Water sticks to itself and to other materials. That's why water forms drops and the drops can stick to your windows. When they get so heavy they can no longer 'stick' to the glass, they roll off. Water in soils sticks to itself AND to soil particles. The sum of this 'stickiness' is called capillarity. At times, the capillary attraction is greater than the force of gravity acting on the water, so it is retained in the soil. This is primarily driven by particle size - the finer the particles the greater their surface area and the more 'stickiness' those particles have. Increasing the size of soil particles in a given volume of soil reduces the surface area and therefore water's attraction to the soil.

Media that support a layer of saturated soil near the bottom of the container are problematic for 2 reasons. A) they lack the oxygen needed to burn the fuel (food) that serves as the energy source that powers root metabolism. B) the soggy layer of soil promotes growth of the anaerobic fungaluglies that cause root diseases.

There is an inverse relationship between the size of particles in your soil and the ht of the saturated (perched) water table. As particle size increases, the ht (and therefore the volume) of the perched water table decreases, until at a consistent particle size just over 1/10" it disappears entirely.

Plants don't drink water. They absorb it a molecule at a time from water vapor and from a microscopic film of water on the surface of soil particles. The gritty mix is adjustable for water retention, and the basic recipe includes 2 components that are internally and openly porous. This means the particles can be sieved to a size large enough that the soil cannot hold water between its particles (except at the interface where particles touch each other), but do hold water inside of particles that's readily available to plants. The fact that the large air spaces between soil particles (macropores) are never full of water for more than seconds at a time during watering sessions means there is always excellent aeration to provide the oxygen that thwarts anaerobic fungi/bacteria and drive root metabolism.

How much water a soil can hold inside of soil particles w/o holding water between soil particles can be closely controlled by varying the ratio of ingredients that hold water internally to those that don't, while always making sure particle size is large enough that it disallows water retention between particles.

MUCH more detail here.

Al

Al, I have to figure out how to set my notifications because I just came back here last night and saw that you had posted this. Thank you, it really is very helpful. I also read the post you linked to and that answered many questions I've had as well. After the hundreds of posts I've read with people arguing and being snippy about their particular mixtures being best... it all finally made sense :)

So this brings up a new question for me. I just purchased a cereus peruvianus (or peruvian apple cactus) in a 12" pot, so it's pretty large. It was growing in central Florida (apparently cut off a much larger one), in the sandy Florida soil and with the frequent afternoon rainstorms. I am planning to put it in a very large container and make my own soil concoction, but these apparently like a lot more water than the traditional cactus, so I don't think your original gritty mix would be the right choice. I live in North Florida, so we have the rainstorms and humidity, but we don't have the sandy soil in my specific area. I have the "reject" particles of turface, grit, and pumice (photo attached) I saved in hopes of finding a use for them; do you think those would work well as a substitute for sand when trying to make a "sandy soil"? If I'm understanding your post above, the smaller size of those particles would allow the soil mixture to hold more moisture near the roots? Or is this going to simply promote bacteria growth here like it would anywhere else? If you have a mixture recommendation I'd love to know.

The nursery, which is a wholesale nursery selling in cacti and succulents only, just said put it in a sandy, well-draining soil when I asked. Well, for a person like me who would prefer to know what I'm doing and get it right the first time, that doesn't help too much.

Hi, AR. Thanks for the kind words. I don't get snippy about what people use for a soil because I don't care beyond the fact that I'd like everyone to get all they can from their growing experience. I do care when, during the course of a conversation, misleading or erroneous information that has the potential to limit your experience is claimed or inferred to be true. I'll argue those points with gusto, for no reason other than the fact I think it's important for (the collective) you to have accurate info if you're to make good decisions.

I think it can safely be said that the number of plants that would do well in soils in which they naturally occur would be somewhere between very few to practically none, with the overwhelming number of them crowding each other for space at the 'practically none' end of the comparative scale. Putting mineral soils in a pot changes the way water physically behaves in them due to the fact it eliminates the wicking effect of soils deeper than pots. I said all that to say it hardly matters a whit, what type of soil the plant prefers growing in situ, 'cause it ain't gonna like it in a pot.

