Introductory soil physics

markyscott

markyscott

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Questions about soil physics come up all the time and I find that I repeat myself a lot on this topic. I'm going to try and write a reference for folks to refer to on this topic. I'm expressly NOT going to offer you any advice on what soil you should use in your garden, whether you should use organics or inorganics, or anything on that line (that's where these soil threads go south). Instead, What I'm going to do is offer some basic information on which you might base your own choices. I'll try to do this at a high level, but this is related to my profession, so it's easy for me to go overboard. If you want references, I'm happy to supply them. I'll try to do this in a few notes so that it's easier to follow.

Scott
 
The two most important physical properties of a potting mix are:
  1. Air-filled porosity (AFP - the percentage of a potting volume that is occupied by the air after irrigation), and
  2. Water holding capacity (WHC - the percentage of a volume of potting mix that is occupied by water after irrigation).
When we say "good drainage", what we really mean is "good AFP". It's the single most important physical measure of a good soil mixture. We say "good drainage" because AFP is hard to see when we water, but we can see when a soil allows water to pass through rapidly. Soil which has this characteristic typically has a high AFP. But "good drainage" is not a measurable quantity and AFP is. There are no guidelines on how fast an optimal soil should allow water to pass through it because its affected by too many other variables - such as the number of holes on the bottom of the pot (as you already pointed out), the shape of the container and many other factors.

Scott
 
AFP and WHC are controlled by the capillary properties of the soil. They are a result of the small pore spaces and due to the fact that soils particles are "water wet". In other words, water tends to adhere to the soil particle much more strongly than air. When you irrigate, all the pore space is temporarily filled with water. When you stop, the water that's not held by capillary forces drains out of the pot due to the force of gravity. When it stops, there is still water held in the pore spaces due to these capillary forces (we'll touch on this several times).

So what factors impact AFP?
  • Grain shape - rounded particles have lower porosity, angular fragments have higher porosity. Higher porosity tends to mean higher AFP and lower water-holding capacity.
  • Grain size - porosity is not strongly affected by particle size, but the size of the pore space is. As you increase the grain size, the porosity remains the same, but the AFP increases and the water-holding capacity goes down.
  • Sorting - a uniform grain size has a high porosity, high AFP, and lower water-holding capacity.
  • Height of the container - the same soil in a deep pot has a lower AFP than in a shallow pot
Scott
 
Good potting mixes generally have an AFP of at least 15-20% and a water holding capacity of at least 30%. And you don't have to take my word for it - there are several decades of horticultural and agricultural research that will tell you the same thing. Here are some facts:
  • As a general rule, less than 10% AFP and your soil is waterlogged - plant growth suffers. Too long under these conditions and plants will die (minus a couple of notable exceptions that have adapted to surviving these conditions).
  • Increase the AFP and increase the growth rate, but you'll need to provide more water and nutrients. But increase it too much and the trees will not have access to enough water when you take the hose away. Decrease it too much, and plants will stay wet and growth rates slow down dramatically. As AFP approaches zero, growth rates are minimized and plants can easily succumb.
  • The recommended AFP can be pretty species specific too - some plants (maples, pines) grow best with a higher air-filled porosity. Others can tolerate a much lower AFP than most plants. Bald cypress and Water elm, for instance, can tolerate long periods of water logged conditions, but they are specialized to do so. In general though, if your soil stays too moist it encourages soil pathogens such as phytophthora and soil collapse making it difficult to care for your plants.
Scott
 
