What is this organic matter forming a mulch layer?
It's citrus peel, mostly oranges
Its a new mulch Ive started adding to the garden - orange peel. (And grapefruit peel too).
In gardening books, they tell you not to add orange peels to compost because worms dont eat them and don't like them. Probably true. So people throw them in the bin.
But you can put them on the surface as mulch, and they will rot away. A fungus that produces a black-coloured spot initially and then eats up the whole peel will eat them. For it to go to work, it needs to be wet peel. Watering the peel as part of watering the pot will do this, especially if one waters at the evening for overnight, as we have to here on very hot days (+32deg).
It is remarkably fast decay once the fungus gets going. I think I put this peel in about 2 months ago. The white side goes first then the orange side later.
Citrus peel makes a good mulch, about 1 inch square pieces. You can even arrange them white-side-up to shelter the soil from strong sun, which I have been doing in winter.
That's one less thing in the rubbish, and one more sort of organic matter reentering the system. One less need to buy fertiliser.
The other take away point here is that maybe the fungus works in compost (at least on its side or something) and means the conventional advice is wrong - has any one tried to adapt placement strategies in a compost bins to allow citrus to be integrated? To what extent are gardening books just reiterating what other books have said, I wonder...? (I know it happens to some extent, but things like this do make one wonder...)