Organic Fertilizer Guide: What to Use and When for Vegetables, Flowers, and Lawns

Understanding NPK Numbers

Every fertilizer label displays three numbers separated by dashes: nitrogen (N), phosphorus (P), and potassium (K). These represent the percentage of each nutrient by weight. A 50-pound bag of 5-3-3 fish meal contains 2.5 pounds of nitrogen, 1.5 pounds of phosphorus, and 1.5 pounds of potassium. The remaining 44.5 pounds is organic matter and filler.

Nitrogen (N): Leaf and Stem Growth

Nitrogen drives vegetative growth. Leafy crops (lettuce, spinach, kale, cabbage) need the most nitrogen. Fruit-producing crops (tomatoes, peppers, squash) need moderate nitrogen and more phosphorus and potassium. Lawns need steady nitrogen throughout the growing season. Nitrogen deficiency appears as yellowing of older leaves (chlorosis) and stunted growth. Excess nitrogen produces lush, soft foliage that is susceptible to disease and pest damage, and delays flowering and fruiting in tomatoes and peppers.

Phosphorus (P): Roots, Flowers, and Fruit

Phosphorus supports root development, flower formation, and fruit production. It is most critical at planting time and during the flowering and fruiting stages. Phosphorus does not move readily in soil. It stays near where it is applied, which is why it should be incorporated into the root zone at planting, not surface-applied as a side-dress. Phosphorus deficiency appears as purplish discoloration of leaves and poor flowering. Most garden soils have adequate phosphorus from years of previous applications. A soil test is the only reliable way to determine if additional phosphorus is needed.

Potassium (K): Disease Resistance and Overall Vigor

Potassium regulates water movement within the plant, activates enzymes involved in photosynthesis, and strengthens cell walls. Adequate potassium improves disease resistance, drought tolerance, and winter hardiness. Potassium deficiency appears as brown scorching on leaf edges and weak stems. Sandy soils are most prone to potassium deficiency because potassium leaches more readily than phosphorus but less readily than nitrogen.

12 Organic Fertilizer Sources: NPK, Cost, and Application

1. Compost

NPK: 1-1-1 (variable). Cost: free (home-made) to $30 to $50 per cubic yard (bulk). Compost is the foundation of organic fertility. It adds humus, improves soil structure, increases water-holding capacity by 20 to 30 percent, and provides a broad spectrum of macro- and micronutrients. Apply 1 to 2 inches as a top-dress around established plants in spring and fall. Work 2 to 3 inches into new planting beds before planting. Compost releases nutrients slowly over 3 to 6 months. A soil test every 2 to 3 years tells you whether compost alone is providing sufficient nutrients or if targeted amendments are needed.

2. Blood Meal

NPK: 12-0-0. Cost: $15 to $20 per 5-pound bag. Blood meal is dried, powdered slaughterhouse blood. It provides fast-acting nitrogen for leafy crops and lawns. Apply at 1 to 2 pounds per 100 square feet. Blood meal releases nitrogen over 4 to 6 weeks. It can burn plants if over-applied. Do not exceed 3 pounds per 100 square feet in a single application. Blood meal also repels rabbits and deer when sprinkled around the garden perimeter. Store in a sealed container; blood meal attracts flies and rodents if left exposed.

3. Bone Meal

NPK: 3-15-0. Cost: $10 to $15 per 5-pound bag. Bone meal is steamed and ground animal bones. It provides phosphorus and calcium for root development, flowering, and fruiting. Apply at 1 to 2 pounds per 100 square feet, mixed into the top 4 to 6 inches of soil at planting time. Bone meal releases phosphorus over 3 to 4 months. It is most effective in soils with a pH below 7.0. In alkaline soils (pH above 7.5), phosphorus from bone meal becomes locked in calcium phosphate compounds that plants cannot absorb. Add 1 tablespoon per transplant hole for tomatoes, peppers, and eggplant.

4. Fish Meal

NPK: 5-3-3. Cost: $18 to $25 per 5-pound bag. Fish meal is dried, ground fish waste. It provides a balanced NPK profile plus trace minerals. Apply at 2 to 3 pounds per 100 square feet. Fish meal releases nutrients over 4 to 8 weeks. It has a strong fishy odor that dissipates within 3 to 5 days after application. Work it into the soil to reduce odor and prevent animals from digging. Fish meal is an excellent all-purpose fertilizer for vegetable gardens and flower beds.

