A field guide to soil amendments for Western Washington gardens. Start from what you're seeing, follow the branches, and end at what to add, how much, when, and why. Numbers come from WSU's Pacific Northwest Gardener's Handbook, chapters 3–5.
Where pH should land
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West of Cascadestopsoil guideline
Most garden plantsvegetables, ornamentals
Acid loversblueberry, rhododendron
Lime and wood ash push pH upElemental sulfur and ammonium fertilizers pull pH down
Before you amend anything, test
Every branch below starts from a soil test. Guessing at lime or sulfur rates is how gardens end up overshooting in the other direction.
Sample each area separately: vegetable beds, lawn, and berry patch get different recommendations.
Take at least 10 subsamples per area, from the surface down to 12 inches. Skip odd spots like old burn piles or compost heaps.
Air-dry, mix well, and send about a cup to a lab. A basic garden test runs $25–50 and covers pH, lime requirement, P, K, Ca, Mg, and often boron.
Ask for the lime requirement test if pH is low. It tells you exactly how much lime to add instead of an average.
Retest every 2–5 years, and stick with the same lab so results are comparable.
WSU keeps a list of Pacific Northwest soil labs; pick one that uses WSU or OSU methods and gives garden recommendations. Labs can't reliably predict nitrogen, so N rates come from the plant tables further down.
Is this bed for acid lovers like blueberries or rhododendrons?
Yes
Don't lime. These plants want pH 4.5–5.5. Liming them invites iron deficiency. Leave this area out of any liming you do elsewhere.
No, it's vegetables, lawn, or general landscape
Does the soil test show low magnesium?
Yes, Mg is low
Dolomitic lime
What
Ground dolomitic limestone (calcium plus magnesium carbonate).
How much
The rate from your lime requirement test. Without one, about 50 lb per 1,000 sq ft is an average application.
When
Fall, so it has winter to react before spring planting. Lime is slow.
How
Till or dig it in if you can. Finely ground works faster but is dusty; pellets cost more but spread cleanly.
Why
Raises pH and supplies both Ca and Mg, which are often short together in acidic west-side soils.
No, Mg is fine
Ground limestone
What
Ground limestone (calcium carbonate). Fine for organic gardens.
How much
Per the lime requirement test; about 50 lb per 1,000 sq ft on average.
When
Fall, for spring crops.
Why
Neutralizes acidity, adds calcium, and keeps aluminum locked up. Below pH 6, aluminum becomes soluble and can be toxic to roots.
Alternative: wood ash
How much
No more than 15–25 lb per 1,000 sq ft.
Why
Raises pH and adds K, Ca, and Mg, but heavy doses cause short-term salt injury. Never use on alkaline soil.
Not gypsum. Gypsum adds calcium and sulfur but barely moves pH, and it only helps structure in sodium-heavy soils, which are rare here. Watch for overshoot: liming can trigger iron, zinc, or manganese deficiency, so retest before adding more.
Less common on the west side, but some soils still aren't acidic enough for good blueberries. The giveaway is yellow young leaves with green veins (interveinal chlorosis): the plant can't get iron at that pH.
How big is the area, and how far does pH need to move?
An existing bed that needs a significant drop
Elemental sulfur
What
Elemental sulfur, labeled 90% or 99% S.
How much
2–4 lb per 1,000 sq ft per application. Your soil test and texture set the real number.
When
Ahead of the season; soil microbes oxidize it into acid slowly. Retest before each repeat.
Why
Lower pH frees up iron. Iron fertilizer alone won't fix the yellowing; the soil locks it up again unless pH comes down.
Stop when you hit target. Too much sulfur drops pH below where you want it, and that's slow to undo.
Only a small nudge, or ongoing maintenance
Acidifying fertilizer
What
Ammonium sulfate (21-0-0) or urea; some organic fertilizers like cottonseed meal also acidify.
Why
Lowers pH gradually while feeding nitrogen, which acid lovers need anyway.
A new planting where native soil won't cooperate
Build an acidic raised bed
What
A sandy mix amended with peat or sphagnum moss plus elemental sulfur.
Why
Peat is more acidic than other organic amendments, and a bed lets you control the whole root zone.
West of the Cascades, topsoil should test above 5% organic matter. Organic matter improves structure, drainage in clay, water-holding in sand, and feeds soil life. It's the single most broadly useful amendment.
Is this a new bed or an established one?
New garden or landscape bed
Compost, worked in
How much
1–3 inches. One cubic yard covers about 300 sq ft at 1 inch deep.
When
At bed preparation, before planting. Dig or till it in, or leave it on top for landscape beds.
Season
Fall if the soil is still dry enough to dig, giving it months to break down before spring planting. Otherwise spring, once a handful crumbles instead of rolling into a wire. Based on the handbook's advice to apply organic materials weeks to months ahead and never to work wet soil.
Why
Builds structure fast while you still have open access to the soil.
Established bed
Compost top-up
How much
About ½ inch per year.
