Organic Garden Fertilizers

Once your soil quality is where it needs to be, you probably won’t need organic garden fertilizers other than the continual reapplication of compost. In the meantime, this page will help you keep your plants happy and fruitful…

N, P, K & Others - Nutrients Essential to Plant Growth


  • Carbon
  • Hydrogen
  • Oxygen
  • Calcium
  • Magnesium
  • Sulfur


  • Boron
  • Copper
  • Iron
  • Manganese
  • Molybdenum
  • Zinc

Nitrogen (N), phosphorous (P) (also called potash) and potassium (K), along with the nutrients listed to the right, are all needed in relatively large quantities in order for plants to grow. Micronutrients, as the word implies, are required in lower amounts. Trace elements are required by plants in even smaller (but equally essential) amounts.

The reason N, P and K are singled out so frequently is their lack of abundance in most soils. Whereas your garden plants can always get plenty of carbon, hydrogen and oxygen from the air, their growth quickly depletes N, P and K (along with some of the other nutrients) from the soil. Enter fertilizer.

Plants require a specific balance of macronutrients, micronutrients and trace elements in order to thrive, so when amending your soil with any fertilizer, it is extremely important to NOT overdo it. Only apply the amounts designated on the label. When in doubt, add less, wait then add more if necessary.

Most commercial fertilizers are sold with an NPK percentage rating such as 10 – 20 – 10. A fertilizer with these NPK percentages would have 10% nitrogen, 20% phosphorous and 10% potassium.

The easiest way to determine which nutrients your garden soil has and which it lacks is through at-home soil testing kits. You can also contact your local Cooperative Extension for a more detailed test.

In the meantime, here’s a quick reference for identifying and fixing N, P and K deficiencies…

Nitrogen (N) deficiency signs:

  • Leaves - light green or yellow (usually starts with lower leaves)
  • Growth – poor

Nitrogen (N) deficiency fixes:

  • Add compost to your soil in the fall after the growing season
  • Plant green manure (i.e. alfalfa, clover, mustard, fava beans), then till the entire plant back into the soil as soon as the flowers begin to form at least a few weeks before planting your garden crop
  • Temporary fix: add blood meal (dry fertilizer) or fish emulsion (liquid fertilizer) (see Types of Organic Garden Fertilizers section below for more information)

Phosphorous (P) deficiency signs:

Leaves – turn purplish/bluish color. Usually starts with lower leaves (leaves usually will NOT turn yellow with phosphorous deficiency).

Phosphorous (P) deficiency fixes:

Potassium (K) deficiency signs (often difficult to detect; especially common in sandy soils):

  • Leaves and stems: weak; thin
  • Leaves sometimes have curling tips and/or space between leaves’ veins turn yellow
  • Fruits: less than ideal (smaller, poorer taste)
  • Pests and disease more abundant

Potassium (K) deficiency fixes:

But we’ve gotten ahead of ourselves.

What about all of the synthetic (chemical) fertilizer options? Shouldn’t you consider them as a valid option, especially since scientists are able to “allocate” much more specific plant-required nutrient percentages into them?

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Organic vs. Chemical Garden Fertilizers

Chemical fertilizers appear miraculous, giving you the most beautiful and fruitful plants imaginable. Unfortunately, “if it seems too good to be true, it usually is” is a very fitting cliché.

There’s more going on with chemical fertilizers than simply giving the plants exactly what they need. If that were the case, we could just stick some seeds in a container of fertilizer, water it and call it a day! Even with the highest-tech synthetic fertilizer, plants still require good old fashioned soil in order to grow.

Why is soil so important for plant growth in the presence of nature-conquering chemical fertilizers? Here are a few key reasons:

  1. Healthy soil contains organic matter on which soil organisms feed, thus breaking down the matter into particles that are small enough to be absorbed by plant roots
  2. That organic matter gives soil the ability to hold, or store, nutrients for future plant use
  3. The broken down organic matter allows gradual water drainage and proper water retention which allows plant roots to absorb water and water-soluble nutrients at their own pace

All of this is entirely made possible by the living soil organisms that break down the organic matter (food for the soil organisms) and turn it into soil in which plants can be grown. Without the soil organisms and their broken down organic matter, the dead soil would lose its texture and not be able to retain water or nutrients that plants need to grow. In other words, chemical fertilizer applied to dead soil would flow right through (or sit on the soil’s surface) and render it useless to plants.

Where organic garden fertilizers replenish the soil with organism-feeding organic matter with each application (organic fertilizer comes directly from living things), chemical fertilizers contain only the nutrients that the plants require to grow. With nothing left behind for the soil organism to feed on, soil that is exclusively fertilized with chemicals will eventually die.

