Plant Care
Why Pacific Northwest Soil Is Acidic + How to Fix It
Acidic soil in the Pacific Northwest comes from decades of heavy rain leaching out calcium and magnesium. Here's the chemistry behind it and how to fix it with lime.
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Quick Answer
Pacific Northwest soil turns acidic because heavy rain leaches out calcium and magnesium. Apply dolomitic lime in fall, 5-10 lbs per 100 sq ft (2.3-4.5 kg), to raise pH.
TL;DR
Pacific Northwest soil turns acidic because heavy winter rainfall dissolves atmospheric carbon dioxide into carbonic acid, which leaches calcium and magnesium out of the root zone. When pH drops below 5.5, soluble aluminum becomes toxic and phosphorus gets locked up. The fix is lime—dolomitic lime in the magnesium-depleted maritime west, applied in fall so it has months to react. Test first; some plants actually prefer the acidity.
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Frequently Asked Questions
Most maritime gardens benefit from liming every two to three years, but the only reliable schedule is one based on testing. Re-test pH every two to three years and lime when it drifts below about 6.0. Because the same heavy rainfall that acidified your soil keeps leaching base nutrients, pH will slowly fall again after each correction. Think of liming as ongoing maintenance rather than a one-time fix, and keep records of your readings so you can see the trend.
Wood ash does raise pH and adds potassium, and it works faster than lime because it is more soluble. However, that speed is also its risk—it is easy to overshoot pH and to introduce salt stress. Apply wood ash sparingly, no more than 15-25 lbs per 1,000 sq ft (6.8-11 kg) per year, and only where a test shows you need both higher pH and potassium. For larger corrections in PNW soil, dolomitic lime remains the safer, more controllable choice.
Lime is only slightly soluble and reacts with soil gradually, typically taking three to six months to meaningfully shift pH. This is exactly why fall application is recommended in the Pacific Northwest. Applying lime in autumn lets the wet winter season carry it into the root zone and complete most of its reaction before spring planting. If you lime in spring, expect little benefit until the following season.
If your blueberries are yellowing in genuinely acidic soil, suspect either waterlogging or an inland alkaline pocket rather than a need for more acid. Saturated roots cannot absorb iron regardless of pH, and that produces the same yellowing. Confirm with a fresh soil test and check drainage. East of the Cascades, alkaline soil is the more likely culprit, and the fix there is elemental sulfur, not lime.
It depends on what you filled the beds with. Beds built from acidic native soil or unbuffered peat-heavy mixes can still test low and benefit from lime. Test each bed individually, because raised beds can drift in pH just like ground soil—especially under the PNW's heavy rainfall. Annual compost and periodic testing usually keep a well-built bed in range with only occasional liming.
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Sarah earned her doctorate in plant biology and spent time working in botanical garden education before transitioning to freelance writing and consulting. Now based in Portland, Oregon, she teaches plant identification workshops at local community centers and maintains a modest collection of over 60 houseplants in her small apartment. Sarah specializes in helping beginners understand plant science without the jargon—her approach focuses on practical observation over theory. She's killed her fair share of fiddle leaf figs and finally cracked the code on keeping them alive.
