Also known as poysulphate thenJuniper is a British fertiliser from up north (I want to say polyhalite, but not certain). You can get it from your Fibrophos supplier, it is sold through Hatcher Fertilisers.
There are two grades, a 13% potash (organic approved) and a 25%, both with lots of sulphur and a few other goodies.
I put a load on back in March for an organic farmer and it spread well. It looks similar to Derbyshire lime, a gritty white dust.
I got home and when servicing the machine there was maybe a couple of kilos stuck in nooks and crannies so I sprinkled it on about a 6m square right in front of our kitchen window on a mowing field. I can see it stands out , no bother, so it works there at least!
Potash has, but there's masses of Polyhalite, sold as polysulphate, there will be more if the other mine gets openedI though the potash had run out there , they are testing phosphate at our farm
If your furthest block gets no/little muck or slurry, does the nearest get too much?
A common problem and if you get high P/K it can cause a lot of problems (plus high P not good for water quality).