Fertilizer & Nutrition

Kelp Has Earned a Special Classification

Kelp (Ascophyllum nodosum), commonly referred to as Norwegian Sea kelp, is a perennial brown algae or seaweed that grows in the cold waters of the north Atlantic Ocean. It is commonly found around the coasts of Norway and down through the coast of Northern Europe on the east. And on the west is stretches from the northeastern coast of Canada down the North American coast line as far south as New York. The kelp species Ascophyllum nodosum is not the only type of seaweed used in gardening. There are several products on the market today made with different species of kelp such as Nereocystis, more commonly known as Pacific Bull Kelp, however the Ascophyllum nodosum species is the most widely used and nearly all of the testing and studies published on the use of kelp for crop production in the last 40 years have used this type. Therefore, this article will be focused upon only this one species.

Kelp’s many uses range from food to fertilizer.

People all over the world use Ascophyllum nodosum kelp in many different ways and most of the uses have a common thread: to help improve either plant or animal (including human) physical well being. Some common uses of kelp include food, fertilizer, soil conditioners, animal feed, skin and hair products and nutritional supplements. The reason kelp is so widely used and valued is because of its ability to accumulate minerals and nutrients in its stems and leaves as it more or less filters the surrounding seawater. Ascophyllum nodosum kelp has been found to contain a bounty of natural components that can be beneficial to plant growth. It contains the macro elements Nitrogen (N), Phosphorus (P) and Potash (K) as well as numerous micro elements such as Iron (Fe), Manganese (Mn) and Zinc (Zn), but these elemental nutrient levels are at relatively low percentages. Research has shown that kelp also contains natural plant growth hormones called auxins, gibberellins and cytokinins in addition to amino acids and vitamins including vitamin B1. The fact that all of these different materials are contained in one natural package that can be continuously harvested in a sustainable fashion is what has led to the popularity of sea kelp not only for human usage but in gardening as well.

Kelp is often referred to as a “sea plant” however this is unintentionally misleading because Ascophyllum nodosum kelp is neither plant nor animal, it belongs to its own special classification group called Algae. The main characteristic that separates kelp from the plant kingdom is the fact that it has no roots. Kelp remains anchored to rocks and other solid substrates with the help of root-like appendages called holdfasts, which do not absorb water or nutrients. Ascophyllum nodosum kelp does not have the required tissues to move water and nutrients from one location to another (called translocation in vascular plants) so it absorbs minerals and vitamins from the surrounding waters through both its fronds (leaves) and stems. The collective run-off from the entire world ends up in the ocean so it contains all known mineral elements needed for plant growth and the kelp will absorb them all.

The presence of Ascophyllum nodosum kelp is usually an indicator of clean, unpolluted water as kelp is very sensitive to what is contained in the surrounding waters and will not populate an area that has low to poor quality waters that may contain harmful contaminates. Something that is rather interesting about kelp is the fact that the whole plant, both stems and fronds, can perform photosynthesis so it requires as much sunlight as it can possibly get. To assist in the acquisition of sunlight all of the fronds are equipped with circular air bladders at their base, near the stem. Kelp grows in what is referred to as the mid-intertidal zone, where the shifting tides alter the depth of the water drastically each day. When the tides come rolling in the air bladders raise the entire plant closer to the surface of the water where the penetration of sunlight is the strongest. During low tides kelp collapses into a wadded mess on the sea floor but when they are floating tall (around 10ft.) and fully extended it is a marvelous sight to see, one that Charles Darwin once described as “great aquatic forests”. This description of a plot of kelp is not just fun to imagine but it is also down right accurate.  Kelp “forests” provide a wonderful habitat to many different types of sea life which means it is especially important  that harvesting it is done carefully and in as sustainable a way as possible. Ascophyllum nodosum kelp can be harvested by hand from the shore as broken pieces often wash in with the tide but the commercial harvesting of kelp is done by boat. When the kelp reaches a mature size it is harvested by cutting the stem about a foot above the holdfasts. This is done to allow space for the kelp to create new growth. Ascophyllum nodosum kelp, as well as other varieties, is fast growing and within 3-4 years it can reach a mature state to be harvested again. The ability to grow at a rapid rate is advantageous to the kelp because at times the onslaught of waves is so intense and unrelenting that they can leave the kelp battered or broken, the capacity to generate new growth quickly helps to ensure its survival.  After harvesting, the kelp is rinsed with water to remove excess sea salts and put through a manufacturing process.

Commercially, Ascophyllum nodosum kelp is available in a few different forms. Perhaps the most popular being a concentrated liquid form that is easily mixed with water. Kelp in this form is easy to mix and use, allows a grower to use as much or as little as is desired in a solution, has a pleasant aquatic smell and no potentially irritating dust.

