Soil porosity and water retention

Discuss repotting, soil, lighting, fertilizing, watering, etc. in this category.
iann
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Soil porosity and water retention

Post by iann »

Today we have the advanced lesson, but I'll try to include some pictures :)

Porosity is the easy part. Total soil porosity is simply the percentage of the soil volume which is air. Here are some typical values for various soil components:
Peat: 80%-95%
Clay: 50%-70%
Loam: 40%-45%
Sand/grit: 25%-40%
Perlite: 80%-95%
Pumice: 70%-85%
Calcined clay: 60%-70%
Calcined diatomaceous earth: 70%-80%

One easy thing to see is that the maximum possible water content in a saturated soil is equal to the total porosity. A simplistic view is that a certain fixed amount of this "drains" and the soil is left holding a fixed amount of water for plants. The amount that drains is then replaced by air. This air percentage should typically be 10% or more for good plant health, preferably higher for succulents.

In fact the amount of water that is retained in soil depends on the pressure or suction being applied to the water, with more and more water draining away until a fixed amount remains even at extremely high pressures. This can be shown in a water retention curve. From left to right is shown the fraction of water in the soil by volume, and the vertical axis shows the pressure or suction on the water, so at the bottom is saturated soil and at the top is soil which is fully drained (but not dry).
Image

Water which remains in soil at very high pressures cannot be extracted by plant roots. This pressure is called the wilting point because when soils reach this level of dryness then leafy plants will wilt. This is typically at about the 1,000 mark on the water retention graph. Note that although clay soils can hold a lot of water, a relatively large amount is unavailable to plants. In fact, certain loam soil mixtures typically hold the highest level of water available for plants. Coarse fluffy peats as shown on this graph also hold very large amounts of available water, although finer and more decomposed peats behave more like a clay soil.

In real soil, the pressure is applied by gravity and so water will drain to a certain point depending on the height of the water column. The water retention curve is often drawn with the pressure axis labelled as a height or depth and each unit on this axis corresponds to 10cm or 4". In open ground, typical average water retention corresponds to the 10kPa mark on the graph, but in a pot they correspond to somewhat below the 1kPa mark. You can see immediately why potting soils can get so soggy, because there simply isn't enough pressure to drain off a meaningful amount of water. Even pure sand doesn't drain off more than a few percent of the water, although coarse grits are better. Halfway through this document there is a comparison of different peat materials and you can see that fresh sphagnum moss peat would drain quite well in a pot, but older more decomposed peats are very poor:
http://nrs.fs.fed.us/pubs/jrnl/1968/nc_ ... er_001.pdf

You can see that some quite extraordinary substances are needed to allow more than 10% of the water to drain away from a pot. These substances are things like perlite and the various calcined mineral granules. Here is a graph comparing calcined diatomaceous earth granules (Axis and Isolite) with sand. The vertical portion of the sand curve to 10cm means there will be little drainage in a pot, although in open ground most of the water will drain away from sand. The sloped curves of the other diatomaceous earth products means there will be significant drainage even in a pot. Note also the high level of water which remains unavailable to plant roots and is only released very slowly by evaporation. Perlite has similar drainage behaviour, but doesn't strongly retain as much water and will become completely dry more quickly. Coarse grit also drains quickly in a pot but has a low total porosity and so is less able to retain both water and air.
Image

In case it looks like hardly anything will provide drainage and so some air retention in a pot, there is a hysteresis effect in most soils. That is, they do not become completely saturated until a particular water pressure is applied for some time or until excess water pressure is applied to force the soil to become wet. You can read about this in detail, but the main thing to know is that the water retention curve is steeper as the soil is being watered and so contains more air than you might expect. Lengthy soaking from below, standing in water, or blocked drainage holes will overcome this effect.
--ian
daiv
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Re: Soil porosity and water retention

Post by daiv »

Ian,
Thanks for taking the time to post this. I've made it "Sticky" for future readers. I didn't respond right away as I wanted to go through the post in more detail.

