A Gardener’s Guide to Fertilizers

Organic FertilizersIn a perfect world, your garden’s soil would provide all the nutrients plants need. But in the real world, garden and lawn soil — and thus the plants that live in them — often needs a little boost. Improving the soil is the number one thing you can do to improve your garden, yard or landscape and organic fertilizers can help.

All plants need:

  • Macronutrients – nitrogen, phosphorus and potassium
  • Secondary nutrients – sulfur, calcium and magnesium
  • Micronutrients – iron, manganese, zinc, chlorine, boron, copper and nickel (in very small quantities)

Organic fertilizing can be as easy or as technical as you want it to be. For gardeners who don’t wish to spend a lot of time figuring out what individual plants want, there are commercial blends that can be used on all plants.

For those who like to treat each plant as an individual there are singular fertilizers or mixes for every kind of plant. Often fertilizing protocol changes as the plant grows. Keep reading to review the best fertilizing method for you.

Why Organic?

Plants can’t tell if the nitrogen, or other nutrients, they are taking up came from an organic or chemical source, but choosing an organic over chemical fertilizer does have an impact on the health of your soil and ground water.

Organic fertilizers actually improve the soil, while chemical or synthetic fertilizers deplete the soil over the long run.

Organic Fertilizers

Chemical Fertilizers

Release nutrients slowly, providing a steady flow of plant nutrients Release nutrients rapidly
Non-burning (won’t harm delicate seedling roots) May burn plants (and harm delicate seedling roots)
Improve soil structure Leaching can pollute groundwater
Increase water holding capacity Loss of fertilizer due to leaching means soil requires many applications
Increase nutrient holding capacity Can make soil toxic after continuous use
Promotes earthworms and soil micro-organisms Mineral salts can build up over time and kill off soil microbes
Buffers soil from chemical imbalances High nitrogen levels may repel earthworms
Improves soil over time

Chemical fertilizers came about after WWII when the companies that made ammonia gas for explosives needed to find a way to stay in business. So, they figured out how to make ammonia gas (mostly nitrogen) into fertilizer.

Dry FertilizersDry Organic Fertilizers

Dry fertilizers can be made from a single ingredient (such as greensand, blood meal or steamed bone meal) or a blend of nitrogen, potassium, and phosphorous plus micronutrients. There are many commercial blends available or you can make your own (see Fertilizer Recipes: How to Make Your Own).

Applying Dry Organic Fertilizers

  1. Before planting broadcast dry fertilizer across the soil.
  2. Rake or hoe fertilizer into the top 4-6 inches.
  3. Add small amounts to planting holes or rows.
  4. Side dress plants during the growing season.

Liquid Organic Fertilizers

Plants have the ability to absorb liquids through both their roots and their stomata (pores on the leaf’s surface). Liquid fertilizers can be applied to the soil or sprayed on to the leaves (see Foliar Fertilization – PDF).
Liquid fertilizers (whether a commercial blend, fish emulsion, or compost tea) are especially beneficial during critical times in a plant’s life such as just after transplanting, during extreme temperatures or drought, or when the plant is blooming or setting fruit). Foliar fertilizing will also benefit plants throughout the growing season and can be applied every 2-4 weeks.

Applying Liquid Fertilizers

  1. Always follow label instructions.
  2. Using a surfactant (coconut oil or mild soap — 1/4 tsp per gallon of spray) to help get the best coverage.
  3. Check the spray’s pH — a slightly acidic fertilizer (6.0-6.5) is best (lower pH with vinegar).
  4. Use a spray mister with the finest mist possible.
  5. Spray until liquid drips off the leaves, being sure to spray the underside of leaves where pores are most likely to be open.
  6. Spray during the early morning or late evening for best absorption.
  7. OR water liquid fertilizers around the roots of plants.

