Duff is the natural soil of much of Puget Sound country. According to a Puget Sound Action Team report, "Undisturbed sites in the Puget Sound Lowland area consist of up to 3.5 feet of forest duff soil." This soil is so unlike the garden soil I'm working to improve around the house that I'm told that Puget Sound had no red worms in the soil when Doc Maynard came to town; decomposition into soil was handled by a huge array of native bugs, such as earwigs.
I was raking the yard and got quite a pile of needles etc that I was hauling to the back through the narrow north yard, when I thought, "Why not leave some here?" The north yard is low, and over the course of a few years, I can bring it up level with the house windows to improve insultation, plus eliminate an unneeded step-down from the front yard. I poured the first few gallons out and thought nothing of it.
I gradually added to the pile and as it started spreading east, I decided to try making the entire north yard a duff yard. This would free me from some mowing, which is nice, but more importantly, it might improve the biodiversity of our little plot of land. I'm not sure what sort of microcritters prefer Puget Sound duff to a grass lawn, but there has to be something.
The photo shows the results so far. Interestingly enough, the color scheme reminds me of the bark chips that some people like to put down in lieu of grass. However, bark chips are no fun to walk on, whereas this needle duff has the comfortable springiness I remember from camping.