Soup Up Your Soil with Kelp, Molasses, and Guano

By Diane Young
Published: August 18, 2020 | Last updated: May 4, 2021 10:38:11
Key Takeaways

What do kelp, molasses, and guano have in common? Well, it’s kind of a long story, but simply put, they are all organic ingredients that can boost the quality of your soil, and consequently, your harvests.

Rick Weller, the founder of Organically Done in Michigan, likes to talk about “souped-up soil”—soil that’s loaded with organic amendments thoroughly blended in before you plant. The point of souped-up soil is to combine all the nutrients, amino acids, and minerals that plants need to grow to their best potential and produce a high-yield crop by only adding water during the growing season.

There are many ways you can soup up your soil using organic amendments, starting with kelp, molasses, and guano.



kelp, a type of seaweed

What is Kelp?

If you’ve ever walked along an ocean beach after a storm, you’ve seen seaweed—probably kelp–lying tangled along the high-tide mark. Kelp grows in long, leathery strips that are slippery when wet. Many times here on the coast of Maine, I’ve gathered dry seaweed with a pitchfork for my compost pile.

Kelp is almost always mixed with small mussel and clam shells, crab shells and claws, seabird feathers, chunks of weathered white Styrofoam, and frayed bits of orange polyester rope. I only pick out the non-organic bits.


How Does Kelp Soup Up Soil?

Kelp is an amazing gift from the sea for gardeners and growers. It isn’t a miracle plant food, but it can be very important when used in combination with other nutrients.

While not a primary source of the macronutrientsnitrogen, phosphorus, and potassium—kelp contains more than 60 trace minerals, plus aminos, enzymes, and alginates. By adding kelp to your soil, along with the other necessary nutrients, you’ll improve your soil health, seed germination, plant vigor, sugar content, and even how many blooms are set and the size and storage life of your harvest.

Seaweed contains mannitol, a natural sugar that chelates or breaks down micronutrients and makes them available to plant cells. It also stimulates more lateral root growth and larger root mass. Furthermore, kelp bolsters stress resistance from disease, pests, drought, and frost; promotes natural growth hormone development; and stimulates important microbial activity.

“Kelp helps make plants hardier,” says Ann Molloy of Gloucester, Massachusetts. “There’s a lot of stress on plants during their growth, budding, and flowering stages. Kelp really helps with that.” And as for the source, Molloy says the cold waters of the North Atlantic are ideal. “The darker the water, the more nutrients in the kelp and the fish as well,” she says.

Kelp is available as a dry meal and in a concentrated liquid form. Kelp meal doubles in volume when it’s wet, which helps aerate your soil and retain moisture. It should be applied in the spring and fall. One pound will cover 100 square feet. Work it well into the soil.

Liquid kelp, on the other hand, is best used for foliar feeding for both soil-grown and hydroponic plants. Spray the tops of your plant leaves as well as underneath them. Apply early in the morning or late afternoon, as direct sun isn’t good. Foliar feeding every two to four weeks is recommended.

Read More: Kelp - The Wonder Amendment


molasses for gardening


What is Molasses?

Here’s a sweet thought. Another organic ingredient you can use on your garden is molasses. There are three kinds of molasses: mild (Barbados), dark (second boiling), and blackstrap (third boiling). Unlike refined sugars, blackstrap contains trace amounts of vitamins and healthy amounts of calcium, magnesium, and iron. Molasses should always be unsulfured.

How Does Molasses Soup Up Soil?

Molasses is a very valuable addition to your compost pile. In addition to the minerals, the sugar feeds the micro-organisms in the compost and in your garden soil. Mix up to a whole cup off molasses in a gallon of unchlorinated water. You can make an even more potent mixture by using milk instead of water. Raw milk is best, but any kind of milk with do. Molasses mixed with milk is a miracle in the garden and greenhouse. However, it isn’t commonly used in hydroponics because its stickiness plugs up the equipment.