Let's talk about how water saturation within the pot. The most important thing to know is that water saturation is not uniform within the pot. The drainage holes allow the gravitational water to effectively drain, but the water held in the soil by capillary forces is held in the same way that water is held by a sponge. The saturation increases toward the base of the pot and is close to 100% saturated in a zone along the bottom. The shape of the pot and the size or number of the drainage holes do not impact the height of the saturated zone along the base of the pot. In a shallower pot, the saturated zone simply extends closer to the soil surface. The variables that control the height of the saturated zone are almost the same as those that control the AFP:
  • Grain shape - Using angular grains tends to decrease the height of the saturated zone.
  • Grain size - Using finer-grained soils tends to increase the height of the saturated zone at the base of the pot.
  • Sorting - A uniform grain size tends to decrease the height of the saturated zone.
  • Height of the drainage layer - you get higher water saturations in the soil on top of the drainage layer (if you use one). This is sometimes referred to as "perched water". It's counter ntuitive and I'll try to explain more on this below.
  • The relative difference in grain size/shape/sorting between the drainage layer and the soil medium - if you don't use a drainage layer, there is no "perched water". The bigger the difference between the medium you're using for a drainage layer and what you're using for a soil medium, the taller the saturated zone above the drainage layer.
  • Time since watering and environmental factors - over time, the amount of water held in the soil by capillary forces and the height of the saturated zone will decrease due to water being removed from the soil by the plant an due to evaporation.
The drainage layer issue is complicated because there are competing effects. First, the saturated zone in the soil will form at the top of the drainage layer, moving it to a higher elevation in the pot. A second, substatially smaller saturated zone will form at the bottom of the drainage layer. So a drainage layer sort of makes your pot effectively shallower - it reduces AFP and increase WHC. Counterintuitive right? The drainage layer increases soil moisture.

Second, the height if the saturated zone will be impacted by the difference in grain size between the soil and the drainage layer. A large difference in grain size will tend to increase the height of the saturated zone and a small difference will tend to decrease it. In other words, as I said above, the drainage layer only matters if there is a big difference in grain size between the drainage layer and the soil medium.

This picture says a lot about what happens when you water.

IMG_4078.JPG
Ref: http://www.ncbuy.com/flowers/articles/01_10076.html

See that saturated zone at the bottom? It sits on top of the drainage layer when you use one - it makes the pot shallower and increase the water saturation in the soil. But how high the saturated zone is is related to what you're using for a drainage layer. The closer that medium is to your soil medium, the smaller of an impact it has.

Scott
 
More - let's talk a little more about drainage layers. It's such an unfortunate name because it's led to the idea that drainage from containers can be improved by adding a layer of coarse material to the bottom of the container. In reality, this does does exactly the opposite - the saturated zone at the bottom of the pot is simply moved up, reducing the unsaturated, high AFP part of the container. Like this:

IMG_4079.JPG
Ref: http://www.ncbuy.com/flowers/articles/01_10076.html

The one issue that I have with this image is that the height of the drainage layer is the same in both cases. In reality it would shrink a little be for the case on the right, depending on the relative difference in grain size between the soil medium and the drainage medium.

Scott
 
Scott, this is great! May I suggest it be collated into one article and added to the Resources for easy future reference for everyone? I have one question you may be able to answer -
  • Height of the container - the same soil in a deep pot has a lower AFP than in a shallow pot
Why is this? It seems a bit counterintuitive, as I realize much of this does. I'd just like to understand this a bit better. Thank you!
 
Scott,
While you at it. I understand Boon uses drainage layer and pass it down to all his students. As you have pointed out that layer raised the saturation zone instead of lower it. What is your personally practice on that. Do you use drainage layer or not?
 
0soyoung said:
Is there a tabulation of AFP and WHC values for various commonly used substrates somewhere? I can readily find CEC and pH, but not these important numbers.
Click to expand...
As to my understanding of this I think the WHC changes a bit depending on factors related to AFP. AFP seems to be highly variable due to several parameters such as grain size etc. So I'd imagine it would be difficult to provide a generalized substrate properties table. Maybe Scott can speak to this better? I do know that growstone provides these numbers for their products, and compares them to other substrates, but many of them are not common bonsai substrates. http://www.growstone.com/about/products/
 
Solange said:
I think the WHC changes a bit depending on factors related to AFP. AFP seems to be highly variable due to several parameters such as grain size etc.
Click to expand...
Yes, likely so.

Glass beads likely have a WHC=0 and some finite, non-zero AFP. Sand is likely similar.

I'm thinking of 'out of the bag' values for Turface, NAPA oil dry, Akadama, 2mm pumice, ..., for example.
 