This line is quite interesting to me:
iann wrote: you can see that fresh sphagnum moss peat would drain quite well in a pot, but older more decomposed peats are very poor
This would explain the success of commercial growers use of peat and the eventual decay of those same very healthy plants over time. Of course it brings up the question, "How long does decomposition take?" Which I imagine varies on how much it is wetted and the temperature.
All Cacti are succulents, but not all succulents are Cacti
noelbro
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Post by noelbro »

very interesting and informative data! I have read it twice and will refare to it again. Thanks Iann i hope to find similar tecknical reads on cacti care on this w-site . Noel.
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Post by MJPapay »

VERY informative - and USEFUL!

Now I begin to see why good peat is so useful to the nurseryman and why sand has always been so lousy for me (I learnt that long ago and have avoided its use ever since).

Too, I see that I should include perlite in my potting mix for cacti. In the past I have found it useless for non-cacti as the resulting compost never seems to hydrate properly. Or at any rate the plants for which I included perlite in the soil mix never performed well. I wonder why that is?
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sundanz
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Re: Soil porosity and water retention

Post by sundanz »

"Too, I see that I should include perlite in my potting mix for cacti. In the past I have found it useless for non-cacti as the resulting compost never seems to hydrate properly. Or at any rate the plants for which I included perlite in the soil mix never performed well. I wonder why that is?"

I would like to know that too! I just redid some of my plants with a 50-50 mixture of perlite and Miracle Grow Cactus Mix. Now I wonder if that is ok?
Life's a Cactus in the Great State of Texas!!!
iann
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Re: Soil porosity and water retention

Post by iann »

Perlite is very dry. Doesn't hold much water, does hold a lot of air. That makes it a good combination with peat which holds a massive amount of water, and not much air if you get it really wet especially the less fibrous peats.
--ian
Geeklet42
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Re: Soil porosity and water retention

Post by Geeklet42 »

I've been trying to find information on expanded shale as a replacement for pumice or perlite. The nurseries I've found here have it instead as a way to improve porosity in Texas' clay-heavy soil. I found a study meant for concrete development instead of planting, so I'm not certain if I should use it in case the pumice aggregate is mixed with other construction-related materials. What do you think?

http://www.flyash.info/2009/175-hemmings2009.pdf" onclick="window.open(this.href);return false;

If so, it seems to indicate that shale is less porous than pumice. Would that mean a soil mix should involve less of it and more soil/coir?

Also, would it make a difference to overall pH that its elemental composition is lower on carbon and higher in sulfur? Or is that just a newbie reacting to the word sulfur?
Astro
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Re: Soil porosity and water retention

Post by Astro »

Interesting tidbit I just read on the Pacific Bulb Society forum:

http://www.pacificbulbsociety.org/pbsli ... haa3.html#

Gist of it is that there have been observations that roots can get 'stuck' at the interface between different mediums because of the different water retention. Perhaps of less importance in a pot, but definitely more so for plants that are planted out in a bed. Morale of the story is that it is good to break up the root ball when transplanting into a different medium :) I try not to disturb the roots of sensitive species (like Melocactus) too much when repotting, but maybe I should be a bit more aggressive after all... Or just make sure I don't change my mind about potting mixes too often.
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Steve Johnson
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Re: Soil porosity and water retention

Post by Steve Johnson »

Astro wrote:Interesting tidbit I just read on the Pacific Bulb Society forum:

http://www.pacificbulbsociety.org/pbsli ... haa3.html#

Gist of it is that there have been observations that roots can get 'stuck' at the interface between different mediums because of the different water retention. Perhaps of less importance in a pot, but definitely more so for plants that are planted out in a bed. Morale of the story is that it is good to break up the root ball when transplanting into a different medium :) I try not to disturb the roots of sensitive species (like Melocactus) too much when repotting, but maybe I should be a bit more aggressive after all... Or just make sure I don't change my mind about potting mixes too often.
Funny you should mention Melos...

I just completed a rather extensive 3-year learning experience with 2 adult Melo matanzanus, and I'll be posting the results (with photos) on the forum ASAP, so keep an eye out there. In the meantime, adult Melos do seem to resent being disturbed once their roots are established. Therefore I wouldn't recommend repotting them unless it's absolutely necessary. But if you do, here's what I believe is the best course of action:

1. Repot in early spring. This gives the roots as much growing season as possible to recover from transplant shock and re-establish.