So why don’t we take a “best of both worlds” approach? Hit ‘em with chemical fertilizer for neighbor-impressing tomatoes while keeping our garden soil healthy through composting and other organic techniques?

That would certainly be better than a chemical-fertilizer-only route, but unfortunately you’d just be slowing down the inevitable conclusion...

For the sake of argument...

...let’s forget for a moment the damage caused to the planet as a whole by the drilling, refining, transport and burning of fossil fuels required to create, produce and sell chemical fertilizer and just focus on the impact it has on your garden.

Let’s also not weigh in on the effects chemical fertilizer run-off may have on your local water supplies or on wildlife and plant life that are downstream from a chemically fertilized garden.

Let’s also dismiss the peak oil facts and the impact peak oil will have on the cost of all oil-dependent goods (including chemical gardening and farming) and therefore the economy.

  • First, chemical fertilizers are more highly concentrated and can burn both your plants’ roots and directly kill (rather than just not feed) soil organisms.
  • Second, since chemicals kill soil organisms and reduce the soil’s ability to hold moisture and nutrients, the more chemical fertilizer you use, the more you need to apply. And in addition to throwing your garden soil pH out of whack (which makes nutrients less likely to be absorbed, leading to less healthy plants), long-term chemical fertilizer use may lead to a toxic build-up of chemicals in your soil that can be absorbed into your garden crops and harvest.
  • Third, chemical fertilizers do not provide the trace soil elements your plants need to thrive… your plants still need to obtain those from the soil. Over time, the amount of trace elements in the soil will become depleted and slowly reduce your garden’s ability to thrive.
  • Finally, synthetic fertilizers repel earthworms, a huge helper for soil quality in terms of both soil aeration and natural fertilization.

At the end of the day, the only way to make the “best of both words” approach work (at least in terms of creating a sustainable garden) is to keep your soil relatively healthy through the rigorous application of organic techniques. And those techniques would get your soil healthy to the point that fertilizers of any sort – chemical or organic - wouldn’t be needed.

To cap off the discussion… considering the effects that chemical fertilizers and their production and delivery have on the environment and your soil, would you ever consider using chemical fertilizers if there were organic fertilizers that were equally fast-acting and effective?

We’ve got good news… liquid organic garden fertilizers such as fish and seaweed emulsion are that good, while dry organic fertilizers provide a longer-term and more sustainable impact…

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Best Time to Fertilize & How to Apply Liquid and Dry Organic Garden Fertilizers

There are two types of organic garden fertilizers: liquid and dry (we'll get into the specific types of each in the next section).

Siphon Mixers for Safe Fertilizing

organic garden pest control

Siphon mixers attach directly to your spigot and, after your garden hose is attached, allow you to easily mix your liquid fertilizer with the water in the hose.

This siphon mixer will mix your liquid fertilizer at a precise 1 to 16 fertilizer to water ratio. It also has built-in features that prevent clogging.

Liquid organic garden fertilizers are usually sprayed (on “fine” or “mist” sprayer setting) once or twice per month. Apply at the base of the plant above its roots along with both sides of its leaves (called “organic foliar feeding”) until the leaves are covered. Outside of the once or twice per month guidelines, consider applying organic garden fertilizers…

  • Just after you transplant a seedling or mature plant
  • During droughts or high/low temperatures
  • When plants begin to bear fruit for the first time

Do’s & Don’ts with Organic Garden Fertilizers

To side-dress means to apply a dry fertilizer on the ground outside of the plant’s leaf line.

When using this technique, be sure to apply the dry fertilizer well outside the plant’s leaf line, slightly turn fertilizer into the soil after applying (about an inch/2.5 cm or so) and closely follow the label’s instructions and recommended amounts.

Dry organic garden fertilizers should be used much less frequently than liquid organic fertilizers. Apply them once or twice throughout the growing season to strengthen your crops or give them an extra boost. After applying, gently rake them into the top few inches (8 cm) of soil and water the area lightly.

If you apply dry fertilizer more than once throughout the season, try to space out the timing. Following are times when you may want to consider adding dry organic fertilizer…

  • When planting, add small amount to holes
  • Add as side-dressing any time during growing season
  • Broadcast over entire bed when plants begin to bear fruit for the first time
  • Rock phosphate, used to add phosphorous to your soil, is the exception. Add it to your compost pile periodically and to your garden bed when starting your garden (1 lb./0.45 kg for every 10 square feet/0.93 sq m). Follow directions on the label carefully.
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Types of Organic Garden Fertilizers

When Organic Garden Fertilizers Aren't Necessary

As mentioned in the introduction, if your soil is healthy you probably don't need to fertilize. Healthy soil comes from...

