Seaweed as Fertilizer

People have been using kelp and other seaweeds as a fertilizer for thousands of years. Historically, it was collected, dried, and added to the soil by hand.
 
Today’s kelp and seaweed market is a very different critter. You can find seaweed extract, kelp meal, and everything in between.
 
Frequently touted as disease suppressants, fertilizers, and stress reducers for plants, kelp and other seaweeds are also said to contain helpful plant hormones and other biostimulants, but are those claims true?
 
First, let’s learn a little about these underwater plants.

Kelp forest (goodfreephotos.com)

Kelp and other seaweeds
 
There are thousands of different seaweeds, and they are all marine algae. Some seaweeds are microscopic, while others, such as kelp, can create towering underwater forests.
 
If you have ever been to a Southern California beach, you know that kelp is slick and abundant. The leaves look like reddish brown lasagna noodles. And as it grows, kelp absorbs pollution.
 
One common pollutant, nitrogen, happens to be the number one nutrient needed by your garden plants. That should make kelp and other seaweeds excellent sources of plant nutrition. But are they?
 
Kelp claims
 
Many claims have been made about the benefits of seaweed in the garden, including the following:

• Early seed germination
• Significant increases in bean and potato crop sizes
• Suppression of blister disease, clubroot, and many fungal diseases caused by Sclerotinia minor
• Extended post-harvest shelf life

 
In one study, fields treated with kelp fertilizer were found to have slightly higher levels of sulfur. Sulfur has been shown to improve the flavor and quality of crops like tomatoes. The problem with all of these claims is that they are based on laboratory studies and some faulty logic.
 
Just because something works under highly control lab conditions does not mean it will work in your yard, or that sellers are providing you with what they advertise.
 
Quality controls matter
 
Before jumping on the seaweed bandwagon, you should know that there can be significant variability on nutrient content. There is no standardization of contents or effectiveness at this time. Also, kelp holds onto other pollutants besides nitrogen. It can contain undesirable heavy metals along with that NPK.
 
While it is unlikely to occur in your packaged seaweed fertilizer, you should know that rotting seaweed produces a lot of hydrogen sulfide, which is a highly toxic gas that causes diarrhea and vomiting.
 
And we do not yet know how much or which type of seaweed actually benefits our plants. More research is needed.
 
Bottom line, most seaweed extracts are very diluted forms of fertilizer. And they are expensive. Until quality controls are in place, you can get better results with a lab-based soil test and regular fertilizer treatments.