Organic Fertilizers and Soil Amendments -- Harvest to Table

Organic fertilizers and organic soil amendments come from natural sources–plants, animals, and rocks.

Organic fertilizer is a natural soil amendment that adds plant nutrients to the soil, most often nitrogen, phosphorous, or potassium. (NPK analysis is the concentration of major plant nutrients–nitrogen (N), phosphorous (P), and potassium (K)–in fertilizer as a percentage of the whole.)

Soil amendments are used to condition or improve the soil. Most, but not all, soil amendments add some nutrients to the soil.

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Common organic fertilizers and soil amendments

Here is a guide to 35 commonly use organic fertilizers and soil amendments:

Alfalfa meal

Description: Organic matter made from alfalfa.
Benefit: Adds nitrogen, phosphorus, and potassium; primarily used to increase organic matter.
Average NPK analysis: 2-1-2
Application and amount for adequately fertile soil: Till in 2 to 5 pounds per 100 square feet. Release time is 1 to 4 months.
Comments: Contains triacontanol a naturally fatty-acid growth stimulant, and also trace elements. May contain seeds. Available at feed stores.

Bat guano

Description: Bat guano (feces) is harvested from caves and powdered.
Benefit: Adds nitrogen and phosphorus; stimulates soil microbes.
Average NPK analysis: 10-3-1 processed for nitrogen; 3-10-1 processed for phosphorus
Application and amount for adequately fertile soil: Till in 5 pounds per 100 square feet or add 3 teaspoons per gallon of water to make tea.
Comments: Apply directly to the soil or make as a tea for side dressing or foliar spray.

Blended fertilizer

Description: Blend of organic fertilizers; may include plant animal and mineral components.
Benefit: Boosts nutrients overall when soil fertility is low.
Average NPK analysis: Varies widely according to blend.
Application and amount for adequately fertile soil: depend upon blend; add to soil or use as a side dressing.
Comments: Commercial blends are convenient. Mixing your own is less expensive.

Blood meal

• Description: Blood meal is made from dried slaughterhouse waste.
Benefit: Adds nitrogen.
Average NPK analysis: 12-0-0
Application and amount for adequately fertile soil: Where the soil is low in nitrogen, till in 2 to 5 per 100 square feet. Add 1 pound of bloodmeal for soil with adequate nitrogen. Keep bloodmeal a few inches away from plant stems; it can burn plants. Release time is 1 to 4 months.
Comments: Do not over-apply; the excess ammonia can burn plants. Available at garden centers and feed stores.

Bonemeal

Description: Steam-processed bone, a slaughterhouse by-product.
Benefit: Adds phosphorus.
Average NPK analysis: Varies from about 1-11-0 to about 3-15-0, and also contains about 24 percent calcium.
Application and amount for adequately fertile soil: Till or rake in 3 pounds per 100 square feet for soil low in phosphorus; add 2 pounds per 100 square feet for soil with average fertility. Release time 6 to 12 months.
Comments: Fine ground bonemeal makes phosphorus more quickly available to plants. The calcium in bonemeal will raise the soil’s pH. Available at garden centers and feed stores. Bonemeal is more expensive than rock phosphate.

Coffee grounds

Description: Used coffee grounds.
Benefit: Adds nitrogen, phosphorus, and potassium, also contains magnesium and copper.
Average NPK analysis: 2-0.3-0.6
Application and amount for adequately fertile soil: Till or sprinkle on the soil; 5 to 10 pounds per 100 square feet. Do not add more than 25 percent coffee grounds to a planting bed.
Comments: Coffee grounds are slightly or moderately acidic and can be balanced in the soil by adding a limestone supplement. Also, compost coffee grounds as a nitrogen addition to your compost pile.

Colloidal rock phosphate

Description: A by-product of mining rock phosphate.
Benefit: Adds phosphorus.
Average NPK analysis: 0-2-0 (18-20 percent phosphorous, 23 percent calcium).
Application and amount for adequately fertile soil: Add 5 to 6 pounds per 100 square feet in low phosphorus soils; 2 to 2½ pounds in average soils.
Comments: Very slow release over a period of two years. Available at garden centers.

