I have some fruit trees (apple, pear, orange, pomegranate, grapes) in my small backyard and have questions about taking good care of these fruit trees from a nutritional standpoint. I am wondering when is the best time to fertilize the fruit trees and how many times a year I should be doing this. Also, it would be helpful to know if the same fertilizer can be applied to all the fruit trees and throughout the year (vs. different fertilizer for different seasons).
I use solid fertiliser, once a year in winter, and some time I replace it with some manure.
I don't think there is need of more frequent fertilising: tree have deep roots, so it take time for fertiliser to diffuse, and to the root to pick up. Just select slow fertilisers. (I assume a non sandy ground, which will not block the fertiliser, nor fruit-tree in pot)
I use also generic fruit-tree fertilisers: most of my trees are of the same genus (Prunus) (or anyway family: Rosaceae) so I expect a good ratio. Addisionally I have no monoculture nor a dense orchard, so I expect the tree will get the rest from the soil, without depleting it too much.
My vineyard is different: dense and monoculture, so I make soil tests (every 10 years) and I give it the right amount of fertiliser. The lemon trees in pot also requires some more frequent fertilising and regularly (every few years) change of the ground.
Calculating the right quantities of fertiliser is also complex: it depends also on varieties (if one want to be ultraprecise), the size of trees, the amount of fruits, on how do you prune trees, etc. Additionally, the fertiliser norm expect that do you leave the leaves on the ground (it is practical on big orchards, but better to remove them on small orchard, to handle better the diseases).