Using milk on crops and soil was an ancient practice that has been lost in today’s enormous agribusiness. Studies have shown that milk does work as a fertilizer, even for foliar feeding. Another benefit of spraying the molasses-milk combo is that it controls broad-leafed weeds and runs off any hungry grasshoppers.

“We’re big on molasses. It’s not just for plants, it great for the microbes—the more organics the better,” says Eric Olsen of NPK Industries. “We use the entire plant, roots and all, which has a good amount of iron. Our veteran growers have been using molasses a long time.”

Read More: Feeding Your Garden Cane Molasses



bat guano for gardening

What is Guano?

Another great organic amendment for plants is guano. Guano is the accumulated poop of seabirds, seals, and bats, which ends up being some of the most potent, sought-after and expensive natural fertilizers in the world. Most guano comes from caves that have long-established colonies or “camps” of bats. The caves are usually in mountainous areas and in a tropical climate.

In North America, guano is imported from all over the world, including Mexico, Jamaica, Indonesia, Sumatra, and Peru. Mexican guano boosts lots of vegetation, as opposed to promoting fruit and flowers, while Jamaican and Indonesian guano boosts bud production. Seabird guanos are the most balanced.

How Does Guano Soup Up Soil?

The N-P-K values of guano vary depending on what part of the world the guano is harvested from, so read your labels carefully. Some may look like this:

  • Indonesian, 0.5-12-0.2
  • Jamaican, 1-10-0.2
  • Sumatran, 8-3-1
  • Mexican, 10-2-1
  • Mexican liquid, 8-5-0
  • Peruvian seabird, 10-10-2

Whichever source of guano you’ve considered, check to make sure the harvester is going about things as eco-friendly as possible. For example, in Peru, through the Fair Trade Agreement, the sale of guano has been a huge financial benefit for the country’s economy says John Lorey of Sunleaves Garden Products. “We have people there overseeing the operation to make sure everything is being done right.”

There are more than 1,200 species of bats worldwide, which can be broken down into two types based on their diets. Bats that eat moths and other insects produce guano that’s high in nitrogen, while bats that eat fruit produce guano that is low in nitrogen, but high in phosphorus.

There are a few ways to go about introducing your plants to guano. Guanos come as a loose powder in bags and in a liquid. Seabird guano is pelletized. Some customers make guano tea, a potent plant elixir, which has to be used in the first 12 hours. “Guano is popular with our hydroponic growers, and hydroponics is growing by leaps and bounds because it’s so ecologically sound,” says Tony Bayt of Sunleaves Garden Supply.

Read More: Growing With Guano: How Bat Guano Benefits Soil and Plants

A Word on Worm Castings

Another organic option you might want to consider for your garden is worm castings—an organic form of fertilizer products by earthworms and also known as vermicast. In ancient Egypt, Cleopatra proclaimed the lowly worm “a sacred creature” and she made taking worms out of the fertile Nile Valley a capital offense. Charles Darwin thought it probable that worms are the most important creature on earth.

Even the late Robert Rodale praised worm castings as “the finest form of humus known.” In soil enriched with castings, microbial activity is 10 to 20 times higher than in just soil alone. Castings hold two to three times their weight in water, which means you can water less. Other natural fertilizers may have higher percentages of nutrients, but a plant’s ability to use them is limited because they aren’t broken down to the degree of worm castings.

Gardeners appreciate that castings are non-toxic, odorless, won’t burn plants or roots, and have colossal growing power. Just a handful will make your plants happy.

Here’s a quick tip: Earthworms love coffee grounds, so by using them in your compost pile, your worm population will increase, pooping their enriching castings as they go.

Read Next: Worm Tea - The Secret to Organic Gardening


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Written by Diane Young

Profile Picture of Diane Young
Diane Young has had her food and gardening articles published in the Burlington Free Press, The Boston Globe, The Christian Science Monitor, Country Journal and other regional publications. Soon after, Troy-Bilt commissioned her book, Community Gardening in North America. A New England gardener, Diane lives in Maine and Mexico.

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