Kelp Amendments: How to Use Kelp in Your Garden

By Chris Bond
Published: December 1, 2014 | Last updated: May 4, 2021 10:02:01
Key Takeaways

With a whole host of benefits, from improving seed germination and increasing stem strength, to helping plants deal with drought, high temperatures and frost, kelp is an amazing additive to include in your garden.

Source: jeff waibel/

Adding kelp to your garden soil is a great way to boost micronutrient levels organically. Fish or other marine extracts are often mixed with kelp to add macronutrients to the mixture. Kelp is especially beneficial for seedlings and transplants. Kelp contains dissolved ocean minerals and is available most commonly in powder form, a granular form known as kelp meal, or in liquid form.


What is Kelp?

Kelp is a type of seaweed. Large-growing and usually brown, kelp has no roots. It anchors itself to the ocean floor by a part known as a holdfast. Though it seems to act like a root system, a holdfast is different than a root, as it does not absorb nutrients like roots do. The foliage of kelp are called blades and the stems are known as stipes.

The stipe is a flaccid structure and cannot stand on its own. It does so with the help of an organ unique to kelp—an air bladder. The air bladder helps keep the foliage and stems afloat so the kelp can grow towards the sunlight. From there, the plant can photosynthesize and make for itself all of the wonderful nutrients that will help keep your plants healthy and productive. Some of the larger species of kelp, known not surprisingly as giant kelp, can reach heights of up to 150 ft. and grow 1-2 ft. per day.


The Benefit of Kelp Amendments

Kelp contains more than 60 trace elements used by plants, including iron, copper, zinc, molybdenum, boron, manganese, cobalt and alginic acid, to name a few. It aids in the development of extensive root systems and offers natural resistance to harmful nematodes, diseases and pests. It improves seed germination, increases stem strength and helps plants deal with drought, high temperatures and frost. Kelp also increases the nutritional value of fruits and vegetables, and helps soil by improving its structure, aeration and moisture retention. It normalizes pH levels and stimulates soil bacteria.

Liquid Kelp

Liquid kelp is usually highest in nutrients and most available for immediate use by plants compared with the powder or meal form. It can be cold-processed, or enzymatically digested. The digested form is a higher-quality formulation, so expect to pay a bit more. Its use might be best reserved for high-value crops. The digested form also contains more natural growth hormones (more than 60) than either of the other two extracted forms. The cold-pressed form will have less nutrients, but still offer more than powder or meal.

Some of the growth hormones in liquid kelp include cytokinins to aid in the division of cells; auxins to promote root and cellular development; indoles to aid in root and bud development; and gibberellins to promote stem and seed development. This form needs to be reapplied more often, as the nutrients will be used within one month of application. Mix liquid kelp at a rate of 1-2 tbsp. per gallon of water.


Kelp Meal

In meal form, kelp can be incorporated into garden soil at a rate of 1 lb. per 100 sq. ft. Depending on weather and microbial activity in the soil, one application will begin to release nutrients in about four months. Of all kelp products, meal usually contains the least amount of kelp, as there may be other organic materials mixed in with it.

Kelp Powder

As a powder, kelp can be mixed with water and applied as a foliar feed, or put into a fertilizer injector and delivered by irrigation water to plants at a rate of ¼-½ tsp. per gallon. The benefit of powder applications is that nutrients are immediately available to plants and remain available for several months afterwards.



Making Your Own Kelp Amendments

If you happen to live close to the ocean, you may be able to collect and process your own kelp amendments, but before doing so, consult local laws around harvesting kelp from public lands. Collect only fresh-looking kelp—after it washes ashore, it quickly begins to decompose and the beneficial elements start to diminish. If it has been sitting on the shore for a while, it may also have become home to all sorts of insects you don’t want to bring into your car, home or garden.

After you have collected a sufficient amount, decide if you want to create a liquid or dry amendment or if you want to add the kelp directly to your garden or compost pile. Either way, rinse the kelp off first as it likely contains sand from the beach and possibly salt from the ocean. The rinsed material can then be added to the compost pile.

Using Fresh Kelp in the Garden

To use the kelp while it’s fresh, incorporate it into either the compost pile or garden with a spade or digging fork. Depending on your environment, you may wish to dry it first, as freshly collected seaweed may attract animals to your garden. For a dry amendment, hang the kelp somewhere it will receive the full benefits of the sun and breeze. Once it has thoroughly dried, pulverize it into small flakes and spread it across the soil or around the base of plants. The pulverized kelp can also be added to your compost pile if you are not able to apply it directly to the soil.

Make Kelp Tea for Soil Drenching or Foliar Feeding

For a liquid amendment, you can make a kelp tea by soaking the freshly collected seaweed in a bucket of fresh water. Stir it up every few days. It can steep for several weeks to several months depending on environmental factors. Make sure to do this in a well-ventilated area, preferably outside—as the nitrogen breaks down, it will have a pungent aroma. It is ready to be used when it no longer has an ammonia-like smell to it. The tea will be concentrated and water should be added at a rate of three parts water to one part kelp tea.

The solids that remain in your “teapot” can be reused. You can make a second batch of tea with the remaining solids, or boost the nutrient level by adding some fresh kelp to it as well. The second batch will not be as nutrient-dense as the first, but will still contain appreciable amounts of micronutrients and growth hormones. A ration of one part tea to one part water should suffice. After the second brew, the remaining solids can be incorporated into your compost pile. Kelp tea can be used as a soil drench or a foliar feed. To use it as a soil drench, aerate the soil you wish to amend with a digging fork before applying the tea. This will add oxygen to the soil, adding to the aerobic processes, and allow your tea to cover a greater surface area of soil.

When using kelp tea as a foliar feed, consider adding an organic surfactant to help it stick to the foliage. You can use liquid molasses, dry molasses, liquid agave, fish oil or yucca extract. When applying tea with a pump sprayer, clean the sprayer out immediately after applying so any sugars or solids do not clog the hose or sprayer attachment for future applications. Adding sugars to your teas while they are steeping will add carbon to the mix and speed up the decomposition process as well as reduce the strong aroma of the nitrogen.

Buying Kelp Amendments

For gardeners unable to make their own kelp amendments, fear not. Kelp is not a difficult amendment to locate. There are currently more than 45 registered organic kelp products on the market in the United States and an untold number of as-yet uncertified sources. You can also make your own amendments by purchasing kelp in a grocery store. Though it is not a common item in most grocery chains, you can find it in Asian markets and specialty stores.

On a final note, as with any chemical, organic or not, make sure you wear the proper personal protective equipment while applying any amendment, liquid or dry, and when using a commercially prepared formulation, adhere strictly to the application instructions on the label. When it comes to pesticides and fertilizers, organic or not, the label is the law!


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Written by Chris Bond | Certified Permaculture Designer, Nursery Technician, Nursery Professional

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Chris Bond’s research interests are with sustainable agriculture, biological pest control, and alternative growing methods. He is a certified permaculture designer and certified nursery technician in Ohio and a certified nursery professional in New York, where he got his start in growing.

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