In situ composting of seaweed works fine. In fact, rainfall is useful to leach out the salt. It does break down pretty quickly, so won't necessarily add a lot to soil structure. Now, seaweed is pretty bereft of NPK, but it has a lot of other good stuff like trace minerals.

Oh,that is a bit disappointing.I was hoping that it would indeed improve the soil structure (I can dig in a lot) as well as providing a small amount of fertiliser.

Last year I added it to my garden compost heap(weeds plus quite a lot of attached soil scraped up together) and grew things ** directly into that quite successfully despite the v large amount of weed seeds in the mix.

I But I did notice that laying them thick over the raspberry canes has not been successful to date- have given them a very large much if garden compost this year and hope for an improvement in these very old canes next year.

**eg spinach, peas ,fennel,carrots,kale

Adding seaweed is better for soil structure than adding nothing, but the amount of insoluble fiber is pretty low, so it kind of just dissolves. Piles of seaweed will tend to turn into a gel by itself, leading to anaerobic digestion.

Seaweed is great, it's like a multi-vitamin for plants. it also has some beneficial growth hormones that stimulate plant growth.. seaweed by itself doesn't have a lot of mass, so after it decomposes it'll just mostly leave the minerals behind, not a lot to improve the structure of soil unless you compost huge quantities of it.

you're better off planting something like radishes, which will serve both as a cover crop and improve the soil drastically the following year once the radishes die and the roots decompose.. there are specific varieties (daikon) radish that grow like 12in. or more which are even better.

Seaweed attracts worms, and worms will improve your soil structure. I use quite a bit of seaweed---mainly as a mulch layer, but also to give my compost piles a quick fuel.

I can agree that earthworms can improve soil structure by aerating it, but their castings are basically just well-digested organic material. Well digested seaweed doesn't add much to soil in terms of lignins, which is the primary constituent of humus, in the same way that well composted seaweed doesn't have much. That is, compositionally, worms don't add much to what is already there. They'll just speed up the digestion somewhat.

Certainly soil won't be any worse as a result of adding the seaweed. Add some other stuff with more structure if you want.

Yes I see that.Actually I normally do that anyway but sometimes I might have a patch that hasn't been worked for a few years.

I assumed that seaweed would work on it's own and would,as a bonus be weed free but I see now that I will have to keep up the humus content even if I do also use the seaweed.

I wouldn't worry about it too much - seaweed makes it better, if you have other stuff in there, great; if you add that stuff at a different point down the road, also fine. Don't let the perfect be the enemy of the good - especially if you've been adding to the soil in the past, there's something to build on in there anyway.

One way of thinking about it is that seaweed is about 90% water. Most regular compost is more like 50% water. So when you pile on seaweed, you're mostly piling on water. That being said, as noted, seaweed is rich in trace minerals and useful bacteria. So seaweed is good, but I'd hesitate to use it as the only soil amendment.

If you added 5 times more volume of seaweed compared to normal compost might you get a similar amount of humus related benefit plus the added benefit of the trace elements?

Would the soil struggle to accommodate that amount of addition and might that kill the organisms already present?

Also ,what about the kelp stems that I usually just ignore and don't collect ?They are a lot "woodier"

That all sounds sensible, but large quantities of seaweed does present a risk of too much salt. I guess you could wash the seaweed before you dig it in, but if you get a lot of rainfall that might not be necessary. As I said, because of the gel-like behavior of seaweed, anaerobic composting may occur if it isn't well mixed with the soil. That will be a lot slower than regular aerobic composting. I believe many people dig something like straw or leaves in along with the seaweed to ensure air pockets. One big advantage of seaweed is that it will keep the moisture level up in the composting layer.

I would count on the rain to wash off any salt but a quick search indicates that it might not present a problem with the plants I tend to grow such as spinach and kale.

So perhaps a few leaves along with the seaweed and I may have a nice bed to use in the spring.

Sounds like with spinach and kale, you're good to go. Also, you have most of a fall and winter to leach out the salt. Good plan.

I am adding a new comment now as I can't seem to find a way of starting a new thread (is it just me or is that a problem with this site?)

Anyway , I want to ask again whether the woodier stems of the kelp would provide any humus compared to the leaves?

And also might they provide a little more phosphorus than the leaves?

Yes I did throw a lot of it on the compost heap last year and the results were fairly spectacular

I still think I need some sources of phosphorus as my strawberries ,rasps etc have given zilch this year (and hardly better last year)

I am saving up dried banana skin and other similar stuff for next year to give to selected plants at different times and am hoping for better next year