How Nutrients from the Ocean Help Your Garden

By Barbara H. Shaw
Published: September 26, 2017 | Last updated: September 26, 2017 06:19:24
Key Takeaways

Many believe that all life began in the sea, so it makes sense that minerals and nutrients from the sea are beneficial to land-based organisms today, including your plants. Here's how ocean-based nutrients help your garden grow.

The oceans are exceedingly rich in minerals and organic nutrients. Over the past few billion years, zillions of cubic kilometers of minerals weathered from rocks and megatons of organic residues washed down rivers and into the planet’s seas. Want rare earths and trace minerals? Aluminum and boron to yttrium and zinc—every one of the elements on the periodic table is found in any sample of ocean water.


Even a small quantity of mineral-rich water right from the surf has long been known to enhance the soil and promote growth of its life-giving microbes. Fish, shellfish, and seaweed have enriched fields and gardens for millennia. European settlers in Massachusetts learned from the locals to add a small fish to each hill of corn seeds for a bountiful crop. Today, waste nutrients from fish processing make rich fertilizers.

Ocean Minerals

Seawater holds a mix of minerals life needs to thrive. But at first glance, objections may arise. Isn’t the ocean too salty? Isn’t it badly polluted? Isn’t ocean water free to anyone?


Saltiness objection countered: The minerals in ocean water buffer the sodium in the salt. Sea mineral makeup is remarkably close to that of blood, buffered to neutral pH. Having rescued used blood from a medical lab where I worked, I know it’s a fabulous fertilizer. Besides, most producers of sea minerals reduce the sodium content of their product.

What about pollution? Plenty of pristine waters remain, far from major industrial and urban areas, and that’s where producers harvest. In addition, they use GPS to find the cleanest, most nutrient-dense waters.

Why buy what’s free, some may ask. Water is heavy and moving it costs plenty. Besides, most of us don’t live close to the ocean. Harvesters concentrate the good stuff to save us a lot of effort. Though many do neutralize the salt in their product, research on adding seawater to plant media without salt removal showed no problems due to sodium. But removal of sodium allows for higher application rates over a longer period.


Early experiments showed that a single application of a liter of seawater per square foot of garden would last five years. Later, studies demonstrated astounding increases in bacterial populations when sufficient minerals were provided.

Shellfish Waste

The edible shellfish include soft-bodied mollusks, such as clams, scallops, and oysters with hard calcareous shells, and crustaceans, a class of arthropod that includes crabs, shrimp, and lobsters. Crustaceans have jointed legs and a carapace or external skeleton largely of chitin.


Oyster shells act faster than limestone to raise soil pH and come with lots of trace elements. Crushed oyster shell has the following gross analysis: 56 per cent carbonate, 37.7 per cent calcium, 1.2 per cent magnesium, 0.73 per cent sulfur, 0.57 per cent sodium, and 0.21 per cent phosphorus. These amounts can vary.

Overuse of seashells can cause problems, which is why rumor has it pretty shells should not sit in potted plants. The sulfur in shells adds acidity, and high calcium may block uptake of magnesium and important trace minerals. We can do harm with indiscriminant use of any fertilizer or amendment, as most gardeners have learned. Get a soil test, so you know where you’re starting.

Crab and lobster shells are an excellent dry organic source of nutrients, including calcium (approximately 12-23 per cent) and magnesium (up to 1 per cent). Of course, they have varying analysis, depending on where it was caught and what kind they are. They also give the roots something to grab onto and wrap around for a food source, creating a deeper root system. The shells also retain moisture in the soil and build the organic matter.

Since crab and lobster shells are high in chitin, they promote the growth of chitin eating bacteria in the soil. This will help eliminate ants, grubs, fungus, and root nematodes because they are all chitin-based in structure. Put around plants, crab and lobster shells will keep slugs and snails off. They also help choke out weeds and keep moles, voles, mice, etc. from digging things up.

Seaweed Fertilizer

The most common seaweed fertilizer is made from Ascophyllum nodosum, commonly known as rock weed. It’s a fast-growing brown alga that’s oxygen rich and exceedingly dense in important elements such as iron, zinc, copper, manganese, and boron, plus 60 to 80 other trace minerals.