2. Get the roots as squeaky-clean as possible. This should be standard practice with all cacti when they're repotted, although take your time and be gentle with Melos.

3. When the roots are bare-naked, soak them in 1% Hydrogen peroxide for 15 minutes. This kills any potential pathogens which could be hiding in there, plus it promotes faster healing of the roots. Then rinse them gently, but thoroughly in running water and let them dry out before you repot. And finally, repot in fresh, dry mix and let the plant settle in for a week or two before giving it water.

That's it for now, but more later if you'd like to discuss some thoughts on the mix with me.
If you just want photos without all the blather, please visit my Flickr gallery.
My location: Los Angeles, CA (Zone 10b)
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toson
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Re: Soil porosity and water retention

Post by toson »

Steve Johnson wrote:
Astro wrote:Interesting tidbit I just read on the Pacific Bulb Society forum:

http://www.pacificbulbsociety.org/pbsli ... haa3.html#

Gist of it is that there have been observations that roots can get 'stuck' at the interface between different mediums because of the different water retention. Perhaps of less importance in a pot, but definitely more so for plants that are planted out in a bed. Morale of the story is that it is good to break up the root ball when transplanting into a different medium :) I try not to disturb the roots of sensitive species (like Melocactus) too much when repotting, but maybe I should be a bit more aggressive after all... Or just make sure I don't change my mind about potting mixes too often.
Funny you should mention Melos...

I just completed a rather extensive 3-year learning experience with 2 adult Melo matanzanus, and I'll be posting the results (with photos) on the forum ASAP, so keep an eye out there. In the meantime, adult Melos do seem to resent being disturbed once their roots are established. Therefore I wouldn't recommend repotting them unless it's absolutely necessary. But if you do, here's what I believe is the best course of action:

1. Repot in early spring. This gives the roots as much growing season as possible to recover from transplant shock and re-establish.

2. Get the roots as squeaky-clean as possible. This should be standard practice with all cacti when they're repotted, although take your time and be gentle with Melos.

3. When the roots are bare-naked, soak them in 1% Hydrogen peroxide for 15 minutes. This kills any potential pathogens which could be hiding in there, plus it promotes faster healing of the roots. Then rinse them gently, but thoroughly in running water and let them dry out before you repot. And finally, repot in fresh, dry mix and let the plant settle in for a week or two before giving it water.

That's it for now, but more later if you'd like to discuss some thoughts on the mix with me.
i agree on point 1 & 2, H2O2 is quite a new method. How many days u'll dry out them without using H2O2 ? one week or two?
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Steve Johnson
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Re: Soil porosity and water retention

Post by Steve Johnson »

toson wrote:How many days u'll dry out them without using H2O2 ? one week or two?
Nope -- a day or two, maybe three at the most. However, that's when the weather is warm. If you're talking about fall/winter, I'd give it a week to let the roots dry out completely.
If you just want photos without all the blather, please visit my Flickr gallery.
My location: Los Angeles, CA (Zone 10b)
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Edwindwianto
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Re: Soil porosity and water retention

Post by Edwindwianto »

iann wrote: Fri Jul 17, 2009 3:36 pm Image
Just found this interesting post 👍👍👍
I'am particularly interested with this graph...since it shows how many water are retained in a given volume of sand
It is expressed in x-axis in xxx meter cubic of water is retained per 1 meter cubic of sand (m^3/m^-3)

Now, this is my pot
rsz_20200311_072327.jpg
rsz_20200311_072327.jpg (40.14 KiB) Viewed 12269 times
It's dimenssion are
Top surface = 8 x 8 cms
Bottom surface = 6.5 x 6.5 cms
Vertical height = 9,5 cms (not the slope "height")

When i turn my pot up-side-down, it is essentially a pyramid which is losing it's top
With simple geometry and 15 mnts, i could calculate the volume = 502cms cubic
For the sake of simplifying the following calculation, let's just say...it is 500cms cubic

So...my pot can contain 500cms cubic of sands.......................................................................(1)

Now...we go back to the graph...
The graph says that the amount of retained water correlates with the preassure, given to the substrate
The only preassure that is meaningful for me is the atmospheric preassure, since i live on the earth's surface and i don't grow my cacti in a presurized chamber...

