Grandpa's "long winded" answer:
Grandpa was born in the days when horses were the main form of power on the farm. Grandpa liked horses and other farm animals, but his real love was fruit growing. One of the by-products of horses and cows of course was manure, which his father used to spread in the fields and orchards as fertilizer. Grandpa was still young when tractors started to displace horses, and this valuable organic fertilizer became less available. In his later years, we remember Grandpa always trying to find and use manure whenever he could get ahold of it. We weren't sure if it was just the "fragrance" that Grandpa liked or if he actually thought that it was more beneficial than chemical fertilizers.
When Grandpa couldn't get manure, he did the next best horticultural practices and he planted many "green manure" crops which helped build the organic matter in the soil. Grandpa believed in great believed in lots of organic matter, and never "pooh-poohed" its benefits. However, Grandpa never used manure as a back filler in the hole when planting fruit trees or in fact anything other than the natural soil.
What to fill the hole with. Lot's of excellent horticultural "experts" tell you fill the hole with peat or a really good black dirt, sometimes even with well composted manure. Grandpa always thought it is a waste of time, money, and effort. The roots on that newly planted tree are supposed to grow out of the hole and spread into the neighborhood! Sometimes adding a different kind of soil into the hole creates an artificial environment, like trying to grow your tree in a pot. Your tree is eventually going to spread its roots way out into the rest of your site, so encourage it to do so right away. Use the good topsoil you dug up as the filler, and if you run short use a little bit of some from the immediate area. Try to not use too much sub-soil, because it is poor in nutrients and organic matter.
Some soil amendments and additives can cause real problems and burn the roots of newly planted trees. Fresh manures have lots of "hot" organic things in them, besides the interesting smell. Organic additives are good, though, for the orchard soil and site. If you have a very sandy soil, poor in organic matter and water holding capability, then soil amendments should be incorporated and thoroughly mixed with the soil, prior to planting the orchard. If your soil tests indicate a pH problem that you need to correct, now is also a good time to improve the organic matter in the soil. Lime and other additives should be well incorporated into the soil for best effect.
Once you have the tree planted, you will not be able to as easily adjust the deficiencies that your soil may have, until the next planting cycle years later. If you have a season or too to spare (not likely if you are in a hurry to pick home grown fruit!) then "green manure" or "cover crops" such as rye, oats, Sudan grass and other grasses that grow vigorously and then can be turned under really help to build organic matter over time. If you don't have a lot of time, but believe you need to increase organic matter then there are lots of sources. These are good ones to use without having to be too concerned about other affects:
• Well composted leaves and grass.
• Well composted and aged manure.
• Fresh manure, if you can wait a while for the ammonia and higher salt levels to leach away with some additional time. Use common sense and don't use too much fresh manure right out of the critter. (E. coli is also a concern with many manures!) These are sources are ones which can also be good, but you need to consider their potential deleterious affects on the nitrogen level in the soil. Add extra nitrogen to hasten the decomposition process.
Most of these rapidly "mine" rob the available nitrogen from the soil as they are decomposing:
• Fresh leaves and grass right from the lawn. Avoid oak, walnut and other nut tree leaves as they are high in tannin and other compounds that tend to inhibit growth of other plants.
• Bark and wood chips and sawdust--- these require a lot of nitrogen as they are decomposing. Use sparingly.
The "Short and Sweet" from Grandpa:
• Grandpa grew up with horses, but never threw horse "poo" or other manure because it could burn the roots.
• Many soil additives are good, but work best when thoroughly incorporated into the planting site before planting.
• Some soil additives with a high cellulose and wood content will require extra nitrogen to break down and decompose.
Grandpa's "long winded" answer: