The message
from "mark" contains these words:

I've just started a potato barrel.

It is open at the bottom and sitting on wire netting to filter out rats.
I've started it with about 9" of compost then dotted spuds over that about
4" apart.

These have been covered with about 3" more compost.

The compost is from a large and old compost heap. The potatoes are
ones from
the kitchen cupboard that have started sprouting ( no rolling of eyes
please!). I was proposing to build up the compost as the potatoes tops grew.

Potatoes will grow from the stems as they grow, but, if you want a
bumper crop, plant several more layers as the compost level rises.

What nutrients can I add to enhance the crop?

In barrels/tubs I always use tomato fertiliser if I haven't got a
stinking bucket ready.

(Part-fill a bucket with water and toss in weeds as you pull them.
Nettles are best, but anything else does.

Soon, it will pong fit to make you puke. It's ready.)

I have wood ash from a wood burner, would that help if I mixed it with
future additions of compost.

Strawberries, fruit trees, etc might enjoy it, though potash contents in
log ash is rather meagre.

--
Rusty
Men love women, women love children, children love hamsters.
(Alice Thomas Ellis)