Understanding Fertilizer for Pacific Northwest Gardens
Fertilizer provides the nutrients plants need to grow, flower, and fruit. While healthy soil supplies much of what plants require, most gardens benefit from supplemental feeding—especially in the Pacific Northwest, where heavy winter rains leach nutrients from soil.
Understanding what fertilizers contain and how to use them helps you feed plants effectively without wasting money or harming the environment.
The N-P-K Numbers Explained
Every fertilizer displays three numbers—the N-P-K ratio—representing the percentage by weight of three primary nutrients:
Nitrogen (N) – First Number
Drives leafy, green growth. Essential for all plants but especially important for leafy vegetables, lawns, and plants in active growth phase.
- Deficiency signs: Pale green or yellow leaves, stunted growth, oldest leaves affected first
- Excess signs: Lush foliage but few flowers or fruits, weak stems, susceptibility to pests
- High-nitrogen sources: Blood meal, fish meal, composted manure, grass clippings
Phosphorus (P) – Second Number
Promotes root development, flowering, and fruit production. Critical for transplants, blooming plants, and fruiting vegetables.
- Deficiency signs: Purple-tinged leaves, poor flowering, weak roots, slow growth
- Excess signs: Can interfere with uptake of other nutrients; avoid over-application
- High-phosphorus sources: Bone meal, rock phosphite, composted manure
Potassium (K) – Third Number
Supports overall plant health, disease resistance, and fruit quality. Important for all plants, especially those under stress.
- Deficiency signs: Brown leaf edges, weak stems, poor fruit quality
- Excess signs: Rarely problematic, but can interfere with magnesium uptake
- High-potassium sources: Kelp meal, wood ash, greensand
Reading Fertilizer Labels
A fertilizer labeled 10-5-5 contains 10% nitrogen, 5% phosphorus, and 5% potassium by weight. The remaining 80% is carrier material. Higher numbers mean more concentrated fertilizer.
Types of Fertilizers
Organic Fertilizers
Derived from plant, animal, or mineral sources. Release nutrients slowly as soil organisms break them down.
Advantages:
- Feed soil life as well as plants
- Release nutrients slowly over time
- Improve soil structure
- Low risk of burning plants
- Sustainable and environmentally friendly
Common organic fertilizers:
- Blood meal (12-0-0): Fast-acting nitrogen source
- Bone meal (3-15-0): Slow-release phosphorus for roots and flowers
- Kelp meal (1-0-2): Trace minerals and growth hormones
- Fish meal (10-6-2): Balanced nutrition with micronutrients
- Composted manure (varies): Balanced nutrition plus organic matter
- Alfalfa meal (3-1-2): Gentle, balanced feeding
Synthetic Fertilizers
Manufactured chemical compounds that deliver nutrients directly to plants in immediately available form.
Advantages:
- Precise nutrient content
- Fast-acting
- Inexpensive per unit of nutrient
- Easy to apply
Disadvantages:
- Don’t improve soil structure or feed soil life
- Easy to over-apply and burn plants
- Nutrients can leach into groundwater
- Salt buildup with repeated use
Liquid vs. Granular
Liquid fertilizers: Fast-acting, ideal for container plants and quick feeding. Mix with water and apply when watering. Good for foliar feeding.
Granular fertilizers: Longer-lasting, applied to soil surface and watered in. More economical for large areas. Some are slow-release for extended feeding.
When and How to Fertilize
Spring
As plants break dormancy and begin active growth, they need nutrients. Apply balanced fertilizer to established plantings and prepare beds for new plantings.
- Work compost or composted manure into vegetable beds
- Feed perennials as new growth emerges
- Apply slow-release fertilizer to trees and shrubs
Growing Season
Feed actively growing and fruiting plants throughout summer:
- Side-dress vegetables with compost or granular fertilizer monthly
- Feed container plants weekly with liquid fertilizer
- Reduce nitrogen as plants begin fruiting (excess nitrogen delays ripening)
Fall
In the Pacific Northwest, stop fertilizing most plants by September. Late fertilization stimulates tender growth that won’t harden before frost.
