, so the numbers compare from year to year.

Your county Washington Extension office can help you take the sample and read the report.

Results

Once the report comes back, type the numbers into the soil test calculator to see what to add for your bed or lawn. The soil test guide walks through each line of the report.

Other States

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