Plant SupplementsNutrient Supplements

Having little or no N-P-K of their own, nutrient supplements are designed to optimize fertilizers, not act as them. Containing vitamins, minerals, and hormones not found in most commercial plant foods, these give your plants “that little something extra.” The most well known is kelp which:

  • contains at least 60 trace elements that plants need in very small quantities
  • contains growth promoting enzymes and hormones
  • stimulates soil bacteria (which increases fertility through humus formation, aeration and moisture retention)

Kelp is sold both as a dry meal and as a liquid.

Applying Nutrient Supplements

  1. Liquid supplements can be applied following the same method as liquid fertilizers.
  2. Kelp meal should be applied at 1-2 lbs per 100 square feet each spring. Kelp extract can be applied weekly to outdoor plants at a rate of 3 tablespoons per gallon of water.
  3. If you can find fresh seaweed, rinse the salt off and use it in the garden as mulch or throw it in the compost pile.

Types of Organic Fertilizers

Bird and animal manures Good nutrient source and chock-full of microorganisms. Should be well-aged or composted before applying directly to the garden.
Blood meal Slow release source of nitrogen plus trace minerals. Apply just before planting and use sparingly.
Fish meal/ emulsion Source of nitrogen, phosphorus, potassium and trace elements. Releases quickly.
Greensand Rich in potassium and numerous micronutrients. Can be used to loosen clay soils. Apply 5-10 lbs per 100 square feet.
Shellfish meal Strong source of calcium (23%), nitrogen, phosphorus and micronutrients. May also be used to inhibit root-knot nematodes.
Rock phosphate Great for flowering plants and provides a 10 year phosphate reserve.

Once you figure out what you want from your fertilizer, it is easy to pick the right one.

Desired Effect


Promote large blooms and fruits Bat guano
Condition soil Compost
Animal manures
Promote sturdy above-ground plant growth Blood meal
Fish emulsion
Promote root growth in transplants and seedlings Phosphate rock
Bone meal
Enhance composting process Alfalfa meal
Blood meal
Bind sandy soil Colloidal rock phosphate
Loosen clay soil Greensand
Coconut coir

Which Vegetables Need the Most Fertilizer?

Light Feeding Vegetables
Bean, beet, carrot, onion, pea, potato, radish, turnip

Heavy Feeding Vegetables
Broccoli, Brussel sprouts, cabbage, cauliflower, celery, corn, cucumber, kale, leeks, lettuce, melon, peppers, pumpkin, spinach, squash, tomato

Mycorrhizal FungiBiological Approaches that Promote Plant Health

Mycorrhizal Fungi are probably already growing in your soil. They form a symbiotic relationship with plant roots. These fungi benefit plants by:

  • Colonizing plant roots and sending hyphae throughout the soil, essentially extending the plant’s reach.
  • Blocking disease organisms.
  • Absorbing phosphorous, water and trace minerals — and sharing them with the plant.
  • Excreting sticky compounds that bind the soil into aggregates, keeping the soil porous and airy.

You can promote mycorrhizal growth by not tilling the soil (this tears them up) or by inoculating your garden with purchased mycorrhizae products.

Worms improve the soil in many ways:

  • Improve the structure of the soil (aerating clay soils and binding sandy soils together).
  • Burrowing opens channels for root growth.
  • They help regulate water (moving moisture to dry areas and draining water clogged areas).
  • Chomping up leaves and other organic debris.
  • Worms leave behind castings full of nutrients.

It’s not just the worms that are good for your garden, their nutrient-rich castings (basically worm poop) are an excellent soil additive. Castings can be produced commercially, or you can raise worms yourself (see Composting with Worms).

There are a lot of ways to reap the benefits of worm castings, including:

  • Top Dressing: Spread a layer (1/2 inch deep) of castings around plants. Mulch and water.
  • Seed Starting Mix: 3 parts aged compost or coconut coir to 1 part castings.
  • Potting Mix: 2 parts aged compost, 1 part castings, 1/2 part vermiculite.
  • Trees and Fruit Trees: Apply around the base and water well. Reapply when necessary.
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