Compost

Description: Decayed plant and organic materials.
Benefit: Adds organic matter and nutrients to the soil.
Average NPK analysis: 0.5-0.5-0.5 to 4-4-4; about (25 percent organic matter).
Application and amount for adequately fertile soil: Spread finished compost across the planting bed 2 to 3 inches thick twice a year.
Comments: Unfinished compost should be added to the garden in the fall so that it has time to further decompose. Aged compost can be added at any time during the year. Check commercial compost to make sure it does not contain seed or pesticide residue. Compost that smells like ammonia is not completely decayed or finished.

Cottonseed meal

Description: Processed from cotton seeds.
Benefit: Adds nitrogen, also potassium.
Average NPK analysis: 6-0.4-1.5
Application and amount for adequately fertile soil: Till in 10 pounds per 100 square feet
Comments: May contain pesticide residues. Release time 1 to 4 months.

Cow manure

Description: Cow and steer manure.
Benefit: Adds nitrogen and organic matter and some other nutrients to the soil (about 17 percent organic matter).
Average NPK analysis: 0.6-0.2-0.5
Application and amount for adequately fertile soil: Add 2 to 4 inches of manure to planting beds as sheet compost in the fall; turn the manure into the soil. Add fully composted manure as a side dressing during the growing season.
Comments: Use dehydrated manure. Cow manure can be added to the soil or to the compost pile. Manure may contain pesticides and residue of livestock medications.

Eggshells

Description: Crushed eggshells.
Benefit: Adds calcium to the soil.
Average NPK analysis: About 1.2-0.4-0.1.
Application and amount for adequately fertile soil: Apply 2 pounds per 100 square feet.
Comments: Contains calcium plus trace minerals.

Epsom salts

Description: Magnesium sulfate is a chemical compound.
Benefit: Soil balancer, magnesium sulfate is an essential element in the chlorophyll molecule.
Average analysis: 10 percent magnesium, 13 percent sulfur.
Application and amount for adequately fertile soil: Use 1 tablespoon of Epsom salt in a gallon of water as a foliar spray; 1 pound per 1,000 square feet.
Comments: Epsom salt is highly soluble and is not persistent and will not build up in the soil.

Feather meal

Description: Processed from poultry slaughter.
Benefit: Adds nitrogen.
Average analysis: Nitrogen 7 to 12 percent.
Application and amount for adequately fertile soil: 2.5 to 5 pounds per 100 square feet. Slow release, more than 4 months.
Comments: Slow-release fertilizer.

Fish emulsion

Description: Liquid fertilizer processed from fish waste.
Benefit: Nitrogen-rich and adds micronutrients to the soil.
Average NPK analysis: 5-2-2, also contains sulfur.
Application and amount for adequately fertile soil: Add 6 tablespoons per gallon of water. Release time 1 to 4 months.
Comments: Can be foul-smelling. Purchase fish emulsion at a garden center.

Fish meal

Description: Ground and heat-dried fish waste.
Benefit: Adds nitrogen, which can also be a source of phosphorus and potassium.
Average NPK analysis: Ranges from about 4-2-2 to 9-7-0.
Application and amount for adequately fertile soil: Till in 3 pounds per 100 square feet where the soil is poor; add 2 pounds per 100 square feet where the soil has moderate fertility. Release time 1 to 4 months. Also contains many trace elements
Comments: Fish meal can be placed directly in a planting hole early in the season. Purchase fish meal at a garden center or fish-processing plant.

Granite meal

Description: Rock powder mined from granite quarries.
Benefit: Adds potassium, also trace minerals.
Average analysis: 1-4 percent total potash; about 67 percent silica.
Application and amount for adequately fertile soil: Broadcast 10 pounds per 100 square feet to soil low in potassium; apply 5 pounds for soil with average fertility.
Comments: Contains 67 percent silicas and 19 trace minerals. Purchase granite meal at garden centers.

