Everyone on this forum should read that thread, and also the other ones about perched water tables and fertilizers. After reading this you will have a better understanding about why I and others here strongly oppose the use of peat and those bagged mixes that are available in stores.

The only problem as it pertains to cactus and succulents is that many of the species we grow have many fine feeder roots which in an open mix as suggested, they would have difficulty absorbing the water well enough.

x

Ooops...I meant to say houseplants in my post.

Here is the link about perched water tables. Thank you Al
"tapla" from the houseplants forum

Here is a link that might be useful: Perched water tables

OK so that's the basic theory sorted. Now, how does all this apply to highly xeric plants?

Many cacti thrive in very dense mineral soils, it sounds like you have some experience with these. Cactus watering is typically done onto dry soils and, depending on the soil type, will often not produce any perched water table. Cacti are often planted in shallow containers, are they highly saturated with each watering or not? What about bottom watering, which presumably does saturate part of the soil near the bottom of the pot? What do top dressings do? How well do the cactus "rules of thumb" actually fit in with the theory? Such as never overpot. Water until it comes through the holes, then allow to dry completely.

The perched water table happens no matter what is in the soil mix unless all the particles are uniformly large. It's simply a physics phenomenon. If you don't water enough to saturate the soil then you're probably right, a perched water table does not form.

Shallower containers means the PWT has a greater influence on available soil volume. Soil within the PWT is anoxic and is a dead space. Adding "drainage" material only raises the PWT and further reduces available soil volume. Bottom watering is no different than top watering except that with succulents the goal is to try and prevent the top part of the soil from becoming moist and contacting the vulnerable necks of plants, but I personally do not believe that this is prevented because the water will rise via capillary action anyway. I think plants rot more often because the roots are desiccated, not because of over-watering per se.

Take a decent sized pot, fill it with a soil mix, and saturate it thoroughly until it drains from the bottom. When it's done draining, tilt the pot at an angle, you will see more water come out...why, it's the PWT, but by tilting the pot you are manipulating it and allowing excess water to drain.

I'm not a proponent of top-dressing. I believe it prevents evaporation (which may not be a bad thing all the time) but also there is no visual sense of whether the surface is dry.

I too questioned how it relates to the more specialized cactus and succulents. The point the author is trying to make is basically avoid organics (especially peat) because as it decomposes the soil structure is negatively impacted. He also advocates thorough leaching of mineral salts with each watering and at the same time flushing through oxygen. Unless you use slow release fertilizers, that's not an important factor for us. The soil mix I use is 100% inorganic except for the small % that may be present in the garden loam I use. It does not decompose and remains stable forever.

When I water my plants during active growth, they are drenched completely. During dormancy, I don't water enough to drench the soil, only enough to percolate a little moisture. My mix has lots of clay so it binds water for a very long time, reducing the likelihood of root death. Turface, or other fired clay pellets, accomplish the same thing but with added porosity and aeration. I will try it out with some plants and see what the results are.

The author's mix dries out so quickly that I wonder how well it would work for our plants during dormancy, where bone dry means root death...again, that's why I use clayey soil which holds water longer but the downside is you need to be careful of overwatering.

I love it when there is legitimate science behind horticultural practices. So much of what we read is bogus.

x

The perched water table happens no matter what is in the soil mix unless all the particles are uniformly large
I disagree. Watering most soil mixes when they are very dry results in only a limited amount of water being retained. With a second watering after a few minutes, more water will be retained. You can demonstrate this very easily by watering a completely dry pot of soil in a saucer until the saucer is full and then leaving it for an hour. Chance are the saucer will be partially or completely emptied when you come back. Exact results depend on your soil mix but it happens to some extent in any dry soil.

What happens with that first watering is that some water is retained around some particles, saturated at the bottom by some definitions since water drains through, but that doesn't mean zero air. The water them migrates over the following minutes or maybe hours into small pores in the coarse material and into the layer structure of clay material, or into the internal structure of organic materials like peat and bark, even moves to higher levels in the soil as they suck in the water. All of the sudden there is no perched water table, you could add more water to the soil without it draining through. I have never seen any experimental work published on this but it certainly occurs and is easy to verify, I just don't know whether it means we usually do or usually don't have perched water tables in our succulent pots. In some summer-growing succulents which are heavily watered and not allowed to completely dry there almost certainly is a perched water table at some times, but again I don't agree that this means zero air in the soil mixes which are often used. Presumably this is part of the reason why we stick so much grit in our soils. Winter growing mesembs I almost never water to the point of runoff.

