Kelp Fertilizers and Soil Amendments in Organic Gardening

If you’re an organic gardener you have probably seen and used kelp products. They come in a variety of forms from liquid fertilizers to dried kelp meal and powders. So with all of these products out there what benefit do they bring to your garden and are they worth it?
Lets start with taking a look at what benefits Kelp brings to the garden table.
Nitrogen Phosphorus and Potassium are some of the macro nutrients required for plant growth and because they are used in such high quantities are often the nutrients that get depleted in the soil the fastest.
The highest reported Kelp product NPK was Kelp powder. The Colorado State Extensions website reports the NPK at 1 – 0 – 4. [5] A variety of sources state the NPK of the other product as negligible. Often kelp products are mixed with fish products to give them an NPK.
With a low or negligible NPK let’s take a look at the trace elements in Kelp products.
There are generally speaking 18 elements in the soil that are considered to be essential or beneficial to plant growth.[8] The presence or absence of these elements will impact the overall plant growth and production. Of these 18 elements it is common to test for all but Chlorine and Silicon as they are well distributed on earth.
Many species have been used for agricultural purposes as to add essential and beneficial elements. One such species of Kelp was analyzed for trace elements Stoechospermum marginatum. The results are in mg/L and included Copper, Manganese, Zinc, Iron, Potassium, Magnesium, Cobalt, and Sodium. [2] These account for 8 of the 16 essential and beneficial elements required for plant growth that are commonly tested for.
Kelp is commonly marketed not only to add elements to the soil but as having plant growth hormones.
Plant growth hormone can have a number of benefits to vigor and growth in your garden. Most notably is its ability to stimulate root growth. [6] Larger root systems anecdotally result in healthier plants that can resist disease and stress better while producing larger harvests.
Kelp has been found to have high concentrations of plant growth hormone [2]. The hormones found in kelp extracts have been shown to have varying success in aiding the establishment of seedlings and any net gain is lost in the first 5-10 weeks of growth. [6] Over application of the concentrated hormones was also shown to have a net negative effect on seedlings growth.
Kelp products seem to bring some benefit for our gardens. Right? Well I am not sold on the idea of using kelp products in my garden. Let me explain why.
The two main benefits kelp brings to the garden is essential and beneficial elements and plant growth hormones.
Over the last few weeks we have evaluated a number of free and local resources. Specifically fall leaves and comfrey. Both have been shown to have essential and beneficial elements. Fall leaves have 11/16 essential and beneficial elements while comfrey has 15/16. The one element that is missing form all three materials is selenium. As selenium is an essential part of at least 3 amino acids one can assume it is in any soils that have been amended with compost and it is simply below the tests detection limit.[9]
So not only does comfrey have a good NPK in this case fall leaves and comfrey have more trace elements then Kelp does. So what about those plant hormones ?
Earth worms and composting worms provide a number of benefits in your garden. They break down organic material releasing nutrients, beneficial bacteria and plant growth hormones. These rich castings can be made in worm farms or directly in the garden by letting them break down your mulch layer made of things like fall leaves and comfrey among others.
A research paper published in the European Journal of Soil Biology found the same plant growth hormones found in the kelp extracts were also found associated with the humic acid in vermicomposts.[7] So once again a free and local resource has a source of plant growth hormones.

A full transcript including references can be found:
http://www.albertaurbangarden.ca/2015/02/28/kelp-fertilizers-and-soil-amendments-in-organic-gardening/