I think if you can find a low sodium nitrogen source you can fertilize all summer long. I do but I only use a certain fertilizer and I step it down during the hottest times of the summer.
I believe Ryan explained as the tree transpires it pulls mositure from the roots, when doing this it also pulls in both the good and bad of what is in the soil system as it can’t differentiate or pull just water.
Because we are using a small shallow container there is no real place for the excesses to go ( like a much bigger system with a tree in the ground) so they get trapped in the substrate, etc.
So an excess of salts or other nutrients get drawn into the tree with that needed water.
During the spring or rainy season the effect of is diluted because the tree isn’t transpiring at the highest rate, like during the summer.
I could be totally wrong incorrect here, but I believe this is what led to the no fertilizer in the summer “ rule” because people saw negative things happening to their plants under heat stress.
Because deciduous are higher water mobility they transpire more and will pull in more sodium, magnesium, potassium and Cholrides ( all salts) and that is what can lead to health issues.
Of course they will also pull in the other nutrients in the pot such as calcium, phosphate, sugars, micronutrients like iron, etc, etc.
The point being that the tree can’t just take in water and calcium by choice. When it calls on the root system to deliver water, it brings up whatever nutrients are in that water, that soil, etc.
But excess salts during the hottest months can be problematic.
Again I am a beginner who has chosen to go down the path of Apical testing, I only use Eden products and I could be totally wrong/inaccurate here. There are people here who are far more knowledgeable than me.
Let us know where you research and experimentation lead you.
I am sure someone will correct me and then we can all learn together.
I am always open to a easier way to understand this as I am not a scientist.
