In big trouble today....keep called people he instead of she. So I apologise now and for repeating it in the future. I am but a simple Cornish yokel you see.
Michael, I used to put it on very thickly on the soil surface in the autumn. it did a few things......it prevented weeds, its texture deterred slugs n snails as well as its saltyness, it provided alginic acid (I think I have that correct) which helped bind together the sandy particles in my soil. It provided nutrients in small quantities too so plants, veg, etc., weren't over fed and it rotted down beautifully come the spring leaving enough material to continue deterring those molluscs.
I didn't wash the salt out of it....islanders everywhere used it as is as we do/ did here without worrying about the salt content. Yes, used it in my compost heaps too....it's texture mixed well with the other materials there. Putting it down in autumn will take some nitrogen from the soil but since few things grow then it did no harm. Come the spring and summer the nitrogen will have been returned to the soil.
It certainly was best soil conditioner I used....and weed free!