When
Yearly. Can go on as mulch first and be turned in later.
Season
Fall, after harvest, left on the surface as winter mulch. Dig it in or plant through it in spring once the soil is dry enough to crumble. Skip beds getting a cover crop. Based on the handbook's advice to cover bare winter soil, apply organics early, and stay off wet soil.
Why
Organic matter decomposes; this replaces what breaks down.
Does the soil test show very high phosphorus?
Yes
Switch to cover crops
What
Stop adding compost and grow cover crops instead (see the calendar below).
Why
Compost carries P; excess P washes into streams and lakes and feeds algae blooms.
Low-nitrogen materials stay on top. Straw, bark, and sawdust (under about 1–1.5% N) steal nitrogen from plants if mixed into soil. Use them as surface mulch only. Woody compost can do the same in its first year or two; if plants yellow, add some extra nitrogen. Avoid extreme rates like ⅓ compost by volume in permanent beds; it decomposes, settles, and compacts.
Ch. 3, Organic Amendments and Compost; Ch. 5, Improving Soil Quality and Runoff
Water won't drain
Test it: fill a planting hole with water. It should drain within 24 hours. Gray subsoil with orange or red mottling also means a seasonal high water table. In the Puget Sound area, a common culprit is glacially compacted hardpan starting 18–36 inches down, or shallower on graded lots.
Where is the problem?
Near the surface, in fine clay or silty soil
Organic matter
What
Compost worked into the top layer, per the organic matter section above.
Why
Builds aggregates that open up large pores. It won't fix a dense layer deeper down.
A single tree or shrub planting hole holds water
Vertical drain
What
A hole 4–6 inches across and 3–5 feet deep, dug in the bottom of the planting hole with an auger or post-hole digger, filled with coarse gravel.
Why
Gives pooled water a path through the restrictive layer.
Don't amend tree backfill. Research shows no benefit in routine plantings, and the texture mismatch can make the hole hold even more water. Loosening a wider area helps more.
A whole bed or area, with a dense layer below
Raise the root zone
What
Raised beds 8–12 inches high for most garden plants; berms 2–3 feet high for woody plants.
Why
Lifts roots above saturated native soil. The go-to fix for glacial hardpan.
Or a tile drain
What
Perforated pipe buried 1 to several feet down, leading to a ditch or gutter.
Why
Removes excess water from the zone above it. Check local rules before draining.
It's a low spot that collects runoff
Redirect water, or plant for it
What
Divert roof downspouts away from beds, and choose species native to wet sites.
Why
Sometimes the cheapest fix is matching the plant to the spot.
Ch. 5, Poor Drainage and Special Considerations for Trees; Ch. 3, Wet Soils and Compaction
Soil is compacted
Signs: bare worn paths in lawn, dry spots or runoff when irrigating, tree roots running along the surface. Compare how easily a long screwdriver or soil probe goes into moist soil in the suspect area versus a normal one. Compaction suffocates roots more than it physically blocks them.
What's growing there?
Lawn
Core aeration plus topdressing
What
Hollow-tine aerator that pulls cores, then less than ½ inch of fine compost raked or mowed into the holes.
When
At least once a year in heavy-traffic areas; more often where paths form.
Why
Reopens pores for air and water; the compost keeps them open.
Severely worn path
What
At least 1 inch of compost, rototill, then reseed or sod. Keep traffic off while it establishes, or give in and put in pavers or a path.
Established trees or shrubs
Thick coarse mulch
What
A deep layer of wood chips or arborist chips over the root zone.
Why
Stops further compaction and slowly improves the soil without cutting roots.
Vertical mulching, for a declining tree
What
Auger holes 2–4 inches across and 2–3 feet deep, starting 2 feet from the trunk and gridded out to the drip line. Fill with wood chips or gravel; some add slow-release fertilizer.
Why
Lets air and water move under a compacted surface.
Bare ground after construction
Rip, then layer topsoil
What
Fracture compacted subsoil with a deep ripper or heavy tiller while dry. Spread about a third of the topsoil, till it into the subsoil, then add the rest on top.
Why
A sharp boundary between subsoil and topsoil blocks water and roots; the mixed layer softens it.
Prevention: only work soil when dry. If a handful rolls into a wire or worm shape, it's too wet to till. Mulch walking paths, and use plywood under equipment.
Ch. 5, Soil Structure and Compaction and Extreme Practices; Ch. 3, Compaction and Loss of Structure
Plants look hungry
Nitrogen is the most common shortage in Washington. On the west side, potassium, sulfur, calcium, magnesium, and boron (in root crops, brassicas, and caneberries) also come up. Symptoms overlap, so a soil or tissue test beats eyeballing.
Which leaves show it first?
Older, lower leaves
What does the yellowing look like?
Evenly yellow, no dead spots → likely nitrogen
Browned, dead leaf edges; weak stems → likely potassium
Yellow between green veins → likely magnesium (use dolomitic lime if pH is also low)
Younger, newer leaves
What does the yellowing look like?