5. Fish Emulsion

NPK: 5-1-1. Cost: $12 to $18 per quart (liquid). Fish emulsion is a liquid fish fertilizer that provides fast-acting nitrogen. Dilute 2 to 3 tablespoons per gallon of water and apply as a soil drench every 2 to 3 weeks during the growing season. Fish emulsion also works as a foliar spray at half strength (1 tablespoon per gallon) for quick nitrogen uptake through leaves. The odor is strong for 24 to 48 hours after application. Apply in the evening to reduce odor nuisance and avoid leaf burn from midday sun.

6. Alfalfa Meal

NPK: 2-1-2. Cost: $12 to $18 per 5-pound bag. Alfalfa meal is ground dried alfalfa plants. It provides nitrogen, phosphorus, potassium, and triacontanol, a natural growth hormone that stimulates root development and increases yields by 10 to 20 percent in university trials. Apply at 2 to 3 pounds per 100 square feet. Alfalfa meal releases nutrients over 2 to 3 months. It also feeds soil microbes, increasing earthworm populations by 30 to 50 percent within 6 months of regular application. Alfalfa meal is the standard organic fertilizer for roses and perennials.

7. Cottonseed Meal

NPK: 6-2-1. Cost: $10 to $15 per 5-pound bag. Cottonseed meal is a byproduct of cotton processing. It provides nitrogen and acidifies the soil slightly (lowers pH by 0.2 to 0.3 units). Apply at 2 to 3 pounds per 100 square feet. Cottonseed meal releases nitrogen over 2 to 3 months. It is a good choice for acid-loving plants like blueberries, azaleas, and rhododendrons. Purchase organic-certified cottonseed meal to avoid pesticide residues from conventionally grown cotton.

8. Kelp Meal

NPK: 1-0.2-2. Cost: $15 to $22 per 5-pound bag. Kelp meal is dried, ground seaweed. It provides potassium, trace minerals (over 60 micronutrients including iron, zinc, manganese, and boron), and natural plant growth hormones (cytokinins and auxins). Apply at 1 to 2 pounds per 100 square feet. Kelp meal does not provide significant nitrogen or phosphorus. Use it as a supplement alongside nitrogen and phosphorus sources. Foliar spray of liquid kelp (1 tablespoon per gallon) applied every 2 to 3 weeks improves plant stress tolerance to heat, cold, and drought by 15 to 25 percent.

9. Worm Castings

NPK: 1-1-1. Cost: $20 to $30 per 10-pound bag. Worm castings are the excrement of earthworms fed organic matter. They provide a balanced nutrient profile, humic acids, and beneficial microorganisms. Apply 0.5 to 1 inch as a top-dress around established plants, or mix 10 to 20 percent by volume into potting mixes. Worm castings improve soil structure, water retention, and nutrient availability. They also contain chitinase, an enzyme that helps control fungal diseases and nematode populations. University trials show a 10 to 20 percent yield increase in vegetables grown with 15 percent worm castings by volume in the soil mix.

10. Greensand

NPK: 0-0-3. Cost: $12 to $18 per 5-pound bag. Greensand is a mined mineral (glauconite) from ancient seabed deposits. It provides potassium (3 percent) and iron (5 percent), along with 30 other trace minerals. Greensand releases potassium slowly over 3 to 5 years, making it a long-term soil amendment rather than a quick fertilizer. Apply at 5 to 10 pounds per 100 square feet. It also improves soil structure in clay soils by increasing pore space. Greensand is most valuable in sandy soils that leach potassium quickly.

11. Rock Phosphate

NPK: 0-3-0. Cost: $10 to $15 per 5-pound bag. Rock phosphate is mined phosphate rock ground to a fine powder. It provides phosphorus and calcium. Apply at 5 to 10 pounds per 100 square feet, mixed into the top 6 inches of soil. Rock phosphate releases phosphorus very slowly over 3 to 5 years. It is most effective in acidic soils (pH below 6.5) where the phosphorus is more soluble. In alkaline soils, use bone meal instead. Rock phosphate is a long-term investment in soil phosphorus reserves, not a quick fix for deficiency.

12. Soybean Meal

NPK: 7-2-1. Cost: $12 to $16 per 5-pound bag. Soybean meal is ground soybeans from which the oil has been extracted. It provides a moderate nitrogen release over 2 to 3 months. Apply at 2 to 3 pounds per 100 square feet. Soybean meal is an economical nitrogen source for vegetable gardens and lawns. It feeds soil microbes as it decomposes, improving soil biology. Purchase organic-certified soybean meal to avoid genetically modified organisms and pesticide residues.

Feeding Schedules by Crop Type

Leafy Greens (Lettuce, Spinach, Kale, Swiss Chard)

Leafy greens need nitrogen more than any other nutrient. Apply 2 pounds of blood meal (12-0-0) per 100 square feet at planting. Side-dress with 1 pound of blood meal per 100 square feet 3 weeks after planting. For a sustained release approach, apply 3 pounds of soybean meal (7-2-1) per 100 square feet at planting and leave it. The nitrogen releases over 2 to 3 months, covering the entire growth cycle of most greens (30 to 60 days from transplant to harvest). Avoid excess nitrogen on spinach, which can accumulate unsafe levels of nitrates in the leaves.