If your soil is less than ideal, liquid and/or dry organic garden fertilizers will provide a nice helping hand. Even with perfect soil, fertilizing can go a long way towards getting the hungry feeder crops the nutrient levels they prefer.

When should you use liquid fertilizer versus dry fertilizer? Liquid organic fertilizers should be thought of as fast-acting immediate fixes while dry fertilizers work more slowly and consistently by amending the soil.

Following are common types of each (Nitrogen(N), phosphorous (P) and potassium (K) percentages are indicated next to each fertilizer)...

Liquid organic garden fertilizers (short-term quick fix)

  • Aerated compost tea (NPK: 1-1-1 to 4-4-4) – an excellent homemade fertilizer – fast, free and effective! There are two ways to make it...
    • Put some of your best compost into a cloth sack, drop it into a bucket of water, put the lid on and insert an aquarium pump for aeration. In four to five days you can use the tea on your crops.
    • Purchase a compost tea kit - same as the above option, but allows you to purchase all components together instead of purchasing them individually.
  • Liquid seaweed fertilizer (also called seaweed extract or kelp extract/kelp fertilizer) (NPK: 1-0.5-2.5) – this amazing fertilizer provides plants with the macro and micronutrients they crave in addition to being a natural source of growth hormones. Get ready to be impressed.

    Organic Garden Fertilizers Mainly Used for pH Adjustment

    See Soil pH Tester for more information about pH.

  • Fish emulsion fertilizer (NPK: 4-1-1) – an equally impressive and fast-acting all-purpose organic fertilizer. The only drawback is the “fishy” smell it leaves behind, although that will go away within a few days.

Dry organic garden fertilizers (longer term slow-acting fix)

  • Compost mulch (NPK: 1-1-1 to 4-4-4) - not usually called a fertilizer, but definitely helps in this regard.
  • Kelp meal (kelp fertilizer) (NPK: 1-0.5-2.5) – as with liquid fish and seaweed emulsion, kelp meal is excellent at providing all the nutrients plants love.
  • Bone meal (NPK: 3-15-0) - a great source of phosphorous, bone meal is a mixture of ground bones that is commonly used for flower fertilizing (especially bulbs). Generally speaking, the more finely ground the bone meal, the more quickly its nutrients will be released.
  • Organic flower bulb fertilizer - for spring-blooming bulbs that have already been established, scatter over the soil in early spring after the shoots emerge. For planting flowers in the fall, add directly to the planting bed. For summer-blooming bulbs, corms, rhizomes and tubers, mix with soil then plant. (Note: Follow label instructions for amounts)
  • Organic poultry manure (NPK: 5-3-2)- composted chicken manure (don't use it when it's fresh) is an excellent source of nitrogen, potassium and phosphorous and improves soil water retention. If you buy it, be sure that it is guaranteed to be weed-free.

    Organic Garden Fertilizers for N, P and K Deficiencies

    • Blood meal – especially beneficial in nitrogen (N) -deficient soil
    • Rock phosphate (NPK: 0-3-0) – should be added to new gardens (1 lb./0.45 kg for every 10 square feet/0.93 sq m) and to compost piles to prevent phosphorous (P) deficiencies
    • Greensand (7% potassium) - Use with potassium (K)-deficient garden beds. Apply directly to beds (1 lb./0.45 kg for every 10 square feet/0.93 sq m) and compost piles (granite dust and wood ashes are also used for potassium deficiencies, but greensand is preferable).
    • Potash fertilizer – potash is essentially mined salt that contains high amounts of potassium. It is primarily used in gardening and farming to amend soil that is low in potassium.
  • Coffee grounds as fertilizer (NPK: 2-0.3-0.2) – used or unused coffee grounds make a good fertilizer when needing to get bring down the pH of alkaline soils. They’re also free! Just ask your local coffee shop for used grounds and they should be happy to give them to you rather than sending them to a landfill.
  • Earthworm castings are worm excrements. Earthworms ingest larger soil particles and organic material and excrete them in the form of fine pellets called castings that add nutrients to your soil, improve soil structure and improve water absorption and retention. If you have an earthworm-friendly garden (an absolute must; healthy soil = earthworm friendly), these will be added for your on a daily basis as each earthworm will excrete its weight in castings every day.

    You can also...
  • Custom-made organic fertilizers (NPK: specific to your needs) are prepared after receiving a soil sample directly from your garden (allow roughly 6 weeks after soil sample is sent).
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