Soils vary. Sample your soil and have it tested to see what, if anything, your particular site could use.

According to my father, manure is the best fertilizer, the more - the better. He harvests at least 900-1000 pounds from each of his 35-year old standard apple trees every other year so I would trust his opinion.

What type of manure does you father use?

Aged cow manure only. He adds manure early in the spring (when soil is workable), in the late July and in October, digging it to the soil. He also "overwaters" trees once a month if it's dry or twice a month during spring and summer if winter was very cold.

Like all nutrient sources must be used in conjunction with soil testing to know what effects it may be having. Otherwise, it's a crap shoot (especially if you use a manure gun). Father's property could have very different nutrient situation from flowerthyme, it's not even in the same part of the country. Some lots have more than one kind of soil right on the same parcel.

Italian prunes are not that fussy. In fact few plants are so fussy about fertilizer that they must have one particular kind or else. How on earth would anything survive if it were?

It would help to know where you are in more detail than just 'NW'. NW what? If you're on glacial soil common around Puget Sound that's one thing, but the alluvial soils of the Willamette Valley are another. Still, manure and compost are the best all around fertilizers for non fussy plants like fruit trees, no matter what the soil (well ok maybe not peat soils). My fruit trees when I had an orchard loved composted chicken bedding, which was convenient because I also had chickens. The best manures in no order are chicken (aged only!), llama, rabbit, alpaca and horse. Cow manure is fine because it's so common and cheap but it does tend to be salty. Not a problem on glaciated soils but definitely it is on clays. People do say they use llama and alpaca manures fresh with no problems. Have not tried it myself. Mushroom compost is great, it's well aged cow manure, blended with composted straw, that was used to grow shrooms in. Fantastic stuff.

In pretty much any soil, nitrogen is the main limiting factor to plant growth. Nitrogen washes away and volatilizes off quickly, plus it's the one that plants need the most of. Beyond that you should have some idea what your soil actually needs - some are deficient in phosphorus, others not; or iron, or magnesium, or whatever. There you need a soil test. But you know you need nitrogen, that's a given. Just how much is the question. Soils high in organic content don't need so much or often, and some plants 'eat' more than others. Lean glacial soils have very little organic content so need plenty of nitrogen, preferably in the form of organic content so it doesn't wash right out. That's where manures and composts come in.

My fruit trees, on glacial gravelly soil, were mulched with cardboard and several inches of either mushroom compost or composted chicken bedding every year. Unless you have very lean soil, that should be fine. The roses needed more than that because they are fussy - very heavy feeders. Not so with fruit trees, the less fussing you do the better they are.