Grass clippings (green)

Description: Lawn clippings.
Benefit: Grass clippings add organic matter and nitrogen to the soil; they stimulate worms and soil microorganisms.
Average NPK analysis: 0.5-0.2-0.5
Application and amount for adequately fertile soil: Add 50 pounds per 100 square feet for soil low in nitrogen; add 30 pounds for soil with average fertility.
Comments: Do not use grass clippings from lawns that have been treated with herbicides or pesticides.

Greensand

Description: Also called glauconite; greensand comes from a 70 million-year-old marine deposit mined in New Jersey. It is a greenish-colored sand-based mineral.
Benefit: Adds potassium and micronutrients.
Average NPK analysis: 0-0-1 (the total potassium may be up to 7 percent but the release is very slow).
Application: Broadcast in fall; apply 5 to 10 pounds per 100 square feet if the soil is low in potassium; use 3 pounds per 100 square feet in average soils. Greensand releases potassium very, very slowly over a 10-year period. Slow release of 7 percent potash plus 32 trace minerals. Greensand helps bind particles of sandy soil and loosens clay soils.

Gypsum

Description: Calcium sulfate powder, mined or as a by-product of mining.
Benefit: Soil balancer; provides the soil with calcium and sulfur.
Average analysis: 22 percent calcium, 17 percent sulfur
Application and amount for adequately fertile soil: Broadcast 4 pounds per 100 square feet on soil that is low in calcium; apply 2 pounds per 100 square feet on soil with average fertility; add ½ pound on fertile soil.
Comments: Use gypsum to correct a calcium deficiency. Gypsum has little effect on the soil pH. Do not apply gypsum if the pH is below 5.8. Gypsum will loosen tight, clay soil; it will neutralize too much sodium and magnesium in the soil.

Horse manure

Description: Solid waste from horses and ponies.
Benefit: Adds organic matter and small amounts of nitrogen, phosphorous, and potassium.
Average NPK analysis: Analysis varies, about 0.7-0.3-0.6
Application and amount for adequately fertile soil: Spread 2 to 4 inches of manure in the fall and turn it under to allow it to further compost. Add 10 to 20 pounds of composted or dried manure per 100 square feet.
Comments: Add dry, composted manure. Allow fresh manure to compost or age; it is high in nitrogen and salts and can harm plants. Composting manure will help destroy weed seeds. Horse manure collected from stables may contain bedding such as straw or sawdust or larvicides or livestock medications.

Kelp meal

Description: Dried, ground seaweed, an ocean product, also called seaweed meal.
Benefit: Adds potassium and also calcium, sodium, sulfur, and organic matter.
Average NPK analysis: 1.0-0.5-2.5
Application and amount for adequately fertile soil: Add 1 to 2 pounds per 100 square feet and till or turn it into the soil.
Comments: Contains a broad array of vitamins, minerals, and soil-conditioning elements; do not over-apply or the micronutrients can become toxic. Rinse seaweed of salt before adding it to the garden. Kelp meal is available at garden centers.

Langbeinite, sold under the brand name Sul-Po-Mag

Description: A mineral called sulfate of potash-magnesia; sold as Sul-po-mag commercially.
Benefit: Quick-release potassium; also contains magnesium.
Average NPK analysis: 0-0-22; 11 percent magnesium, 22 percent sulfur.
Application and amount for adequately fertile soil: Broadcast 1 pound per 100 square feet.
Comments: Do not use this product often; it may interfere with the absorption of other nutrients. The release of potassium from Sul-po-mag is almost immediate. Do not use dolomitic limestone; it will bring magnesium to a toxic level; instead, substitute greensand or other potassium sources.

Limestone, calcitic

Description: A lime that contains mostly calcium carbonate, with a small percentage of magnesium carbonate.
Benefit: Soil balancer, calcium (38 percent).
Average analysis: 65 to 80 percent calcium carbonate; 3 to 15 percent magnesium carbonate
Application and amount for adequately fertile soil: Add 6 pounds per 100 square feet if the soil is claylike; add 4 pounds if the soil is loamy; add 2 pounds if the soil is sandy.
Comments: Lime raises calcium levels in soil and will slowly raise the soil pH at the same time. It also adds calcium and magnesium to the soil. Perform a soil test before adding lime to the soil. You can purchase lime at a garden center.

