Humans have always preferred to live by the sea.
The ocean offers a bounteous harvest of nutrients, for ourselves and for the soil and plants we cultivate.
Like sea minerals and kelp, fish fertilizer is an excellent source of many nutrients.
As natural, available forms of nitrogen and phosphorus in particular, they’re especially valuable when you don’t have enough humus in your soil yet.
Fish fertilizer products are high in good old NPK – nitrogen, phosphorus, and potassium.
If we believed the line that chemical fertilizer companies try to sell us, we’d think these three nutrients were the only ones that mattered to plants.
Fortunately, we now know that plants need a wide variety of mineral nutrients in specific ratios, as well as healthy soil life, to thrive.
But that doesn’t mean nitrogen, phosphorus and potassium are anything to sneeze at. Having a balanced, continuous supply of these nutrients is essential to plant growth and health.
That’s probably one reason humans have been using fish as a fertilizer at least as far back as the Middle Ages, and possibly even in ancient Egypt. First Peoples of North America buried fish directly in the soil to support their crops.
Besides NPK and many micronutrients, fish also contains important oils, amino acids, vitamins, hormones, and enzymes that support biologically active soil.
Unlike chemical fertilizers, the nutrients held in fish fertilizers are incorporated by the soil microbes and released slowly to plants, rather than leaching away to pollute rivers and lakes.
Types of fish fertilizer
Fish fertilizer is generally available in three forms: meal, emulsion, and hydrolysate.
Fish meal has had the oils removed to use in other products, increase nitrogen content, and keep it from spoiling, so it’s a less complete nutrient source.
The slower spoilage makes it a decent option if you don’t know exactly when you’ll be using it but want to keep some on hand.
Although it’s a solid, fish meal can be mixed into a foliar spray. It’s easier, though, to use one of the liquid forms of fish fertilizer, like an emulsion or hydrolysate.
An emulsion is usually made either from “trash fish,” which are caught only for this purpose and not for human food, or from the by-products of food fish– all the icky bits that people can’t or won’t eat.
After cooking the mixture to kill putrefying bacteria, it’s filtered and stabilized with sulfuric and phosphoric acid.
This process makes it the most soluble fish product, which is convenient for foliar applications. Unfortunately, it can also contain high levels of chlorine from municipal water sources used in processing.
It also means that all the juicy extras that make such a difference to soil and plant health are knocked out by the heat.
So the oils, amino acids, vitamins, hormones, and enzymes are absent in fish emulsion, resulting in a lower quality product compared to hydrolysate.
Hydrolysate is made using enzymes in a cold process, which leaves those beneficial nutrients and biostimulant compounds intact. The oils are especially helpful for supporting fungus, an essential player in the soil community.
Fish hydrolysate is generally made from whole fish – by-catch from food fishing. It’s often less stinky than emulsion, which reduces the risk of attracting annoying little pests like raccoons, or if you live in a rural area of attracting scary, big pests like bears and cougars.
It’s also a little easier on the human nose, and let’s face it: we organic gardeners need to maintain good relations with our neighbors. Even though our healthy compost may smell sweet and earthy rather than foul, fish emulsion doesn’t smell like a rose garden.
All in all, this makes hydrolysate the best option in most situations, since it’s a more complete nutrient source with fewer undesirable side effects.
Both emulsion and hydrolysate are usually stabilized with phosphoric acid. This isn’t necessarily a problem, since plants and soil will generally benefit from the small additional amount of phosphorus, and it’s allowed under organic standards as long as it stays below a certain threshold.
Sustainability is a real concern for anything made from living ocean organisms these days.
The world’s oceans are in a state of near-collapse due to overfishing, destructive harvesting practices, and changing environmental conditions.
This doesn’t mean everything that’s harvested from the sea is necessarily causing further environmental damage.
Some fish products are more sustainable than others, so it’s worth checking out the specifics of the one you’re thinking about buying. If the seller can’t tell you where it comes from, I’d think twice about buying it.
It’s also worth checking whether fish is the only ingredient in a product you’re considering using, since not only beneficial organic additions like seaweed and crab shells, but also non-organic materials are sometimes added to boost NPK.
Using Fish Products
Like all fertilizers, it’s most effective to apply fish products when the microbes are active and able to use the nutrients you’re providing them. This means warm temperatures and sufficient moisture.
Fish fertilizers can be applied directly to soil, or make great additions to foliar spray, especially when mixed with Effective Microorganisms.
Liquid forms can also work well in hydroponic applications. Dilution rates vary widely, so it’s best to check the specifics of the product you’re using.
I sell a high quality liquid fish hydrolysate that's quite sustainably harvested.