So...atmospheric preassure =760mmHg ~ 101.325kPa
Let's just say...100kPa so we can read the graph easily

When i plot the graph, at 100kPa, the retained water in sand is 0.1m^3 per 1m^3 of sand

Now i have 500cm^3 of sand...
1m^3 of sand = 1,000,000 cm^3
1,000,000cm^3 : 500cm^3 = 2,000...............................................................................................(2)

This means that my volume of sand is 2.000 times smaller that that of the graph
So...i have to devide the amount of retained water from the graph (0.1 m^3) by the same number (2.000), to get a meaningful reading in cm^3

So...0.1m^3 of retained water = 100.000cm^3 of retained water
100.000cm^3 : 2.000 = 50cm^3....................................................................................................(3)

That's it...my 500cm^3 of sand can retaine 50cm^3 of water at atmospheric preassure of 100kPa

Conclusion
If i look at the number at (1) & (3)...i can roughly conclude in general...that sand can retain water by the amount of 10% of it's volume (50cm^3 of retained water is 10% of 500cm^3 of sand)

Demonstration
I did a demonstration on water retention ability of sands, here
Sadly, i didnt record how much water was retained (ie. How much water is let out when i tilt and shake the pot)
I'll do the next experiment to record that amount of water
Same procedure, only this time i'll collect and record that amount of water

EDWIN
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Re: Soil porosity and water retention

Post by The_Nikon_Guy »

I used to get into the technicalities of soil porosity, water retention etc. and I learnt 2 years ago, that cacti simply love plain sand - doesn't matter which one - builders, river or playground.
The key here is to remember that you need to plant them in terracotta pots.
I too started off with about 50% gravel mixes an stressed a lot about where I could source 'exotic' ingredients like crushed and size graded pumice or crushed lava rock or diatomaceous earth or such like.
Then one day, quite by chance, I had bought a sickly looking Astrophytum from one of our local warehouses (Bunnings). I didn't bother going to the garage and dragging the bag of specially formulated cacti mix I had there. So, rather lazily, I scooped a couple of handfuls of builders sand from the side of the garden into a terracotta pot and shoved the Astro into it. Its been there for about 2 years now and looks absolutely gorgeous and healthy. Its even beaten other Astros that were grown in these so called speciality mixes.
The point I'm trying to make is - if there's a way to make money from dirt, people will so do - its called capitalism.
Now, I'm not advocating against capitalism, in fact I'm for it. But, why would you succumb to those pressures and buy these products when plain sand in a terracotta pot will do a simply amazing job?
The proof of the pudding is above in one of my earlier posts in this page - check out how healthy those cacti are.
Also, I don't have to worry about accidental over-watering due to the fact I am using terracotta pots - they are amazing products, not to mention completely recyclable (I know plastic pots are too).
I have grown the following cacti in sand:
(1) Pure builders sand (this sand is fine and has a small amount of clay mixed in): various Mammilaria spp., Astrophytum myriostigma var. nudum, and Gymnocalycium mihanovicii
(2) Pure washed find river sand: Discocactus horstii, Melocactus matanzanus, Fenestraria aurantica (babies toes) and Frailea castanea

I can categorically state that the cacti have grown WAY better in pure sand than in any other medium. I am absolutely convinced beyond any doubt that pure sand is the way to grow cacti.

Having said that - I thing I know why a lot of people are petrified of pure sand - they grow their cacti in plastic pots - yes, its the nemesis of all cacti.
Plastic pots are one of the worst ideas when it comes to growing cacti - I grow all mine in terrracota and its porosity helps a lot in wicking any excess moisture.
This post of mine gives more information...https://www.cactiguide.com/forum/viewto ... 15#p373303
Never argue with fools. They will drag you down to their level and beat you with experience...
DaveW
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Re: Soil porosity and water retention

Post by DaveW »

We have been growing thousands, if mot millions cacti in plastic pots since the late 1960's in the UK with no problem. Few here use terracotta pots anymore and ours is a colder and damper climate than many of you have. You modify your potting soil so it dries out at the rate required for your climate whatever they are grown in and some growers in countries where they cannot obtain proper pots even grow them in tin cans.

