Fish Compost: What It Is, How to Use It & Cost (PNW)

Fish compost is the premium tier of bulk soil amendment in the Pacific Northwest, and the most misunderstood of the major composts a home gardener might buy. Two things drive the confusion: first, "fish compost" is often conflated with "fish fertilizer" or "fish emulsion," which are entirely different products (one is a bulk soil amendment, the other is a liquid concentrate). Second, the price tag (more than double cow compost) makes a lot of buyers wonder if it is really worth it. This guide covers what bulk fish compost actually is, how it differs from liquid fish fertilizer, what extension testing shows about its performance, which plants justify the premium, and how to apply it for the result you want.

What is fish compost?

Bulk fish compost is a soil amendment produced by composting fish byproducts (in the Pacific Northwest, almost always salmon processing waste from regional fisheries) with carbon-rich material like bark, sawdust, or wood shavings. The composting process biologically converts a strong-smelling fish waste stream into a stable, dark, near-neutral soil amendment that looks and behaves like high-quality finished compost.

The production process matters more than the source ingredient. Properly produced bulk fish compost in the PNW:

• Uses an aerated static pile method for the active composting phase (28+ days), where forced air keeps the pile at optimal microbial-activity temperature
• Sits inside or behind a 3-foot biofilter of coarse woody debris that captures odor compounds during the active phase
• Is then cured for up to 2 years in open piles to stabilize pH, salts, and any residual volatile compounds
• Is typically Certified Organic by the Washington State Department of Agriculture before sale

The finished product is dark, crumbly, smells earthy (not fishy), and works into garden beds the same way any other bulk compost would. The bark or sawdust component also gives it a slightly fibrous texture that helps with soil structure, which is a real difference from pure cow or mushroom compost.

Fish compost vs fish fertilizer (fish emulsion, fish hydrolysate)

This is the single most common confusion in the fish-amendment market. Bulk fish compost and liquid fish fertilizer are different products that solve different problems.

The practical decision:

• Use bulk fish compost when you are building or amending a vegetable bed, raised bed, or new planting area and want to add organic matter plus slow-release nutrition.
• Use liquid fish fertilizer when you want to side-dress an actively growing crop, supplement a bed mid-season, or feed container plants between waterings.
• The two products are complementary, not competing. A premium vegetable bed will use bulk fish compost as the structural amendment and liquid fish fertilizer as a mid-season supplemental feed.

NPK, pH, salts, and what is actually in fish compost

Fish compost numbers vary by source, feedstock, and producer. Defensible ranges from PNW-published sources:

• Nitrogen (N): ~2 percent on a slow-release basis. The fish protein is the dominant nitrogen source, broken down by microbial activity over weeks rather than hours.
• Phosphorus (P) and Potassium (K): variable by batch. Bone material from fish processing contributes phosphorus and calcium; the bark/sawdust component dilutes the concentration.
• Calcium: elevated relative to plant-only composts because of fish bone. This is part of what makes fish compost particularly useful for tomato-family crops (calcium prevents blossom end rot).
• Micronutrients: magnesium, iron, zinc, and trace elements from fish tissue. Hard to quantify but real, and not always present in cow or mushroom compost.
• pH: 6.5 to 7.5, near neutral after the 2-year cure (same range as cow compost; slightly more acidic on the high end than mushroom compost).
• Soluble salts (EC): kept low by the long curing period. Manufacturers of fully cured product market it as safe for direct seedling contact, though general compost practice still recommends keeping any single application to 1 inch or less per year.

For exact numbers on a specific batch, ask the supplier for a recent compost analysis. Most Certified Organic producers publish or will share their lab reports.

Why fish compost commands a premium price

Fish compost is roughly twice the cost of cow compost at $113.99/yd vs $56.99/yd. Three factors drive the premium:

1. Constrained supply. In Washington, only one facility is permitted to produce bag-and-bulk fish compost at scale, which makes the regional supply a genuine bottleneck. Demand from PNW organic growers and high-end home gardeners is steady; production is not easily expanded.
2. Long, intensive processing. Aerated static-pile composting runs 28+ days under forced aeration with continuous biofilter management for odor. The cure cycle is then up to 2 years, far longer than the 90 to 180 days for cow manure compost or typical green-waste compost. That curing time is what makes the finished product odorless and seedling-safe, and it is what locks up the producer's working capital.
3. Certified Organic compliance. Washington State Department of Agriculture organic certification adds documentation, inspection, and process-control overhead that translates into per-yard cost.

The result is a product that genuinely does more for the soil per yard applied than cheaper composts, but at a price that pushes most general-purpose buyers toward cow or mushroom compost. Reserve fish compost for the projects that justify it.

