Before the questions, I would like to list some principles (P) to see if I got them right.

(P1) The PWT forms when the capilary force of the growing medium becomes larger than gravity, and thus water is held in the soil rather than continue to flow. That's correct.

(P2) By the same 1st principle, putting a layer of pebbles on the bottom of a pot does not work, because there is no capilary action between the pebbles and the soil, and thus, the PWT remains the same height on the soil. So, if for example the PWT is 6 inches, you would continue to have 6 inches over the pebbles. Remember that water sticks to itself (cohesion) and to other materials (adhesion), and these 2 forces that drive capillarity are still in play; so it's not that there is no capillary attraction associated with the layer of pebbles, only that it's greatly reduced, the primary reason being a reduction in adhesion due to the much smaller surface area of the large particles in the pebble layer. The ht of a PWT which perches above the coarse layer would be somewhat less than if it was perching above the bottom of the pot. The reason is, the surface area of the layer of gravel is greater than the surface area of the bottom of the pot, so the increase in adhesion would need to be added into the equation.

(P3) Growth mediums with particles over 0.10 inches in diameter do not exert a capilary force over water, and thus do not have a PWT. Media w/all particles >.10" do have some capillarity, but not enough to overcome the gravitational flow potential of the water column.

Now the questions:

(1) If instead of a layer of pebbles I put a layer of inert particles of, for example, 0.05 inches, would the PWT height go down? Would some of the water from the soil move to the layer of particles that at this size do have some capilary force? If the size of the particles making up the medium above the layer of .05" particles are >2.1X the size of the particles in the lower layer, OR >.10", water will not perch. As a practical matter, adding a layer of fine material below your regular medium is going to be counterproductive as the only time it wouldn't increase the amount of PWT the planting can hold is in the 2 examples cited immediately previous. (2) A related question. If I put such layer of particles of 0.05 inches, and in addition insert some cotton or rayon strands into the layer and the rest of the strand sit on top of the layer (so as to add some wicking action), would that result in me transfering the PWT from the soil above to the layer of 0.05 inches inert stuff? Again lowering the PWTs height.... No. Practical ways of managing excess water in fine-textured media can be found here.

(3) Finally, relating to putting a wicking strand on a container. Why is it that the wick has to go down, rather than just lay horizontally? I don't understand the principle here. I was thinking/understanding that the wick pulled the water from the PWT by capillary action, but if that is the case, putting it on the side of the container should achieve the same effect, or so it seems.... What is going on here? I'm not sure what you mean by "I was thinking/understanding that the wick pulled the water from the PWT by capillary action, but if that is the case, putting it on the side of the container should achieve the same effect, ......." Capillarity is part of how it works, but the primary driver is the fact that a wick increases the ht of the water column which increases it's gravitational flow potential. Maybe it would help to imagine the wick "tricks" the perched water into behaving as though the pot is deeper than it is. As water is pushed down the wick by the weight of water above, water drips off the end of the wick and keeps flowing until an equilibrium is reached between the medium's capillary attraction and gravitational flow potential.

Al

Wow! Thanks Al for the detailed explanations. They are most helpful! In particular, I didn't have clear what adhesion and cohesion meant. It is much clearer now!

I find these concepts fascinating. I am new to gardening at my 60 years of age, but I am truly enjoying learning about this. Actually I did try a few year ago growing tomatoes and lettuce, but that was a complete failure and I abandoned this for a while. Now I started again with figs in containers. Which - I shouldn't be surprised - have their challenge in my climate, particularly the excess rains. Searching for solutions I came upon your posts. I am working my way to try some kind of "gritty mix" with my fig trees.

I did have a question about fertilization when using gritty mix like mediums. I currently use granular chemical fertilizers. Once a month I just add about 1.5 tbsps and that's it. A few times a year I add a handful of lime. I don't know if I will be able to bring my lazy-self to use liquid fertilizers. Do you think it is possible to continue using what I am currently using?

Thanks again for your detailed response.

Thanks for the kind words. I'm always glad to find I've said something helpful or useful.

A medium that puts the concept you're now gaining an understanding of will serve you well in all your container gardening adventures and add significantly to both the tangible and personal rewards you get from the growing experience. I don't say that because I provided an explanation of the concept; rather, I say it because it transformed my own growing experience and the number of other growers using the concept grows daily. Their feedback suggests that virtually all container gardeners will run up against a poor medium and not understand what's occurring in the soil column to thwart their best efforts. Some will intuit that more coarse material will help, but don't go near as far as necessary to ensure their intuition fully delivers on their expectations. The medium needs to contain probably more than 80% of particles >1/8", and the fine material contained in the medium should be added only to make the water-holding ability tolerable to the individual grower. If I was asked to guess at what most growers (who are focused on the plant's best interests) would find the best watering intervals to be, I would answer: assuming the grower is watering to beyond the point of saturation so as to flush the medium when watering, when the plant is growing it's slowest the planting should actually NEED watering before a 5-day interval has passed, and when it's experiencing its most robust growth, no more than 3 days. Shorter intervals would actually be better, but people have lives to live and normally don't want to be tethered to daily watering sessions. It's important to realize I'm not suggesting growers should suddenly alter their watering schedule w/o being certain it's appropriate in combination with the grow medium. I'm saying media that require more frequent watering are by their nature much more conducive to the plant realizing as much of its genetic potential as possible.

I did have a question about fertilization when using gritty mix like mediums. I currently use granular chemical fertilizers. Those would be slow release fertilizers. Once a month I just add about 1.5 tbsps and that's it. A few times a year I add a handful of lime. Highly doubtful you need to do this, and a high probability it will be a limiting factor. Most commercial media is pH adjusted with dolomitic lime which contains Ca and Mg. If you use a lime other than dolomite it can cause an antagonistic deficiency of Mg. Also, the Mg fraction of the dolomitic lime used to adjust media pH is about 125X more soluble than the Ca fraction, so Mg would be the first nutrient to become deficient. The magnesium fraction usually lasts for 2-3 years before needing refreshing, and almost all plantings would need a full repot (includes a change of medium) well within that interval. I don't know if I will be able to bring my lazy-self to use liquid fertilizers. Do you think it is possible to continue using what I am currently using? Yes. The only downside is you can't pull back the reins on fertility during summer's dog days when you want fertility to be low, to facilitate water uptake..... and hot days often find plants in a consequential dormant state (consequence of high temps) where growth is limited. Low fertility when roots are stresses also helps to ward off soil/fungal pathogens that cause root rot.

Al