[Alice]

I wonder how many amateur members have heard of "hydraulic lift" and "hydraulic redistribution".
Hydraulic redistribution is the transport of soil moisture by plant roots from wetter to drier soil areas. The movement can be upwards - termed "hydraulic lift", downwards or horizontal.
During hydraulic lift water enters the deeper tap roots of deep-rooted shrubs and trees, moves up the root system and flows out from the shallower roots into the soil. This happens mainly at night. There is then more water in the upper soil to support daytime transpiration, nutrient uptake and to facilitate microbial activity. But - and this is the neat bit - neighbouring plants can take advantage of this hydraulically-lifted water.
I think what goes on below ground is fascinating and relationships between plants are much more complex than we at first realise. On the practical side, shallow-rooted plants can be kept alive in times of drought if planted close to plants capable of hydraulic lift.
("Roots Demystified" by Robert Kourik, ISBN 978-0-9615848-0-1; www.jstor.org/stable/4221835)

[Bolanthus]

Interesting !

[MikeHardman]

News to me! Always something to learn! Thanks Alice!
I can't help wondering: there must be overlap with root exudation processes as they occur in allelopathy. More generally, perhaps the fluid released by the roots is mediated by the plant, eg. amending the soil to aid beneficial fungi or bacteria, or to top-up nutrients that the plant itself (or other plants) had over-used in that part of the soil. Or it could be a means of making existing nutrients more available - wet the soil then allow the roots to suck it up (a bit like some flies regurgitate stomach acids onto food before sucking it back up).

[Alice]

Yes, I thought it was very interesting too and only heard about it very recently. It seems this phenomenon was first described in the late 1980s but there has been a lot more research in the last 10 years.
Mike, as I understand it, the roots bring up water from deeper layers to surface layers which are generally richer in nutrients and then suck up the nutrients with the water. I like your analogy with the feeding habits of flies!