Liquid fish and liquid kelp get grouped together so often, sold side by side, sprayed in the same tank mix, mentioned in the same breath, that it's easy to assume they're doing the same job. They're not. Fish fertilizer feeds the plant with nutrients. Kelp does something different entirely: it delivers naturally occurring plant growth compounds that influence how the plant grows, roots, and handles stress, largely independent of the nutrient content it also provides.
Understanding that distinction matters for building an effective liquid program, since the two products solve different problems and often work best applied together rather than as substitutes for one another.
What Liquid Kelp Actually Is
Liquid kelp fertilizer is made from cold-processed seaweed, most commonly Ascophyllum nodosum, a brown algae harvested along northern coastlines and processed into a liquid concentrate. The cold-processing method matters, since it preserves the naturally occurring compounds in the seaweed that would otherwise break down under high heat extraction methods, which is part of why processing quality varies meaningfully between kelp products on the market.
Unlike fish fertilizer, which is valued primarily for its nitrogen, phosphorus, potassium, and amino acid content, kelp's real value comes from something else: the natural plant growth hormones it contains, cytokinins, auxins, and gibberellins among them, along with a broad spectrum of trace minerals absorbed from seawater over the plant's growth.
Fish Feeds the Plant, Kelp Directs Growth
This is the core distinction worth understanding. Our guide on fish fertilizer application rates covers how liquid fish functions as a nutrient-dense input, delivering the nitrogen and organic matter that supports overall plant growth and soil biology much the way a traditional fertilizer would, just in organic, liquid form.
Kelp works through a different pathway. Its growth hormone content doesn't feed the plant in the traditional nutritional sense so much as it signals and directs growth processes already underway: cytokinins support cell division and root branching, auxins influence root initiation and elongation, and gibberellins play a role in stem elongation and seed germination. This is why kelp is often described as a biostimulant rather than a fertilizer in the traditional sense, even though it does carry some nutrient value as well.
