A gentler alternative to a chemical source of potash is wood ash, as long as it has no coal in it. We sprinkle it around all our stone-fruit trees, extending to the 'drip line' (how far out the branches reach, which is the same as the roots). Ash from a wood burning stove is perfect, and can also be used as a hen dust bath, around strawbs, onions, tomatoes, and any left overs can be sprinkled across the pasture. Pine needles are supposed to be good around strawbs too.
A liquid feed is inappropriate at this time of year when the trees are dormant, although it's the perfect time to apply well rotted manure.