I would think one has to see how your soil is and how your trees look. If you have good growth and trees look healthy every year then not much to worry with fertilizer.
If you access to compost or old manure you can give them a little boost if need to.

If your soil is healthy, with plenty of humus it will also contain a food web of bacteria, fungus and micro organisms, supporting a good worm population. Vegetative matter in the soil is continuously being consumed by the soil life, and should also be added in the form of mulch regularly. If this is done no synthetic fertilizer is ever needed, and if used, has a negative effect on the soil life. Al

I agree with Calistoga. maintaining the organics in the soil is the best option. People worry far too much about feeding the plant when they should instead just feed and nurture the soil.

Here in the humid warm south with our 300+ day growing season that is a challenge as mulch breaks down so quickly. Here people say the sugar sand eats the organics or that the organics fall right through the sand. Neither of those are correct of course. The long warm moist growing season just lets the bacteria work full tilt. Ten inches of mulch a year here disappears. Over the course of 100's of plants that is I have no idea how many tons of wood chips a year. Mulching is by far the biggest draw on my time.

My experience is with growing blueberry shrubs. Each shrub is heavily mulched with about 6 inches of shredded tree leaves every year. In the spring, each shrub gets 2 or 3 applications of Schultz plant food, one tablespoon, dissolved in 4 gallons of tap water. These applications are in March, April, and May, depending on when the ground is free of frost. Agricultural sulfur is used to maintain soil pH around 4.5. Other than that, the shrubs get watered as necessary. They are healthy and vigorous. The oldest shrub is now 18 years.

Herbivory can deplete the soil, and excessive crops can deplete the tree. Herbivory is you eating all that K in the fruit and not putting it back, or perhaps raking all the leaves in Fall to minimize disease, but failing to put back the equivalent N.

Yes, most of the time all you need is compost (and thinning, to spare the tree), which is organic matter with about 50% C and 50% N removed. Some N is put back into the soil by the N-cycle, but to me it is fairly obvious that in marginal soils (sandy, or shallow, or both) you have to replenish the long term nutrient levels, specially N (which volatilizes) and K (which is present in large amounts in fruits).

Forgive me if I sound naive but would simply planting several crops of green manure during the growing season and bringing in leaves in the fall be adequate? I have not had my soil tested but I did do a soil-in-water test as described by Steve Solomon (I think) and my soil is very high in clay (and rocks). I have focused this year on getting the soil to the point I could use a hoe but before that it was not possible. I had to rent a commercial machine to dig the holes for my trees. Every year the soil improves a little so I know eventually it will be good. I got a rototiller this year and have really been happy with how the soil has improved.

I am reticent to bring too much into the orchard because I don't want weed seeds or diseases I don't already have.

Green manure (it needs to be legumes, not oats) will bring N but not K or other micronutrients. Not sure how to advise you re: diseases. I am partial to wood chips, but I am the first to say that pathogens can come in the chips. I guess that there is more to gain in having a strong tree, than to lose on the chance of disease, and pile on the chips.

A second for the chips. If you are worried about disease try to get chips from the company that trims for your electric company. Tree companies that take down trees deal with a lot of dead dying and diseased trees. The trims from around your electric lines are more likely to come from healthy trees. Another advantage is the trims are mostly smaller limbs and the smaller limbs and leaves are just better as they contain more nitrogen and minerals.

Email Bob Purvis and ask for his fertilizer recommendations. He seems to have a handle on things and has captured your imagination. Find a guru, stick with his advice and follow through all the way with it. Why gather up a hodgepodge of information from a number of different resources that are marginally helpful and lacking accountability?

Speaking only for myself using backyard orchard culture principles, I am concerned with controlling vigor by managing water & fertilizer, and by pruning for size control. For the most part I apply low nitrogen fertilizers (composted steer manure, compost, and/or fish emulsion) in the spring, and keep the trees well mulched through out the year. See the video below starting at 14:24.

Here is a link that might be useful: Backyard Fruit Tree Basics

Milehigh, all of these opinions are great and all become options as to what you have available in your area. I happen to like all of the different opinions. I usually select one or two suggestions and go with them. This is a great forum, which is why opinions rule! I am following this thread as I have the same soil concerns as do you. Mrs. G

I think it really depends on your mindset, as well as local conditions. Get to know your soil, get it tested, and read up on "prefered" soil compositions for each tree (including rootstock).

I think the problem with most places maybe they dont think of long term fertilizing (as in building soil). Tossing manure and compost on will help for sure.

Lots of people are starting to get into cover crops. These are sown, and sheered twice a year, then lightly raked into the soil. These are ususally nitrogen fixing plants (legumes for example). Ontario fruit farmers use white clover as a perrenial cover crop and cool weather grains in winter. These build the soil every year, slowly but surely. Most will attract beneficial insects as well.

Milehigh, given your clear interest in taking your fruit tree gardening to a high level of understanding I suggest you begin dealing with nutrient issues in a professional way. The first step is a complete soil test which would be a test that includes separate top and subsoil analysis. .

If there is a particular concern about certain trees in your orchard because of a possible deficiency that isn't explained in the soil you can move up to leaf analysis.

You can find a reputable lab through your county extension and they will provide you with recommendations you can follow for amending the soil and providing essential nutrients on a regular basis. I agree with others here that those nutrients would probably be present in sufficient quantity in the right mulch and/or compost, if that's how you want to go, and adding organic matter is generally beneficial.

I don't agree that all fruit trees need to have replaced whatever is removed, such as potassium, in every soil, as sometimes there are reserves that can last for many decades. The point is that you can't know what a soil needs just by what you take out of it.

Purvis's analysis doesn't seem terribly useful to me in itself and I've never heard the theory of a tree dying because it has used up available nutrients in the vicinity of its roots even if it is logical speculation. I've certainly seen apple trees survive for well over a century in areas of complete neglect and strong competition from larger trees. At any rate,valid research on fruit tree nutrition is generally about productivity of high quality fruit and not tree longevity.

A tree kept moderately vigorous and protected from serious leaf and root damage by pests will probably live as long as the environment allows it. I've never seen a tree die because of a known nutrient deficiency besides maybe a radical pH issue.

Interestingly, some of the longest lived oaks in the world are ones that have been pruned annually over the centuries to harvest wood for tool handles. This meant the trees were inadvertently pruned for stoutness. Fruit trees are usually maintained to a similar stoutness.