Limestone, dolomitic

Description: A lime that contains about half calcium carbonate and half magnesium carbonate.
Benefit: Soil balancer, calcium (25 percent), magnesium (8 percent).
Average analysis: 51 percent calcium carbonate; 40 percent magnesium carbonate.
Application and amount for adequately fertile soil: Add 6 pounds per 100 square feet if the soil is claylike; add 4 pounds if the soil is loamy; add 2 pounds if the soil is sandy.
Comments: Lime raises calcium levels in soil and will slowly raise the soil pH at the same time. It also adds calcium and magnesium to the soil. Perform a soil test before adding lime to the soil. You can purchase lime at a garden center.

Mushroom compost

Description: Used growing medium to produce mushrooms; usually consists of wheat straw, hay, corn cops, cottonseed hulls, gypsum, and chicken manure.
Benefit: Adds nitrogen and organic matter
Average NPK analysis: About 2-1-1
Application and amount for adequately fertile soil: Add 2 to 3 inches of mushroom compost as sheet compost in the fall and work it into the soil. Use during the growing season as a side dressing.
Comments: Make sure mushroom compost contains no seed or pesticide residue. It is usually pasteurized before it is sold.

Oak leaves

Description: Oak leaf mulch.
Benefit: Adds organic matter.
Average NPK analysis: 0.8-0.4-0.1
Application and amount for adequately fertile soil: Add 100 pounds per 1,000 square feet. Add oak leaves to the garden in the fall and allow them to decompose before spring.
Comments: Oak leaves can be used to add organic matter to the soil. Oak leaves can be used as mulch. They will not make the soil more acidic by any great degree. Oak leaves break down very slowly; shred leaves before adding them to the garden.

Peat moss (Peat moss is a non-renewable resource and is not sold in many countries)

Description: Plant debris that has been decomposed underwater without oxygen. Peat moss comes from bogs, mostly from Canada but also from Europe.
Benefit: Adds organic matter to the soil and increases the soil acidity.
Average analysis: Peat moss has a pH range of 3.0 to 4.5.
Application and amount for adequately fertile soil: Add about 2½ pounds of peat moss per 100 square feet as needed.
Comments: Peat moss is best used around acid-loving plants. Moisten dry peat moss thoroughly before adding it to the soil then mix it into the soil. If left on the soil’s surface peat moss will dry out and repel water. Most peat moss comes from Canada; it is not a renewable resource. Peat moss can be purchased in bales from garden centers.

Pine needles

Description: Pine needle mulch.
Benefit: Adds organic matter.
Average analysis: Soil needles have a pH of about 6.5.
Application and amount for adequately fertile soil: Add 3 to 5 inches of pine needles as mulch around plants. Add pine needles to the garden in the fall and allow them to decompose before spring.
Comments: Pine needles can be used to add organic matter to the soil. They will take 3 to 5 months to begin decomposing. Pine needles can be used as mulch; they allow water to reach the soil. They will not make the soil more acidic by any great degree. Pine needles break down very slowly; shred needles before adding them to the garden.

Poultry manure

Description: Droppings from chickens, turkeys, and other domesticated birds.
Benefit: Adds nitrogen and lesser amounts of phosphorus and potassium and organic matter.
Average NPK analysis: 1.1-0.8-0.5
Application and amount for adequately fertile soil: Apply 5 to 10 pounds of composted manure over 100 square feet. Fresh manure can be applied to planting beds in fall along with high-carbon materials such as shredded leaves to aid in its decomposition.
Comments: Compost poultry manure before adding it to the soil; fresh manure is high in nitrogen and can “burn” plants. Dry and dusty manure can contain fungal spores; wear a respirator or face mask when applying dry manures.