What plants love fish compost?

Heavy-feeding crops and crops that benefit from the calcium-and-micronutrient profile respond strongest:

• Tomatoes. The signature use. Slow-release N for sustained vine growth, calcium for blossom-end-rot prevention, micronutrients for fruit development.
• Corn. Notorious nitrogen-hog crop. A fish compost amendment can reduce or eliminate need for supplemental fertilizer through the season.
• Brassicas (broccoli, cabbage, kale, brussels sprouts, cauliflower). Heavy nitrogen demand for leaf and head development.
• Squash and cucumbers. Sprawling vining crops with high overall nutrient demand.
• Peppers and eggplant. Calcium helps with fruit set; slow-release N keeps the plants productive.
• Leafy greens (lettuce, spinach, chard, arugula). Steady N delivery supports lush leaf production. The low salinity matters here.
• Fruit trees (non-acid-loving). Apples, pears, plums, cherries. A 1-inch ring out to the drip line each spring.
• Berries (non-acid-loving). Strawberries and raspberries (in moderation) appreciate the calcium and N. Avoid for blueberries (acid-loving).
• Roses. Heavy feeders that respond to the slow-release N and micronutrient profile.
• Lawns. A 1/2-inch topdress in spring or fall feeds the lawn and improves soil structure.

For comparisons across the compost lineup, see the mushroom compost guide and the cow manure compost guide.

Plants that don't like fish compost

The same near-neutral pH and high nitrogen that help vegetables hurt plants that need acidic, lean soil.

Avoid or limit fish compost for:

• Blueberries (require pH 4.5 to 5.5; excess N reduces fruiting)
• Rhododendrons, azaleas, camellias (require pH 5.0 to 6.5)
• Heathers and heaths (require pH 4.5 to 6.0)
• Gardenias (require pH 5.0 to 6.5)
• Most conifers (prefer slightly acidic soil; high N pushes soft, vulnerable growth)
• Native PNW ericaceous plants: salal, evergreen huckleberry, kinnikinnick
• Mediterranean herbs (lavender, rosemary, sage, thyme, oregano). These plants want lean, fast-draining soil. Heavy compost amendment encourages soft growth and root rot.
• Carnivorous plants (sphagnum-based acidic media only)

For acid-loving plants, use aged fine bark, conifer needle mulch, or a peat-based amendment instead. See the PNW soil pH testing guide for plant-by-plant pH ranges.

How to apply fish compost

Application rates draw from Oregon State Extension and PNW compost-use guidance. Fish compost is potent — keep annual application at 1 inch or less to avoid salt or nutrient buildup over time.

New vegetable bed (in-ground)

• Rate: Up to 20 percent fish compost by volume — about 3 to 4 inches worked into the top 8 to 12 inches of native soil
• Pair with: 2 to 3 inches of 3-Way Topsoil Mix ($32.99/yd) as a base layer if you are also raising the bed level
• See: best soil for vegetable gardens (in-ground) for the full planting-bed plan

Existing vegetable beds

• Rate: 1/2 to 1 inch annually as a topdress, lightly forked into the top 2 to 4 inches
• Timing: Spring before planting, or fall after harvest
• Do not exceed 1 inch in any single year. Even cured fish compost can build up salts over multiple heavy applications.

Raised beds (new fill)

• Rate: 20 to 25 percent of total fill volume
• Recipe for a new 4 by 8 by 1.5 ft raised bed (1.78 yards total):
  • 55 to 60 percent 3-Way Topsoil Mix ($32.99/yd) as the soil base
  • 20 to 25 percent Fish Compost ($113.99/yd) for premium fertility and micronutrients
  • 10 to 15 percent aged bark fines or coarse Washed Sand ($29.99/yd) for drainage
• Lower-cost alternative: use 25 to 30 percent Cow Compost ($56.99/yd) instead. The bed will perform well; fish compost is the upgrade for tomato-focused beds.
• See: raised bed soil mix recipe for the full breakdown

Tomato beds specifically

• Pre-plant: 1 inch of fish compost worked into the top 6 to 8 inches of the bed in early spring
• At planting: 1 to 2 cups of fish compost mixed into each planting hole
• Mid-season: 1/2-inch topdress around the base of each plant when fruits begin to set
• Why it works: the calcium content directly addresses blossom-end rot risk; slow-release N keeps vines productive through a long Pacific Northwest growing season

Lawn topdressing

• Rate: 1/2 inch thinly spread, raked to soil level
• Timing: Spring or fall. Best after a core aeration.
• Note: for general lawn care, cow compost ($56.99/yd) or mushroom compost ($74.99/yd) is the more economical choice. Reserve fish compost for tired or specialty lawns where the micronutrient profile justifies the upgrade.