0soyoung said:
Yes, likely so.

Glass beads likely have a WHC=0 and some finite, non-zero AFP. Sand is likely similar.

I'm thinking of 'out of the bag' values for Turface, NAPA oil dry, Akadama, 2mm pumice, ..., for example.
Click to expand...
It seems some experimenting is in order. :) would be great if we could measure these for the 'standard' substrates and have them listed to be shared. I suspect it would still only be a very general table. I've seen some good variation in grain size in different bagged lava for example. Also, as Scott pointed out, it appears that the size and shape of the container changes the AFP. Apparently there is a complex mechanism involved I am not understanding, unless AFP is related to perched water. But I always thought deeper containers had better drainage and a lower water table. Which puzzled me and is why I posited my inquiry.
 
Unless stabilised will not glass release Na or K or Ca into the water ------------ as high alkali ?

If you use fired clay [ Lecca ] will there be free water ?
Good Day
Anthony
 
Anthony said:
Unless stabilised will not glass release Na or K or Ca into the water ------------ as high alkali ?

If you use fired clay [ Lecca ] will there be free water ?
Good Day
Anthony
Click to expand...

Hi Anthony. Soil chemistry is a whole other kettle of fish and I'm not going to get into that in this thread. I'll get into how the composition of the soil changes the AFP and the WHC.

Scott
 
Solange said:
Scott, this is great! May I suggest it be collated into one article and added to the Resources for easy future reference for everyone? I have one question you may be able to answer -
  • Height of the container - the same soil in a deep pot has a lower AFP than in a shallow pot
Why is this? It seems a bit counterintuitive, as I realize much of this does. I'd just like to understand this a bit better. Thank you!
Click to expand...

Let's think about this more because it is a bit counter intuitive. Let's first define some terms.
  1. The porosity is measured when the medium is completely dry. It's the fraction of your soil that's air. So if you have a liter of soil and 50% of it is air space, it's said to have a 50% porosity. Most soils have 50% to 80% porosity.
  2. The air-filled porosity (AFP) is the portion of that porosity that is air after the soil has been irrigated and the gravitational water has drained away. In other words, if you completely saturate the soil and then let it drain, some of the pore space will be occluded by water due to capillary attraction binding the water to the soil against the force of gravity. So it's a number less than the total porosity and is an average for the whole volume of soil.
  3. The water-holding capacity (WHC) is just the opposite - it's the portion of the porosity that is water after the soil has been irrigated at the gravitational water has drained away. AFP + WHC = total porosity. You'll also hear this number referred to as the field capacity. This is the water available to the plants after you turn the hose off - so it's what the plant has to live on until the next time you water.
  4. The water saturation is the fraction of the pore space that's occupied by water. This number changes over time - it's high when you water and get's lower after the gravitational water drains away.
So now to your question. For a given soil medium the porosity is constant. Let's say 50% for the sake of argument. Doesn't matter how tall or short the container is - it will always be 50%. After you water, you'll be left with a profile that looks like this (depending on the size of the pot):

IMG_4081.PNG

See? Right after you water, the saturation is high at the bottom of the pot and decreases upward until it reaches what's called the irreducible saturation - that's the amount of water bound to the soil grains by capillary forces. Since the profile is controlled only by gravity, the only thing that matters is what the soil is and how high above the bottom of the pot you are. As you can see, the figure on the left (the taller pot) has a lower average water saturation (and higher air-filled porosity) than the shorter pot on the left. I integrated the hypothetical curves above and got 26% average water saturation (or 24% average AFP) for the pot on the left and 32% average water saturation (or 18% average AFP) for the pot on the right. In other words, a shorter pot is wetter than a taller pot. Weird.

Scott
 
namnhi said:
Scott,
While you at it. I understand Boon uses drainage layer and pass it down to all his students. As you have pointed out that layer raised the saturation zone instead of lower it. What is your personally practice on that. Do you use drainage layer or not?
Click to expand...

I do tend to use a drainage layer, especially in deeper pots. I find it easier to keep the plants watered in our climate.

Scott
 
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