Rock phosphate

Description: Finely ground mineral skeletal remains of prehistoric animals
Benefit: Add phosphorus.
Average NPK analysis: 0-3-0; about 32 percent phosphate, also about 32 percent calcium.
Application and amount for adequately fertile soil: Apply 6 pounds per 100 square feet for soil low in phosphorus; for average soil add 2 to 2½ pounds per 100 square feet. Release time is about 3 to 5 years.
Comments: Rock phosphate is best added to acidic soil of pH 6.4 or less; also contains calcium, iron, and nine other trace elements. Rock phosphate can be purchased at garden centers or farm supply stores.

Sawdust

Description: Wood shavings and dust from sawmills or woodworking.
Benefit: Adds organic matter.
Average analysis: Can be acidic depending on the wood.
Application and amount for adequately fertile soil: Sawdust is often used as summer mulch; add 1 to 1½ inches around plants.
Comments: Add nitrogen to the soil when adding sawdust to the garden; this will aid in its decomposition or be sure sawdust is well-rotted before incorporating. Compost sawdust with nitrogen-rich materials.

Soybean meal

Description: Processed remains from soy.
Benefit: Adds nitrogen.
Average NPK analysis: 7-2-1
Application and amount for adequately fertile soil: Add 8 pounds per 100 square feet. Release time 1 to 4 months.
Comments: Available at a feed store.

Sulfur

Description: A naturally occurring mineral.
Benefit: Sulfur lowers the pH of soil decreasing the alkalinity of a soil with a high pH.
Average analysis: A soil amendment to lower soil pH.
Application and amount for adequately fertile soil: Broadcast 1 pound per 100 square feet to lower the pH by one point.
Comments: Apply sulfur to the soil in the fall; mix the sulfur into the top 3 inches of soil. Additional sulfur may be required if the soil is high in organic matter. Add additional sulfur after testing the soil. Sulfur is available at garden centers.

Wheat straw

Description: Straw residue from harvested wheat.
Benefit: Adds organic matter.
Average NPK analysis: 0.7-0.2-1.2
Application and amount for adequately fertile soil: Add 2 to 3 inches as a mulch; 100 pounds per 1,000 square feet.
Comments: Straw can be used to reduce erosion.

Wood ashes (leached)

Description: The residue from wood fires has been allowed to sit outdoors.
Benefit: Adds potassium, depending on the type of wood; also contains calcium.
Average NPK analysis: 0-1.2-2
Application and amount for adequately fertile soil: Add 2 pounds per 100 square feet every 2 to 3 years. Turn leached ashes into the soil after applying.
Comments: Mix ashes with compost to spread coverage. Use ashes from untreated wood.

Wood ashes (unleached)

Description: Fresh residue from wood fires.
Benefit: Adds potassium, depending on the type of wood; also contains calcium.
Average NPK analysis: 0-1.5-8
Application and amount for adequately fertile soil: Add 2 to 3 pounds per 100 square feet every 2 to 3 years. Apply in the fall and allow fresh ashes to sit through the winter before mixing them into the soil.
Comments: Mix ashes with compost to spread coverage. Use ashes from untreated wood.

Worm castings

Description: The manure of earthworms; it is dark, crumbly, and odorless.
Benefit: A source of organic matter and small amounts of many nutrients.
Average NPK analysis: 0.5-0.5-0.3
Application and amount for adequately fertile soil: Apply 25 pounds per 100 square feet if the soil is low in organic matter; add 10 pounds per 100 square feet if the soil has average fertility.
Comments: Worm casting are about 50 percent organic matter and contain 11 trace elements. Worm casting can be produced in a home worm box with red worms or brandling worms feeding on kitchen scraps. Worm casting can be purchased at garden centers.

How to apply organic fertilizers and amendments

Organic fertilizers and soil amendments are commonly applied by a technique called side-dressing. Side-dressing is accomplished by spreading fertilizers in a shallow furrow or band along the side of vegetable row crops or in a circle around individual plants.

Side dressing delivers extra nutrients to vegetable crops so that they can produce to their full potential.

How to side dress

To side dress create a furrow 1 to 2 inches deep along a row of plants or in a circle around each individual plant. Create the furrow just beyond the plant’s drip line (the drip line is the distance from the central stem of a plant to the end of its widest branches). Apply the fertilizer to the furrow and cover it with soil.