Also, I don't think you should be thinking about trying to use the fines left over from screening Turface as a significant fraction of a soil for plants that happen to be vulnerable to ill effects caused by wet feet. You can increase water retention w/o introducing a perched water table by screening the ingredients to a size large enough to ensure the medium can't hold perched water, but also increasing the volume of ingredients that hold water INSIDE of the soil particles. IOW, screened ingredients equaling

4 parts Turface

3 parts bark

2 parts grit

will hold a lot more water than the standard 1:1:1 screened parts by volume w/o sacrificing any notable amount of aeration or drainage, and it won't invite a perched water table to take up residence in the lower reaches of the pot.

Al

Hi Al,

Thanks for the advice; I have begun the process of making the combination you recommended. I have had a hard time, as everyone seems to, with ingredients. I'm in Tallahassee, FL and have searched high and low for small sized bark. I found a place a little over 2 hours away that sells 3/8" pine fines, so I am planning a drive over to get some bags. I also picked up some #89 crushed granite from a local company, but it seems to be mostly on the larger end (it says 3/8" & down). I'm going to be making a really large amount of the 1:1:1 gritty mix for several large planters for my husband's office, so I'm working really hard to find the most cost effective way to do this. I've been buying Manna Pro Poultry Grit from Tractor Supply for home, but I'd really like to find a less expensive alternative for this project.

So I have two questions I was hoping you'd answer for me. First, I read an old post of yours that said it didn't really matter if you used larger bark pieces in the mix, so do you still think that's the case? I ask because I already have some bags that I could use. I ran over the bark with my car (and got some frustration out in the process), so it's broken up some, but definitely not all small. I could go ahead and replant my peruvian apple cactus if those are acceptable, but no big deal if not.

Second, if I were to mix the larger granite pieces (3/8 inch) and the poultry grit, would that just reduce the water holding capacity of the overall mixture? The planters will be filled with cacti, so I don't think less water holding is necessarily a bad thing.

Thank you!

Allison

Oh my God, that is the best idea for breaking up bark I have ever heard!!!

i have a gravel driveway, too! That should pulverize the bark! I am so going to try that right now.

Will report back! You are a genius.

I got the idea from someone else on here, but it did actually work pretty well! Good luck!

By the way, I also read someone who uses an electric Toro blower/vacuum combo that mulches. I saw it at Home Depot for $55 or so for the less expensive one, or $70 something for the more expensive one. I didn't buy it, but if that works it would be great!

Allison

Okay, it kind of worked , but I chickened out after two passes over the bag of bark. I'm going to try again today when it's light out!

I read an old post of yours that said it didn't really matter if you
used larger bark pieces in the mix, so do you still think that's the
case? You'll probably find some sort of qualifying statement that pivots on the thought that you can have 1 of the 3 ingredients (1/3 of the mix) slightly larger than ideal and not change the way the mix performs markedly, but if 2 of the ingredients (2/3 of the mix) are larger than ideal, it will significantly change how much water the mix can hold. You can probably visualize that when 1/3 of the mix is too large, the appropriate size fraction will flow between the too large particles, so drainage/aeration will be affected much less than if the situation was such that 2/3 of the mixture was too large and the remaining 1/3 fraction was insufficient to fill in all the extra large spaces between the too large particles.

Second, if I were to mix the larger granite pieces (3/8 inch) and the
poultry grit, would that just reduce the water holding capacity of the
overall mixture? The main effect of the larger pieces would be to increase macropore size and reduce water retention. What drives the reduction in water retention is the inverse relationship between particle size and o/a surface area and the number of interface points where soil particles contact each other. As particle size of a given material decreases, o/a surface area of a given volume of particles increases. This means there is a much larger total surface area that can be coated by water, so more water retention. Also, since water is held at the interface where soil particles contact each other, larger particles = fewer particles in a given volume = fewer contact points between particles = less water retention.

Al

Al,

Thank you for being so patient and so willing to help all of us! I know you've probably answered the same questions 100s of times :)

Allison

Lauren, I noticed you are zone 9a, so I think that must be Florida somewhere. On Thursday I drove to Starke (near Gainesville) and bought bags of 3/8" pine bark from a Starke Landscape Supply, which is the retail supplier for Elixson Wood Products. They were around $2 a piece and, once sifted, left me more bark than I expected. The bag is 2 cu.ft. before sifting, so if you are looking for more than a little bark, it may be worth the trip. Really nice people, too!

Allison

Thanks, argreen, but I am actually on the other side of the country in sunny Arizona! But I'm glad you found a good bark product, nevertheless:)!