Liquid kelp concentrates are created through an extraction process called hydrolysis. Hydrolysis extractions involve the use of high temperatures and usually the addition of a chemical like Potassium Hydroxide to achieve such levels of concentration. Some growers do not use liquid kelp concentrates and often cite the manufacturing process as a reason, due to a belief that the high heats and the chemical additions harm the beneficial aspects of the kelp, in turn creating an inferior product. However the majority of the research available has been done using a liquid concentrate of Ascophyllum nodosum kelp, so that topic is still being dated.

Liquid kelp concentrates have an NPK analysis that is low, often less than 1% of nitrogen (N) and phosphorous, making it ineffective in terms of a complete fertilizer. Ascophyllum nodosum kelp is also available as a water soluble powder. By boiling the kelp and then allowing any remaining moisture to evaporate the end product is dry, powdered kelp that is water soluble and can be mixed at the desired concentration rate. This type of kelp, though relatively easy to use, can be extremely dusty, especially when handled in larger quantities. So it may be a wise decision to utilize the assistance of a dust mask or some other form of additional ventilation when using it. Kelp can also be purchased in a dry non-water soluble form called kelp meal. Kelp meal is created by first rinsing the harvested kelp with purified water to remove excess salt, allowing it to completely air dry and then crushing it into finer particles. When mixed into the soil or growing medium it acts as both a fertilizer and a soil amendment. Kelp meal generally has a higher NPK value than the liquid and water soluble concentrates but the nutrients are slow to release due to their organic form. Kelp meal can be moderately dusty so a dust mask may be of need when spreading or mixing.

The newest form of Ascophyllum nodosum kelp that has gained popularity in the last 10 years is an interesting product called “cold-processed” kelp. Cold processed, or enzymatically digested, is a liquid concentrate that is manufactured without the use of heat and harsh chemicals, presumably helping to better preserve and maintain higher levels of minerals, vitamins and growth stimulators. Little field testing has been done utilizing this form of kelp thus far so it is difficult to judge whether it is more effective with regards to influencing better plant growth than the other liquid concentrate form.

For years growers have seen and experienced the benefits of using Ascophyllum nodosum kelp in both small and large scale gardening or farming. Many of which have been tested, to varying extents, in a research setting. The important thing to remember when it comes to examining field testing and research is the fact that researchers may have only had the means to run the test one or two times due to the amount of time being devoted and high costs that are inherent to testing. With that said, if the documented results come from a test that was performed only once, there is no guarantee that the same results will occur in subsequent tests.  This may be something that should be kept in mind when addressing the many possible benefits of kelp.

Possible Benefits of Kelp

Increased or More Rapid Germination of Seeds

Some research has shown that soaking seeds in a kelp solution prior to planting can speed up the germination process as well as help a grower to reach a higher successful germination percentage. This result is contributed mainly to the plant growth hormone gibberellins which play an important role in triggering an exit from seed dormancy to germination.

Aid in the Rooting of Cuttings and Increasing the Root Mass of Seedlings

The plant growth hormones called auxins and the vitamin B1 found in kelp can help stimulate the growth of roots on a cutting or seedling. For cuttings, simply dip the cut end in a kelp solution before planting or placing into a propagation system. For seedlings, prepare a light solution of kelp and water and apply directly to the rooting zone when the plant has at least two or three true leaves.

Lessen Plant Loss due to Transplanting

Dipping the roots of a seedling in a kelp solution can reduce transplant shock. By reducing the shock of transplanting a grower can potentially have less plant failure due to the stresses put on a plant during this critical phase, resulting in higher transplant success rates and overall better early growth and performance.

Better Resistance to Pests and Fungal Attacks

Plants treated with kelp during the early vegetative growth cycle have been found to be more resistant to attacks from insects such as aphids and mites as well as some molds and fungi.

Increased Bud Production in Flowering Plants

Applying kelp, often through foliar feeding, when a plant is transitioning into its flowering stage, may not only help to induce the flowering cycle more quickly but may also encourage heavier lateral flower bud formation.

Better Fruit Set and Stronger Fruits and Flowers

Kelp has been shown, likely due to the presence of plant growth hormones in addition to its potassium (K) content, to increase successful fruit set and help prevent fruit drop off. Applying kelp during fruit and flower development can result in firmer fruits that have a longer shelf life when harvested. Foliar feeding with kelp late in the season, when the temperatures are beginning to drop, can also be a way to provide maturing fruits with some protection against the cold. 

Ascophyllum nodosum kelp, in all its forms, can generally be used throughout the entire plant growth cycle. Its relatively low mineral nutrient content makes it an excellent supplement to both organic and synthetic fertilizer programs. Kelp’s organic composition also makes it a good addition to nearly any soil or compost pile as it will act as a food or energy source for both worms and micro-organisms. Its ability to stimulate biological activity within the soil makes it a beneficial addition for gardeners and in landscaping and turf management as well.

I encourage any grower or gardener that has yet to try Ascophyllum nodosum kelp in their garden to give it a shot and see if it is a good fit in their current fertility program. The odds are it will be.

Kyle Ladenburger is an avid indoor and outdoor gardener and a MyGardenAndGreenhouse.com contributing editor.

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