Fruit trees, perennial borders, roses

• Rate: 1-inch ring out to the drip line each spring (trees) or 1-inch topdress (borders, rose beds)
• Keep clear: pull the compost ring 4 to 6 inches back from tree trunks to avoid bark rot

Cost and coverage

At Harbor Soils, Fish Compost is $113.99 per yard at pickup. Delivery is a separate flat fee based on distance from our Gig Harbor yard.

Coverage per yard:

• 324 sq ft at 1 inch deep
• 162 sq ft at 2 inches deep
• 108 sq ft at 3 inches deep

Worked examples:

• 200 sq ft vegetable bed, 1-inch topdress: 0.62 yards of fish compost. At $113.99/yd, about $71 in product before delivery.
• Two new 4 x 8 raised beds with 20% fish compost (~0.7 yards total fish compost): about $80 in fish compost plus topsoil base and drainage layer.
• Premium tomato bed (8 x 20, 1-inch amendment plus planting-hole boost): about 0.6 yards of fish compost, ~$68 in product. Compare to expected yield uplift over a season.

Use the compost calculator and buying guide to size your specific project.

Fish compost vs cow vs mushroom: which to buy

Decision rule:

• Default to cow compost for most vegetable gardens — gets you most of the benefit at the lowest price.
• Upgrade to mushroom compost for tomato beds, perennial borders, lawn topdressing, or when you want maximum uniformity and reliable weed-free product.
• Upgrade to fish compost for tomato-focused beds, intensive vegetable plots, when calcium-and-micronutrient profile matters, or when you specifically need a Certified Organic amendment.

Frequently asked questions about fish compost

What is fish compost?
Bulk soil amendment made by composting fish byproducts (PNW: salmon processing waste) with bark and sawdust. Aerated static pile composting (28+ days) plus up to 2 years of curing produces a stable, virtually odorless, near-neutral amendment.

Is fish compost the same as fish fertilizer or fish emulsion?
No. Fish compost is a bulk soil amendment sold by the yard. Fish emulsion and fish hydrolysate are concentrated liquid fertilizers sold by the bottle. Different products, different uses, complementary in a serious garden.

What is the NPK of fish compost?
About 2 percent nitrogen on a slow-release basis, with variable P and K depending on source. Concentration is low compared to synthetic fertilizers but the nutrient release is steady over the growing season.

What is the pH of fish compost?
6.5 to 7.5, near neutral after the 2-year cure.

Does fish compost smell?
Properly cured fish compost is virtually odorless. Strong odor is controlled during the active composting phase by aerated static piles and a 3-foot biofilter of woody debris, and the 2-year cure stabilizes all volatile compounds.

What plants love fish compost?
Tomatoes (signature use), corn, brassicas, squash, peppers, leafy greens, fruit trees, roses, and lawns. Best for heavy-feeding and calcium-loving crops.

What plants should not get fish compost?
Blueberries, rhododendrons, azaleas, camellias, conifers, native PNW ericaceous plants, and Mediterranean herbs. Use a slightly acidic amendment for acid-loving plants.

How much fish compost should I apply?
New beds: 20 percent by volume. Existing beds: 1/2 to 1 inch annually max. Raised beds: 20 to 25 percent of mix. Lawns: 1/2-inch topdress.

Why is fish compost more expensive than other composts?
Constrained PNW supply (only one WA facility permitted for bulk production), long 2-year curing cycle, and Certified Organic compliance costs. At Harbor Soils, $113.99/yd vs $74.99 mushroom and $56.99 cow.

Is fish compost worth the higher price?
For tomato-focused beds, intensive vegetable plots, and organic-certified projects, yes. For general garden use, cow or mushroom compost does the same basic job for less. Reserve fish compost for projects where the calcium and micronutrient profile justifies the upgrade.

Related compost and soil guides

• Mushroom Compost: What It Is, How to Use It & Cost (PNW)
• Cow Manure Compost: What It Is, How to Use It & Cost (PNW)
• Peat Moss vs Compost: Which Is Better for Your Garden?
• Best Soil for Vegetable Gardens (In-Ground)
• Raised Bed Soil Mix: The Best Recipe
• How Much Compost Do I Need? Calculator & Buying Guide
• How to Improve Clay Soil
• PNW Soil pH Testing Guide

Order Fish Compost from Harbor Soils. $113.99/yard pickup, with delivery throughout Gig Harbor, Port Orchard, Bremerton, Silverdale, Poulsbo, Bainbridge Island, and the rest of Kitsap County. Most weekday orders deliver same-day or next-day. Order Fish Compost online or call 253-857-5125.