Evenly yellow → likely sulfur
Yellow between green veins → likely iron, usually a pH problem (see pH too high)
Whole plant stunted, dark or purplish → likely phosphorus
Often in cold spring soil; plants may grow out of it as soil warms. Rare in beds with a history of manure or fertilizer.
Never more than 1 lb N per 1,000 sq ft at a time, or you risk burn. Split the yearly total into two or more doses.
Heavy feeders
Half at planting; the rest banded 6–8 inches from the plants and 2–3 inches deep once they hit rapid growth.
The math
lb of N wanted ÷ %N on the bag. For 2 lb N from 30-10-0: 2 ÷ 0.30 = 6.7 lb of product per 1,000 sq ft. About one pint of dry fertilizer weighs a pound.
Organic or not
Synthetic fertilizers are available within days, so apply at planting. Organic fertilizers depend on warm soil microbes; apply them weeks to months ahead, and the slow ones (cottonseed, leaves, horse manure) the fall before.
No phosphorus on lawns. Washington has banned P-containing fertilizer on turf since 2011. Trees and shrubs rarely need P either. No late-season nitrogen on perennials: it delays winter hardening and invites cold damage, and leftover nitrate leaches into groundwater over the wet winter.
Salty soil makes plants wilt and brown as if dry, even when watered. On the west side the usual cause is deicing salt running off sidewalks and driveways; the guideline is under 0.5 dS/m on a soil test. Repeated heavy manure or manure-based compost can also add up.
Does water drain through this soil?
Yes
Leach it out
How much
To clear the top foot: 6 inches of clean water removes about half the salt, 12 inches about 80%, 24 inches about 90%.
When
Spread over several days, keeping soil moist so water keeps moving down. Western Washington winters often do this for you.
Why
Carries salts below the root zone. It also flushes nitrate, so don't fertilize right before.
No
Fix drainage first. Leaching only works when water moves through the soil instead of running off. See Water won't drain.
Prevent it
Deicer
Calcium magnesium acetate costs more but barely affects plants. Sand or dark grit helps too.
Layout
Curbs and raised planters steer salty runoff away from roots; salt-tolerant plants go in the splash zone.
Ch. 5, Salinity; Ch. 3, Soil Salinity
Possible contamination
Urban soils consistently carry more lead, arsenic, cadmium, copper, and nickel than undisturbed soil, from old lead paint, leaded gas, and treated wood. Most landscape plants don't mind; the concern is people, especially children and anyone eating what grows there.
What's the worry?
Heavy metals near an older house, or for a food garden or play area
Heavy metals test
Cost
About $50 per sample.
If high
Grow food in raised beds of clean imported soil, keep exposed soil covered with mulch or plants to cut dust, or replace the soil in annual beds.
Leftover herbicide (new development, ex-farmland, imported topsoil)
Ask first, test if needed
Cost
$200+ per sample, and the lab needs to know which herbicide class to look for.
If found
Wait for it to break down, remove the soil, or add activated carbon to reduce its activity. When buying topsoil, ask the supplier what weed treatment was used and its replant restriction.
Petroleum, pesticides, PCBs
Testing runs hundreds to thousands of dollars and needs a specific suspect. Badly contaminated beds are usually candidates for soil replacement.
Ch. 5, Soil Contamination and Soil Removal and Replacement
The amendment calendar
Western Washington timing, pulled together from the trees above. Slow materials go down early; fast ones go down when plants are hungry.
Shaded entries are my inference from the handbook's general rules, not timing the handbook states outright.
Legumes need an early start to cover the ground before cold. Fall results leave time to lime.
Oct–Nov
Lime or dolomitic lime. Slow organic fertilizers. Cereal rye cover crop. Winter mulch on beds with no cover crop.½ inch compost on established beds, left on top as that winter mulch.
Lime and slow organics need winter to react. Rye can still go in through October. Bare soil erodes and loses nutrients in winter rain.
Dec–Feb
Nothing. Stay off wet soil.
Nitrogen applied now just leaches. Working wet soil wrecks structure.
Mar–Apr
Turn under cover crops before they flower. Compost into new beds. Synthetic or fast organic N at planting. Sulfur for acid lovers if the test calls for it.
Cover crops turn woody and seed after flowering. Plants start drawing nutrients as soil warms.
May–Jul
Side-dress N for corn, potatoes, onions, and other heavy feeders. Mulch 3+ inches between rows.
Matches their rapid growth phase. Thick mulch holds water and smothers weeds.
Aug
Last N for vegetables only. Nothing for perennials, trees, or shrubs.
Late N on perennials delays hardening off for winter.
Any time
Core-aerate heavy-traffic lawn, then topdress with under ½ inch fine compost right after.
The compost fills the aeration holes and keeps them open.
The highlighted row is right now. If your soil test is pending, there's still time to get lime down before winter rains.
Ch. 3, Increasing Soil pH, Green Manure, Organic Mulch, Wet Soils; Ch. 4, Application Timing and Nutrients in Excess