At Compost Happens!, Marion Owen writes,
Pound per pound, kelp supplies more minerals than any other material on the planet. In the garden, it also aerates the soil and makes an excellent mulch around potato plants, fruit-bearing shrubs, bulbs and perennials. And, contrary to popular belief, seaweed does not add harmful salts to the garden.
Kelp is what I call a 'neutral' ingredient, in that it doesn't fit in the nitrogen or the carbon category. Yet, it benefits every compost pile by adding fluff.
I wrote to the city of Huntington Beach and asked if I could collect kelp on the beach for my compost. They replied, "You may collect the kelp that washes up onto the sand but not while it is in the water."
Good to know.