Stan Kengai
Omono
A little soil mechanics lesson, if I may. The idea struck me that I should share this when I read an exchange on another thread about inexpensive Turface.
Imagine two identical bonsai pots filled with Turface (or any soil component), where pot A has only 1/4" sized particles and pot B has 1/16" particles. These two different particle sizes have the same water holding properties per individual particle: they absorb the same amount of water per cubic inch of particle, and water adheres to the surface of the particles in the same amount per square inch of particle surface area. However, these different size particles behave very differently as a planting medium simply because of their respective sizes.
There are 2 things at play here. The first is the density with which the particles can be packed into a given volume of space. If you imagine the individual particles as spheres (large bearings and Bbs, respectively), it's easy to see that you can pack a greater volume of the small particles together, whereas the larger particles have larger air spaces (voids) in between them. Something like this:
OOO
OOO
OOOOOO
OOOOOO
OOOOOO
Hopefully you can see that there is a larger Turface-to-air ratio in the smaller particles. This higher density of the smaller particles means that more water can be absorbed in a given pot space. Now, some soil components absorb little to no water, but that's where our second principle, adhesion, comes into play.
Looking at the illustration above, you should also be able to see that there would be an exponentially greater surface area per pot volume in the smaller particles. This greater surface area in the smaller particles gives water much more surface to adhere to than in the larger particles. This is where the lower limit of particle size comes into play. If the particles are too small, the water can stick to itself (cohesion), filling all of the voids and not draining from the pot, as in fine sand. The smallest screen I would recommend using is 1/16” (1.5mm).
This illustration also points out something that some people take for granted, and that is, soil components need to be sieved to a uniform size. Using particles of differing size causes the voids in between the larger particles to be filled. This makes it hard for the soil to dry out when wet, hard to wet the soil when dry, and hard for air to move in or out. Basically soil compaction.
I have just recently begun using the same 3 basic ingredients (Turface, scoria and coarse sand) 1:1:1 for most of my soil, adding pine bark or charcoal to adjust pH where necessary. (No scoria in Azaleas!) To retain more water, I can simply use smaller particles, instead of changing ingredients or percentages. However, there are no universal formulas for mixing soil because material cost, pot size, stage of development, local climate, fertilizer regimen, and watering habits all play a roll in what properties we want or need in our soil.
[Disclaimer: I’m not advocating using these specific soil components. This thread was initiated by a question on another thread originally about inexpensive Turface. If you can afford akadama and pumice, by all means use them. But I think we as a group tend to make mixing soils more complicated than it needs to be. This is meant to be a discussion of soil mechanics, not soil chemistry. Don’t even get me started on cation exchange capacity: if you use humic acids, it doesn’t matter. Also, I understand that there are indeed other factors at play, like particle surface geometry, dissimilar particle interfaces, porosity and capillary action. However, these factors are very minor in comparison to the above particle size principles.]
Imagine two identical bonsai pots filled with Turface (or any soil component), where pot A has only 1/4" sized particles and pot B has 1/16" particles. These two different particle sizes have the same water holding properties per individual particle: they absorb the same amount of water per cubic inch of particle, and water adheres to the surface of the particles in the same amount per square inch of particle surface area. However, these different size particles behave very differently as a planting medium simply because of their respective sizes.
There are 2 things at play here. The first is the density with which the particles can be packed into a given volume of space. If you imagine the individual particles as spheres (large bearings and Bbs, respectively), it's easy to see that you can pack a greater volume of the small particles together, whereas the larger particles have larger air spaces (voids) in between them. Something like this:
OOO
OOO
OOOOOO
OOOOOO
OOOOOO
Hopefully you can see that there is a larger Turface-to-air ratio in the smaller particles. This higher density of the smaller particles means that more water can be absorbed in a given pot space. Now, some soil components absorb little to no water, but that's where our second principle, adhesion, comes into play.
Looking at the illustration above, you should also be able to see that there would be an exponentially greater surface area per pot volume in the smaller particles. This greater surface area in the smaller particles gives water much more surface to adhere to than in the larger particles. This is where the lower limit of particle size comes into play. If the particles are too small, the water can stick to itself (cohesion), filling all of the voids and not draining from the pot, as in fine sand. The smallest screen I would recommend using is 1/16” (1.5mm).
This illustration also points out something that some people take for granted, and that is, soil components need to be sieved to a uniform size. Using particles of differing size causes the voids in between the larger particles to be filled. This makes it hard for the soil to dry out when wet, hard to wet the soil when dry, and hard for air to move in or out. Basically soil compaction.
I have just recently begun using the same 3 basic ingredients (Turface, scoria and coarse sand) 1:1:1 for most of my soil, adding pine bark or charcoal to adjust pH where necessary. (No scoria in Azaleas!) To retain more water, I can simply use smaller particles, instead of changing ingredients or percentages. However, there are no universal formulas for mixing soil because material cost, pot size, stage of development, local climate, fertilizer regimen, and watering habits all play a roll in what properties we want or need in our soil.
[Disclaimer: I’m not advocating using these specific soil components. This thread was initiated by a question on another thread originally about inexpensive Turface. If you can afford akadama and pumice, by all means use them. But I think we as a group tend to make mixing soils more complicated than it needs to be. This is meant to be a discussion of soil mechanics, not soil chemistry. Don’t even get me started on cation exchange capacity: if you use humic acids, it doesn’t matter. Also, I understand that there are indeed other factors at play, like particle surface geometry, dissimilar particle interfaces, porosity and capillary action. However, these factors are very minor in comparison to the above particle size principles.]
