Kelp is a popular plant additive and understanding what it is and how it benefits plants can help a gardener make the best use of it.
Kelp (seaweed) are types of algae that resemble underwater plants. Much like vascular terrestrial (land) plants they grow in plant-like structures, use photosynthesis, and absorb nutrients through a nutrient solution (the seawater they are submerged in). Compared to a plant; a kelp thallus has fronds or blades instead of leaves, a stipe instead of a stem, and a holdfast instead of roots, and their internal structure is much simpler and less specialized. The holdfast (literally “hold fast”) of a kelp plant is only used to anchor the plant in place, and nutrients are absorbed through the fronds (leaf analogs).
Because kelp relies on photosynthesis, it only grows in nutrient rich water shallow enough to receive needed light (depths of less than 100 feet or so). Kelp is primarily found along the coastlines of temperate oceans. One common Atlantic kelp is named Ascophyllum nodosum which is a brown algae kelp that uses air bladders to maintain the buoyancy of the upwardly growing stipes. This kelp is one frequently used as a plant nutrient source.
Kelp forests supply food and protection for a wide variety of species, and the health of the forest is reflected in the health of the creatures that live on and around it. Sea urchins are herbivores that graze on debris from the forest, and predators such as sea otters eat the sea urchins. If the predator is removed (such as when sea otters are over hunted by people) then the sea urchins will overpopulate and graze on the kelp holdfasts, unanchoring the kelp. If left unchecked long enough, the urchin population will destroy the kelp faster than the kelp can repopulate, eventually converting the forest into an urchin barren (a sea desert with sea urchins but without kelp).
Fortunately, kelp grows fast, and popular varieties can be farmed. The tops of ascophyllum nodosum can be harvested and it will regrow from the holdfast and remaining stipe, allowing for many harvestings from the same kelp if allowed to regrow between harvests. Modern kelp farming techniques use mechanical harvesters that intentionally leave a substantial portion behind so the same area can regenerate and be re-harvested as soon as the following year. This practice is both more sustainable and productive than the previous hand harvest method which had more in common with the urchin mentality of leaving little behind for later until nothing is left.
Kelp plants feed by drawing nutrients out of the seawater they are submerged in. If they were true plants they would be the ultimate foliar feeders, as not only do they absorb their nutrient solution through their fronds, but tolerate being submerged and left in their nutrient solution as an application method. As the kelp draws nutrients from the seawater into itself, it helps purify the water. If the kelp is removed, it is itself a source of the nutrients it has collected, much in the same way terrestrial plants can be a source of nutrition for other terrestrial plants through grinding into meals or composting. The harvested kelp can be processed and used to feed garden plants.
Kelp doesn’t have high concentrations of nitrogen, phosphorus or potassium, and is generally not used as a major source of those plant nutrients. It does however have a wide spectrum of micronutrients and some key growth hormones which contribute to its reputation as a superior general plant tonic.
Since kelp supplies a small amount of many micronutrients it can be used to prevent or correct a variety of different micronutrient deficiencies. If a micronutrient deficiency is suspected, an application of kelp may well correct it even if the particular deficiency is undiagnosed, and including kelp in a feeding regimen may well prevent such deficiencies from happening in the first place. Many plant processes rely on trace amounts of micronutrients to act as catalysts for chemical reactions. Only a trace amount is needed since catalysts are not used up by their reactions, and kelp is a trace source of many needed catalysts. Kelp also contains amino acids which help chelate these micronutrients into forms available to plants.
Kelp is also a source of cytokinins (CK) which are are plant hormones that promote cell division in growing shoots. Added to the soil or sprayed on the plants, cytokinins help the plant make efficient use of existing nutrients and water in drought conditions. A low ratio of auxins (root growth hormones) to cytokinins (cell growth hormones) stimulates and promotes tip and shoot formation. Kelp has both auxins and cytokinins, but generally in a ratio that favors the cytokinins.
Another hormone found in kelp is gibberellin (also known as gibberellic acid or GA), which encourage stem growth and elongation. At high concentrations gibberellin can be used to encourage germination of seeds or manipulate gender expression, but at the levels found in kelp it is generally considered a growth stimulant.
Kelp has been used to improve plant growth for so long the origin is lost to history, but as understanding of how it helps improves, so does the ability to use it best.