Fruiting Vegetables (Tomatoes, Peppers, Eggplant)

These crops need balanced nutrition with emphasis on phosphorus and potassium for flower and fruit production. At planting, add 0.5 cup of bone meal (3-15-0) and 0.25 cup of kelp meal (1-0.2-2) to each transplant hole. Side-dress with 1 cup of compost per plant when the first flowers appear. Side-dress again with 0.5 cup of alfalfa meal (2-1-2) per plant when the first fruits set. Avoid excess nitrogen after flowering begins. High nitrogen produces lush foliage at the expense of fruit. If leaves are deep green and fruit production is slow, reduce nitrogen and increase potassium with a kelp meal application.

Root Vegetables (Carrots, Beets, Potatoes, Onions)

Root crops need phosphorus for root development and potassium for overall vigor. Excess nitrogen causes forked carrots and excessive foliage at the expense of root development. Apply 2 pounds of bone meal (3-15-0) and 2 pounds of greensand (0-0-3) per 100 square feet at planting. Work these into the top 6 inches of soil. Do not side-dress with nitrogen. If foliage appears pale, apply 1 pound of compost per 10 linear feet of row as a mild nitrogen source. Potatoes benefit from an additional 2 pounds of kelp meal per 100 square feet at hilling time to support tuber development.

Legumes (Beans, Peas)

Legumes fix their own nitrogen through a symbiotic relationship with Rhizobium bacteria in their root nodules. Do not apply nitrogen fertilizer to legumes. Excess nitrogen inhibits nodulation and reduces the plant's ability to fix nitrogen. Apply 2 pounds of bone meal (3-15-0) per 100 square feet at planting for phosphorus. If the soil is deficient in potassium, apply 2 pounds of greensand (0-0-3) per 100 square feet. Inoculate bean and pea seeds with Rhizobium inoculant ($5 per packet, treats 8 pounds of seed) at planting time if you are planting in a new garden area where legumes have not been grown before.

Flower Beds (Perennials and Annuals)

Apply 2 to 3 inches of compost as a top-dress in early spring before new growth starts. For heavy-feeding annuals (zinnias, dahlias, sunflowers), side-dress with 1 pound of fish meal (5-3-3) per 100 square feet at planting and again at mid-season. For perennials, apply 2 pounds of alfalfa meal (2-1-2) per 100 square feet in early spring. Alfalfa meal's triacontanol stimulates root growth and increases flower production. For roses, apply 1 cup of alfalfa meal per bush in early spring, 0.5 cup in mid-June, and 0.5 cup in mid-July. Stop all fertilization 6 weeks before the first expected frost.

Lawn

Apply 10 pounds of soybean meal (7-2-1) per 1,000 square feet in early spring (April), early summer (June), and early fall (September). Soybean meal provides a steady nitrogen release that avoids the rapid growth flush of synthetic lawn fertilizers. This reduces mowing frequency by 20 to 30 percent. In fall, replace the soybean meal with 10 pounds of alfalfa meal (2-1-2) per 1,000 square feet to promote root development without pushing top growth before winter. Apply lime only if a soil test indicates the pH is below 6.0. Most lawns need lime every 3 to 5 years at 40 to 50 pounds per 1,000 square feet.

Quick Reference: Which Fertilizer for Which Problem

Timing Rules That Prevent Waste and Plant Damage

Apply organic fertilizers 2 to 4 weeks before the plant needs the nutrients. Microbes need time to break down the organic matter and release the nutrients in plant-available forms. If you wait until deficiency symptoms appear, the correction takes 2 to 4 weeks to take effect, during which the plant has already suffered reduced growth or yield.

Do not fertilize when the soil is saturated or frozen. Saturated soil cannot absorb nutrients, and fertilizer applied to frozen soil runs off when the ground thaws. Apply fertilizer when the soil is moist but not wet, and when soil temperature is above 50 degrees Fahrenheit. Below 50 degrees, microbial activity slows to the point where nutrient release from organic sources nearly stops.

Stop fertilizing 6 to 8 weeks before the first expected frost date. Late-season nitrogen pushes new growth that will be killed by frost, weakening the plant and reducing winter survival. In zone 5, the cutoff is August 15. In zone 6, September 1. In zones 7 and 8, September 15. Fall applications should be limited to phosphorus and potassium sources (bone meal, greensand, rock phosphate) that support root development without stimulating top growth.