- Exception: Fall-planted garlic and cover crops benefit from compost application
- Add compost to beds being put to rest for winter
Fertilizing Specific Crops
Tomatoes
Heavy feeders that need consistent nutrition:
- Add compost and balanced fertilizer at planting
- Side-dress with compost when first fruits set
- Reduce nitrogen once plants are fruiting—too much promotes leaves over fruit
- Watch for blossom end rot (calcium deficiency)—often caused by inconsistent watering rather than lack of calcium
Leafy Greens
Nitrogen lovers that benefit from regular feeding:
- Work nitrogen-rich amendments into bed before planting
- Side-dress with fish emulsion or blood meal monthly
- Consistent moisture helps nutrient uptake
Root Vegetables
Need phosphorus and potassium more than nitrogen:
- Avoid fresh manure or high nitrogen (causes forking and excessive tops)
- Add bone meal at planting for phosphorus
- Grow in soil that was heavily composted the previous year
Beans and Peas
Legumes fix their own nitrogen from air:
- Don’t add nitrogen fertilizer—it suppresses nitrogen fixation
- Inoculate seeds with rhizobia bacteria for best nitrogen fixation
- Light phosphorus application at planting supports root development
Squash and Cucumbers
Very heavy feeders:
- Incorporate plenty of compost before planting
- Side-dress monthly with balanced fertilizer
- Liquid fertilizer gives quick boost if plants look hungry
Pacific Northwest Soil Considerations
Acidic Soil
Our rainfall naturally acidifies soil. Most Pacific Northwest soils test acidic (pH below 7.0). While many plants tolerate slight acidity, vegetables generally prefer pH 6.0-7.0. Add lime to raise pH if needed based on soil test results.
Nutrient Leaching
Heavy winter rains wash water-soluble nutrients out of soil. This makes spring fertilization particularly important and favors slow-release organic fertilizers that resist leaching.
Micronutrient Deficiencies
Some Pacific Northwest soils lack micronutrients, particularly:
- Boron: Important for brassicas and root crops
- Magnesium: Often deficient in acid soils
- Calcium: Can be low in very acid conditions
Kelp meal and complete organic fertilizers provide trace elements often missing from simple N-P-K formulations.
Compost: The Complete Fertilizer
Well-made compost provides balanced nutrition, improves soil structure, and feeds beneficial soil organisms. It’s the foundation of organic fertilization.
- Nutrient content: Varies but generally around 1-1-1; lower than concentrated fertilizers but more complete
- Application rate: 1-2 inches worked into soil annually; can’t over-apply
- Benefits beyond nutrition: Improves water retention, drainage, and soil biology
Many Pacific Northwest gardeners find that heavy composting alone provides adequate fertility for most crops, with targeted fertilizers used only for heavy feeders.
Signs Your Plants Need Fertilizer
- Pale green or yellow leaves (nitrogen deficiency)
- Purplish coloration, especially on leaf undersides (phosphorus deficiency)
- Brown leaf edges (potassium deficiency)
- Stunted growth despite adequate water
- Poor flowering or fruiting
- Older leaves affected before newer ones (mobile nutrient deficiency)
Avoiding Fertilizer Problems
Over-Fertilization
More isn’t better. Excess fertilizer:
- Burns roots and leaves
- Creates salt buildup in soil
- Promotes weak, pest-susceptible growth
- Pollutes groundwater
Best Practices
- Start with a soil test to know what your soil actually needs
- Follow package directions—measure rather than guess
- Use organic fertilizers for gentler, longer-lasting feeding
- Feed plants during active growth, not dormancy
- Water before and after applying granular fertilizers
- Build soil health with compost first, then supplement as needed
Understanding N-P-K numbers and fertilizer basics empowers you to feed your plants effectively. Combined with compost-building and attention to Pacific Northwest soil conditions, proper fertilization helps your garden reach its full potential.
