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By Luke Sumpter Reviewed by: Silvia Maroto

The use of high-quality nutrient sources is key during the cannabis grow cycle. Keeping an accurate flow of nutrients into the soil of your plants will ensure maximum health, vitality, and yield. It will mean that you avoid the risk of your plants encountering deficiencies, which will wind up damaging their health, slowing their growth, and reducing the prized end result of the harvest.

There are numerous options to select when it comes to nutrients for your crops, from waste food items all the way to commercial product lines. Well, we want to introduce the idea of using a unique yet effective choice of nutrients: seaweed.

As it happens, seaweed is absolutely crammed with beneficial nutrients that your cannabis plants will happily utilise to achieve optimum health, size, and growth. Seaweed is available to growers in numerous forms. The first form is easily the cheapest and most accessible for those who live near the coast. We’re talking about fresh seaweed, which can simply be harvested during a trip to the beach.


Many different types of seaweed have achieved great interest due to their applications in the pharmaceutical industry, in feeding fish, animals and humans, as well as in agriculture. They have potential as biostimulants, biofertilizers and can be utilised in different ways such as foliar, soil amendment and seed imbibition applications.

Fresh seaweed contains a plethora of major and minor plant nutrients and traces of minerals that it absorbs from the ocean. These include minerals such as sulphur, magnesium, boron, and calcium. Besides, seaweed is also rich in diverse bioactive compounds like lipids, proteins, carbohydrates, amino acids, phytohormones, osmoprotectants, mineral nutrients and antimicrobial compounds.

When ground up and incorporated into meal, seaweed may also contain beneficial vitamins such as A, C, B12, K, and E[1]. Seaweed also contains substances that regulate plant growth, contributing to rapid and explosive development of crops.

Seaweed also helps soil[2] to retain moisture and may reduce the amount you have to water your plants. When used as a mulch, seaweed may also reduce or eliminate the need to weed your garden, acting as barrier against the formation of intrusive plants.

It’s not only weeds that seaweed offers defence against; the lifeform also produces compounds that are effective against certain fungi, viruses, and bacteria. It’s almost like a medicine for your cannabis plants. As well as fighting off bad bacteria, seaweed can help to condition the soil and encourage the growth of beneficial bacteria.



Kelp is a type of algae seaweed that is particularly nutritious[3]. They form underwater kelp forests and grow in shallow, nutrient-rich waters. It’s this environment that makes kelp so beneficial to cannabis and other plants. The seaweed absorbs a vast array of ocean minerals which are dispersed into the soil when added as a fertiliser.

Kelp is capable of meeting most of the nutritional needs of the cannabis plant. Overall, it contains over 60 trace elements essential to the growth of the herb. These include iron, manganese, cobalt, zinc, and molybdenum. Many of these minerals are crucial micronutrients for cannabis. Although only required in small quantities, they fulfil vital physiological processes. For example, iron is necessary for energy production and the formation of the green pigment chlorophyll. Additionally, the micronutrient manganese is used by plants during photosynthesis, respiration, and nitrogen assimilation. Kelp is a superb source of organic nutrients to fuel these processes.

Kelp offers more than just nutrients. The seaweed also helps to stimulate beneficial microorganisms in the soil. These life forms form synergistic relationships with plants. In return for sugary exudates, they break down organic matter and help their plant allies access vital nutrients.

Kelp also helps to regulate soil pH, an essential variable during cannabis cultivation. Cannabis needs a soil pH of between 6–7 to grow correctly. The roots won’t be able to effectively uptake nutrients if the pH fluctuates above or below this sweet spot. Kelp’s vast supply of beneficial molecules also means the seaweed helps plants during the germination process and allows them to develop sturdy and robust stems and branches.


Kelp is available in several forms. Growers can add liquid kelp during watering. Simply pour around two teaspoons into five litres of water. Liquid kelp also boasts the presence of growth hormones that promote the development of healthy roots.

Kelp meal is a second form of the supplement. Add it to the soil at a volume of 0.5kg/9m². It will take around four months for the nutrients to release into the soil, so add it well before the outdoor growing season commences.

Finally, kelp powder can be used to make a foliar spray as the nutrients are directly available for absorption. Add half a teaspoon of powder into 5 litres of water and spray away!


Considering you live relatively close to the ocean, the easiest way to obtain seaweed is to forage it for yourself. This eliminates any cost and is a perfect chance to go and immerse yourself in nature for a while.

Before you do so though, it’s worth doing some quick research into the area you plan to forage at. Taking seaweed from a polluted area is not advised, as this may introduce undesirable substances into your garden. It’s also worth checking if there are any restrictions in place that prohibit taking seaweed from the local environment.

If you’re in the clear, head to the beach with a bag and collect a bounty. Use a species guide to identify what you take. Kelp is a very popular choice among gardeners, along with knotted wrack.

Once harvested, head home and give your bounty a wash. Next, it’s time to decide what to do with it.

Seaweed As A Fertilizer Of Soil For Organic Cultivation


Most seaweeds can be directly applied to the soil via mulching and will break down quite fast. Simply layer seaweed over the top of your soil and let nature takes its course.


Given all of the nutritional benefits, it's quite obvious that seaweed will make an excellent addition to any compost bin. Be sure to mix in the seaweed well with the rest of the materials to prevent any slime from reducing airflow.


Liquid fertiliser is very easy to make and extremely easy to apply to plants. It can also be made highly concentrated, so less volume needs to be used at a time. Simply add your seaweed and other composting material into a barrel with a small amount of water, and wait for it to break down over the course of several months.

Once ready, it can be sprayed onto your crops to help defend against disease and to assist during the flowering phase.

External Resources:
  1. Seaweed Vitamins as Nutraceuticals https://www.sciencedirect.com
  2. Desiccation stress and tolerance in green algae: consequences for ultrastructure, physiological and molecular mechanisms https://www.ncbi.nlm.nih.gov
  3. Las algas como alternativa natural para la producción de diferentes cultivos http://scielo.sld.cu
This content is for educational purposes only. The information provided is derived from research gathered from external sources.

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