Wood Ash has an NPK (Nitrogen/Phosphorus/Potassium) of 0-1-3 which may aid you and it is easily available for many folks. I really like the suggestion of utilizing composted or pelleted
Chicken Manure as it may help as well but it also contains its share of nitrogen.
Before you amend anything, let me ask you... how are the plants doing in the orchard? Are the trees having any issues with growth or fruiting? I have seen people get tunnel visioned because a particular soil report says the sample was low in some factors but that doesn't mean the tree's roots don't have access to the nutrients somewhere else. I have had quite a bit of success reading my plants and adjusting if I see any odd symptoms developing. An annual cover of compost does a lot of work for me in my growing spaces with the occasional amendment if the plants seem to be having an issue.
This thread on
Potash might interest you as well.