Products include kelp meal, soluble seaweed powder, or liquid extract, all containing organic compounds that feed soil bacteria. It’s also a rich source of growth regulators, stimulants, cytokinins, auxins, and gibberelins.

Kelp’s hormones survive processing and trigger elongation and cell division for increased growth in all plants. When applied to soil, seaweed hormones increase root bulk 67-175 per cent. It’s a fine soil conditioner and helps retain moisture in the soil too.

A number of manufacturers produce liquid concentrates ideal for hydroponic growing.

Seaweed extracts used as a regular foliar spray make nutrients available fast and thus quickly reverse nutrient deficiency. Foliar spray also promotes earlier flowering, stronger fruit set, and stronger, faster stem and leaf development. It also gets rid of powdery mildew in days.

The mannitol in seaweeds increases plant nutrient absorption in soil and other growth media and appears to play a role in responses to stress from any source. Other compounds it contains speed breakdown of sugars and carbohydrates and stimulate nitrogen-fixing bacteria in soil, which indirectly increases absorption of nitrogen as well as other nutrients. The result is a boost in plant vitality and a significant increase in yield.

Overall plant protection is another benefit of using seaweed products. Antitoxins they contain repel some insects, as well as mildew and bacteria, by using the same chemical defenses that protect algae from bacteria and virus in the ocean. Because kelp contains compounds that reduce or prevent mold growth, shelf-life increases for fruits and vegetables grown with seaweed enhancement.

Is there a down side? Though fabulous, seaweeds alone don’t provide enough nitrogen or phosphorus for good growth, so that would need to be added. For homesteaders, wood ash not only lowers soil acidity, but also is a good source of phosphorus, plus potassium, calcium, and magnesium. But not all plants like it.

Fish Fertilizers

When starting or renewing beds in my garden, I stop by the local fish market with a couple of large buckets and bring home bony carcasses of wild-caught fish from pristine Alaskan waters, free in late afternoon. We bury the bounty under a knee-deep layer of soil so that raccoons and rodents won’t dig it up. The rich trove of phosphorus, calcium, and magnesium in bones and the nitrogen in flesh do wonders for building soil. Most growers prefer an easier way.

The smell of fish fertilizers puts some people off, but the larger downside can be toxins including mercury and cadmium if the fish came from polluted water, offshore from urban and industrial centers.

Most fish fertilizers are emulsions. Emulsions are made from so-called “trash fish” (fish that Americans won’t eat), usually menhaden. Menhaden are caught in harbors and rivers on the coast and thus are exposed to coastal pollutants.

Also, lake fish used can contain mercury and PCB’s, and they won’t contain the sea minerals and other nutrients found only in saltwater fish. If you see a nitrogen level higher than 2 on the N-P-K, it’s either an emulsion or has been fortified with another nitrogen. Read labels carefully.

The best fish fertilizer is made using a hydrolysis process. This means instead of being cooked, the fish is liquefied by enzymes. They are usually made from several species of edible fish. The gurry (the part that is left after the fillet is removed for human consumption) is used to make these products. You’ll get a full range of nutrients, vitamins, amino acids, enzymes, and growth hormones.

Look for products that use fish that are caught a minimum of three miles from shore. The North Atlantic Ocean is a great source. The hydrolyzed fish products smell much better than emulsions too.

Fish fertilizers make good side dressings, foliar sprays, and irrigation feeds. The nitrogen is less prone to leaching than more soluble forms because it is enzymatically digested and so is more available than the nitrogen in cover crops and compost, and is more readily available.


Share This Article

  • Facebook
  • LinkedIn
  • Twitter

Written by Barbara H. Shaw

Profile Picture of Barbara H. Shaw

Barbara Shaw gardens, writes, and makes junk art in Oregon. She earned degrees in zoology, physiology, and journalism, and writes about science, health, growing things, and energy management. She also delights in reading, cooking, photography, eco-travel and has visited 60 countries. Married to a sports journalist, she embraces being a grandmother.

Related Articles

Go back to top
Maximum Yield Logo

You must be 19 years of age or older to enter this site.

Please confirm your date of birth:

This feature requires cookies to be enabled