Side dressings include solids such as granular fertilizers, aged compost and manure, bone meal, and rock phosphate, as also liquid fertilizers and fish or seaweed emulsion or compost and manure tea.

Side dressing is commonly done when plants need extra nutrients for growth–at flowering or fruiting time and in the second half of the growing season. From midseason on vegetable crops make rapid growth and quickly use nutrients put in place before sowing or transplanting.

Side dressing also resupplies nitrogen, phosphorus, potassium, and trace elements leached from the root zone by irrigation.

How much fertilizer to side-dress vegetables

One handful of good compost is sufficient to side-dress most vegetable plants. That is equal to about one tablespoon of 5-10-10 fertilizer. Adding liquid fertilizers to your watering can is an easy way to side-dress. A cupful of compost tea is a good side dressing.

It is important to apply commercial fertilizers according to the instructions on bags or bottles. Over-fertilized vegetables tend to be less productive.

Fertilizers should not be placed right next to the stem of a plant; they might burn the roots. Fertilizers side dressed outside the drip line will encourage roots to extend beyond the drip line.

When to side-dress vegetables

One side dressing at midseason is usually enough to sustain and encourage a vegetable plant to harvest. Heavy feeding crops–mostly summer fruiting crops that take a long time to mature such as tomatoes, peppers, and eggplants–will benefit from more than one side dressing.

Here are suggested times to side-dress vegetables and fruits with fertilizer:

Beans: not necessary.
Beets: add bone meal or phosphorous at planting time; side dress when plants are 4 to 5 inches tall.
Beet greens: 2 weeks after leaves appear.
Berries: add bone meal or phosphorus at plant time, or sprinkle bone meal around established plants in spring; add aged compost around plants any time of the year.
Broccoli: 3 weeks after transplanting and again when heads begin to form.
Brussels sprouts: 3 weeks after transplanting; again when sprouts are marble size.
Cabbage: when the head begins to form; side dressing is not necessary if the soil was well fertilized at planting.
Carrots: add bone meal or phosphorous at planting time.
Cauliflower: when heads begin to form; not necessary if the soil was well fertilized at planting.
Celery: 3 weeks after setting out starts; again 6 weeks later.
Chard: after each cutting when substantial leaves are taken from the plant.
Corn: 3 weeks after planting or when plants are 8-10 inches tall; again when tassels appear.
Cucumbers: just before vines start to spread and again when blossoms set.
Eggplant: 3 weeks after transplanting and again at blossom time.
Kale: when plants are 6-8 inches tall.
Leeks: when plants are 12 inches tall.
Lettuce, head: a few weeks after planting.
Lettuce, loose-leaf: after second and third cuttings for cut-and-come-again crops.
Melons: just before vines begin to spread; a week after blossom set; again 3 weeks later.
Mustard greens: when plants are 6 to 8 inches tall and after each cutting when leaves are taken from the plant
Okra: at blossom time.
Onion: 3 weeks after setting out; when tops are 4 to 8 inches tall; when bulbs begin to form.
Peas: not necessary.
Peppers: 3 weeks after transplanting; again at blossom time.
Potatoes: before the second hilling; when plants bloom.
Pumpkins: when plants start to run; again at blossom time.
Radishes: no need to side dress.
Spinach: when plants are about 1/3 grown; at second and third cuttings for cut-and-come-again crops.
Squash, summer: when plants are 6 inches tall; again at blossom time.
Squash, winter: just before vines begin to spread; again at blossom time.
Strawberries: add bone meal or phosphorus at plant time, or sprinkle bone meal around established plants in spring; add aged compost around plants any time of the year.
Sweet potatoes: before second hilling.
Tomatoes: 2 to 3 weeks after transplanting; at blossom time; before first picking; 2 weeks after first picking–always light on nitrogen.
Turnips: add bone meal or phosphorous at planting time.
Zucchini: at blossom time.

Related articles:

Epsom Salt, Milk, and Organic Fertilizers for Tomatoes and Peppers

Organic Plant Nutrients for Vegetable Gardens

Vegetable Garden Organic Pest Control

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