I have tried the various seaweed extracts and found them quite expensive for what they do, apart from them travelling half the world. If I lived near the coast I would probably make my own if it was legal to take the raw ingredient off the beach.
What I have been doing for the past 3 years is purchasing seaweed meal from the animal feed store and adding it to my worm farms and the nutrients come out in the leachate along with all the enzymes the worms seem to add too. Very nice on everything in the garden.
I've used seaweed meal too and it is good stuff..but, it is used very differently and takes long time to take effect. Although liquid and meal comes from same source, I could not compare them as equals....different end products and different uses.
I used to get some 'meal' from work for nothing and used it lavishly too....then the 'good days' came to end and I started looking for supply to get some more ....OUTCH..buying buy the sackfull was REALLY expensive. I just looked other day and it would cost me about £40-50 per sack..about 20 kg. I used to use it as 'soil conditioner', taking long time to start taking effect as it needed to be digested by the soil creatures first.
Liquid seaweed is slightly cheaper, particularly if bought in larger quantity rather than small retail bottles...and the advantage of the liquid is that it is consentrated, little goes loooong way, more precise to measure, mixable with other 'stuff' to make liquid fertilizer and the effect to the plant/soil is almost instant.
I get some liquid seaweed from work...little 'perk' of mine
, but I wish they would go back using seaweed meal too..using both same time would be even better job