Or maybe cacti don't mind a perched water table. Possible reasons would be that they area adapted to saturated soils for relatively short periods, some certainly experience periodic flooding in habitat. Or the structure of a "good" cactus mix may be such that sufficient air remains in the mix even in the area of the perched water table to prevent the kind of root rots which would occur in an anoxic soil. Or maybe we use watering practices which only create a perched water table when the plant is thirsty enough to drain it, or weather conditions will evaporate it.

I absolutely use top dressings. I don't allow the bodies of succulents like Lithops to be in contact with the proper soil, that's death in my climate. Not being able to see the soil is something I just have to live with. But top dressings vary a lot. Some wick up water much more than others although obviously all retain a high proportion of air spaces. Some types of top dressing seem to increase evaporation. Experiment with fine clay granules on the surface of the soil, they continually wick up water and remain moist until the soil mix is completely dry.

I also use loam based soils. They do degrade over time. The most obvious thing is that the initial fertiliser load runs out after a few months or maybe a couple of years depending on the plant and how you water it. The soils also do compact down over weeks or months. It is easy to see the soil level drop if you don't compact it down when you pot the plant, less obvious if you squashed it down to start with. Lastly, over a period of years the clay component of the soil washes out. Unpot a Lithops after ten or twenty years in the same pot and there will be just be sand and gravel, it won't be like soil at all.

Although I use a loam-based soil, the make-up of that soil includes something like 20% organic material and in practice that is usually peat. Add in an equal amount of aggregate and I have a soil mix that is 10% peat. At these levels the soil does not behave like an organic soil but it does provide for better retention of nutrients, as if I needed it with the clay in the loam and the baked clay granules!

Salt buildup can occur regardless of how you fertilise, simply from minerals in the water. For most of us, the vast majority of the water we put on our succulents simply evaporates, any fertiliser or minerals in hard water are just left in the soil. I have quite soft water and the soil I use has a decent initial fertiliser load so I don't fertilise much either. I have never experienced salt buildup, even with the non-flushing watering that I use on most mesembs most of them time.

While completely dry soil may mean root death, after a sufficient length of time anyway, this is almost inevitable in my climate and just something I have to deal with. It is simply not practical to provide water every few weeks to most cacti over winter, and certainly not to Lithops. Lithops lose their fine roots and regrow them in the spring, they seem to be very good at it. Many cacti deal with it well, some don't. Some species don't seem to be adapted to going completely dry for months at a time. When people talk about cacti that "lost their roots" over winter, they are usually talking just about the ones which rotted because they didn't grow their roots back in spring :)

s-n-b

About the perched water table, we seem to be discussing 2 separate things, I think that what you describe is yet another factor that hasn't been touched upon, which is "wettability", in other words, how long does it take for a particular substrate to actually become fully saturated. Perhaps a PWT is never fully realized since often we do not even saturate the soil mix? I'd bet several consecutive waterings, or something like watering in the evening and then again in the morning, should be enough to saturate the soil.

Mineral and salt build-up is dangerous, but cactus and succulents more than most other plants tolerate this well. All container mixes have a tendency to become alkaline. I generally add 1-2 tbs vinegar per gallon of water to help neutralize these salts. But my water is definitely mineralized so I have no choice. pH is critical in that plants that prefer alkaline will tolerate acid, but plants that need acidity usually suffer in alkalinity. Frithia pulchra and some Conos come to mind.

I have never noticed a drop in soil levels, I have some things that haven't been repotted for years. It just goes to show how your "loam" and my "loam" are probably quite different. The end result though is probably the same, once again a testament to the adaptability of plants, and growers alike.

Whenever I do repotting in the spring after a long dormancy, I continue to be amazed at the vitality of the root systems in the pots despite all those months of inactivity, so much ramification that it almost looks blurry. It must be the clay, I can't find any other reason.

There is obviously no absolute correct way because many of us have excellent results despite differing ways.

By the way shrubs, isn't it really early in the morning in the UK? Get some sleep dude :-)