What you are actually doing growing in pure sand is in effect growing in a form of hydroponics where the substrate provides no nutrients but you add these as fertiliser. Cacti can in fact be grown in just water with no substrate. Some substrates used for hydroponics range from rock wool to plastic granules, the substrate merely being there to support the plant, which is all your sand is doing.

https://www.youtube.com/watch?v=0uH_-IUxmQw

All methods of cactus cultivation depend on your climate. Soils that dry out slower are ideal for hotter climates where the heat of the climate dries them out quickly enough, but in our cooler climate potting soils have to be more porous so they dry out just as fast in our cooler conditions.

Regarding sand, depending how fine it is it will retain water for much longer than a gritty potting soil, but no doubt your climate is warm enough to dry it quickly? However evidently you still need a porous pot to get water retaining sand it to dry quickly enough, whereas due to our better draining potting mixtures plastic pots dry out quickly enough for us.

"Some sandy soils are composed of fine sand. Fine sand actually holds nearly as much water, that plants can get at, as good quality loam."

I am also a "Nikon Guy", still using a Nikon D200 with a 70-180 MicroNikkor on it and used a manual Nikon F2 Photomic for 35 years before going digital.
The_Nikon_Guy
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Re: Soil porosity and water retention

Post by The_Nikon_Guy »

DaveW wrote: Mon May 18, 2020 9:06 am We have been growing thousands, if mot millions cacti in plastic pots since the late 1960's in the UK with no problem. Few here use terracotta pots anymore and ours is a colder and damper climate than many of you have. You modify your potting soil so it dries out at the rate required for your climate whatever they are grown in and some growers in countries where they cannot obtain proper pots even grow them in tin cans.

What you are actually doing growing in pure sand is in effect growing in a form of hydroponics where the substrate provides no nutrients but you add these as fertiliser. Cacti can in fact be grown in just water with no substrate. Some substrates used for hydroponics range from rock wool to plastic granules, the substrate merely being there to support the plant, which is all your sand is doing.

https://www.youtube.com/watch?v=0uH_-IUxmQw

All methods of cactus cultivation depend on your climate. Soils that dry out slower are ideal for hotter climates where the heat of the climate dries them out quickly enough, but in our cooler climate potting soils have to be more porous so they dry out just as fast in our cooler conditions.

Regarding sand, depending how fine it is it will retain water for much longer than a gritty potting soil, but no doubt your climate is warm enough to dry it quickly? However evidently you still need a porous pot to get water retaining sand it to dry quickly enough, whereas due to our better draining potting mixtures plastic pots dry out quickly enough for us.

"Some sandy soils are composed of fine sand. Fine sand actually holds nearly as much water, that plants can get at, as good quality loam."

I am also a "Nikon Guy", still using a Nikon D200 with a 70-180 MicroNikkor on it and used a manual Nikon F2 Photomic for 35 years before going digital.
Absolutely right - you could consider this a form of hyrdoponics where the sand is merely a substrate, though, that was hardly my intention :D
Yes, my climate is quite hot an dry though that has not played any role in my decision to use sand in terracotta pots.
Due to capillary effect of sand, the water gets "wicked" (for lack of a better word) quite nicely into the bulk of the sand substrate. The terracotta pot is merely an insurance policy to mitigate accidental overwatering.
All said and done, I do have very coarse gritty mixes that I use - mostly for Lophophora and Sulcorebutia (rauschii form).
I have been growing in sand for close to 3 years coming up and am still evaluating whether its the right thing in the long run.
Am I convinced about its suitablility? Absolutely.
Is it right by the cacti? Maybe, Maybe not. I think there are benefits in having various other grits and rocky materials in the soil.

I have a Nikon D90 that I purchased second-han about 9 years ago - love it to bits!! :lol:
Never argue with fools. They will drag you down to their level and beat you with experience...
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