from what I understand about seaweed and the seaweed based products you can buy is that they are classed as a plant stimulant rather than a fertilizer. That is to say that the NPK are very low but trace elements are there. If applied by dilute liquid by watering can or superspray hose end sprayer it helps the plant get more out of whats in the soil. A bit like drinking your daily yoghurt drink to keep you in good shape incase infection hits etc.

Seaweed helps build up plants resistance to drought and pest attack.

One should still you base fertilizer or farmyard manure to keep the fertility levels high.