I went to the wiki-link: Phloem - Wikipedia, the free encyclopedia ...
There I read that
nutrients which have been converted to sugars (plant food or sap) flow in both directions in the plhoem depending on where the food/sap is needed most.
I am beginning to realize that you are telling me (I think) there is the flow of nutrients and sap generally speaking in most plants, during transpiration or only when there is light. You also would have me believe that generally speaking in most plants, there is nothing produced at the roots from within the plant under favorable conditions. No reason for root exudate, therefore no root exudate normally without being triggered by some various mechanism.
I'm trying to correlate this with what I've read concerning crop rotation. If I am to believe what you are telling me, then crop rotation is not necesary if enough
nutrient/fertilizer is used to replace the mineral content that has been depleted by the previous crop. You would have me believe that crop rotation has nothing to do with anything a plant might put back into the ground (generally speaking even though there are exceptions) under ideal conditions. But wait, you say plants in general excepting certain examples, do not release anyhting from the roots under ideal conditions.
Hmmm ... that previous link I included speaks of a viscous substance the roots exude mostly at night apparently, to help with taking in nutrients during daylight when transpiration is occuring. I still can't make it all add up in my head. For some reason I still belive there must be some sort of waste product besides O2 from the leaves, speaking in general terms for I realize there are exceptions, that the plant needs to rid itself of in order to be healthy, grow and produce blossoms. The waste would be put into the ground via the roots.
At this point, I'm trying to find any information about whether this does or does not occur generally speaking with plants in general even though I'm sure there are probably exceptions to the rule.