Introductory soil physics

[markyscott]

To explain where the notion that porosity is independent of grain size comes from, I have to tell you all about packing. If you imagine that you wanted to arrange them in a box, you could imagine a number of ways the marbles could be stacked.

The simplest way is like this



The top image is looking down on the marbles, the bottom image is looking from the side. This is called "cubic packing" because if you imagine drawing lines that connected the centers of all of the spheres, it would form a little cube.

The porosity of this very simple system can be calculated mathematically without too much difficulty - it's 47.6%. And it's completely independent of the size of the spheres. Make them twice as large and you get exactly the same number. Kind of interesting, isn't it.

Scott

 

[markyscott]

But I'm sure you can also imagine that there are a bunch of different ways that you can pack the marbles. Here are some examples:



Each arrangement has a bit different porosity and it can be easily calculated mathematically that the porosity is independent of the diameter of the marbles.

Seems pretty cut and dried, doesn't it?

Scott

 

[markyscott]

For the math geeks out there (I'm proud to count myself among them). The calculations of these ideal systems is pretty simple for the cubic packing case. For example, taking the cubic arrangement of identical spheres of radius r occupying a cubic unit cell of length L:



We can show the following:


Similar calculations can be done for all the packing arrangements (albeit a bit tougher) with similar results - porosity is not a function of grain size for all these regular packing arrangments.

Scott

 

[Anthony]

The Ball bearing principle thanks you mightily Scott

 

[markyscott]

So for the normal folks who checked out with the math I just showed that porosity is not a function of grain size for cubic packing. It can be shown that this is also true for the other ordered packing lattices. However, it is not true for the random arrangement of spheres. Grains making up substrates in the bonsai pot are not arranged in ordered packings because they are energetically unstable, and the grains are randomly distributed.

For RANDOMLY packed grain, the porosity is controlled by the frictional and cohesive forces operating between the grains. For these kind of systems, porosity is a function of the grain surface area.

But here's the wierd thing - relative to the grain size, surface area is large for small grains and small for large grains.
So, all other factors are equal, a given weight of coarse grains will be stabilized at a lower porosity than the same weight of finer grains.

Say what? That's right - randomly packed coarse grains will have a lower porosity than randomly packed small grains. Here is the data for randomly packed angular vs rounded grains. See the porosity decreases as grain size increases toward some theoretical limit.



Now the effect gets smaller and smaller as the grain size grows - and these are pretty fine grains. An 1/8th of an inch is about 3000 microns. So this is a little theoretical. But interesting nonetheless.

Scott

 

[0soyoung]

Because knowing the names of rocks is empowering. It can tell you the geologic history of an AREA!!!

I can attest to that. For several years I hiked weekly with a retired WSU geology prof.
Great stuff, even though some it turned out to be just the same ole schist.

 

[markyscott]

I can attest to that. For several years I hiked weekly with a retired WSU geology prof.
Great stuff, even though some it turned out to be just the same ole schist.

Who was it? I got my PhD at UW and I know quite a few of the WWU professors.

Scott

 

[0soyoung]

Who was it? I got my PhD at UW and I know quite a few of the WWU professors.

Scott

Gene Kiver - know him?

He was at EWU (Cheney), not WSU, nor WWU (sorry for the confusion)

 

[markyscott]

Gene Kiver - know him?

Hmm - afraid not. But if you spent time with someone who knows Western Washington geology, it's a great opportunity.

 

[jacob letoile]

For the last graph you showed does that assume a consistent particle size?

 

[markyscott]

For the last graph you showed does that assume a consistent particle size?

No sir. Particle size is the independent variable.

S

 

[jacob letoile]

Then I'll ask the obvious question. If porosity increases as soil particle size decreses Why don't we use a nice silty loam, maybe like this one:
soilseries.sc.egov.usda.gov/OSD_Docs/H/HADLEY.html

I have started using my dirt to pot houseplants and it works well, provided I let it dry out between waterings. I have been thinking of trying a trial tree in it. The sacrilege.

 

[markyscott]

Then I'll ask the obvious question. If porosity increases as soil particle size decreses Why don't we use a nice silty loam, maybe like this one:
soilseries.sc.egov.usda.gov/OSD_Docs/H/HADLEY.html

I have started using my dirt to pot houseplants and it works well, provided I let it dry out between waterings. I have been thinking of trying a trial tree in it. The sacrilege.

It's the pore size that matters. Not the number of pores. Pore size scales directly with grain size. With fine grained, loamy soils your water saturation will be very high and the air-filled porosity will be very low and there are some limits for what is optimal for your trees. There are some experiments in this thread and the associated resource to guide you on deciding how small is too small for you and your garden. In general, I'd advise using a deeper pot if you want to use a finer soil.

 

[petegreg]

Then I'll ask the obvious question. If porosity increases as soil particle size decreses Why don't we use a nice silty loam, maybe like this one:
soilseries.sc.egov.usda.gov/OSD_Docs/H/HADLEY.html

I have started using my dirt to pot houseplants and it works well, provided I let it dry out between waterings. I have been thinking of trying a trial tree in it. The sacrilege.

I agree. with Scott.

Look, a good gardener can keep trees alive and happy? in whatever soil. But the more species you keep the more you will need something different as for substrate...to have everything under control. I mean basically watering and fertilizing, that can be really difficult in some different climates...

 

[jacob letoile]

Thank you.

I have been planning on trying those experiments, out of curiosity. I will be sure to post my results.

 

[jacob letoile]

@markyscott before I try to do those experiments myself do you know of a resource that may list typical values? I would imagine the experiments have been done on the Hadley series soils as they are economically important in this area. All google is giving me is variations on the page I linked earlier.

 

[markyscott]

@markyscott before I try to do those experiments myself do you know of a resource that may list typical values? I would imagine the experiments have been done on the Hadley series soils as they are economically important in this area. All google is giving me is variations on the page I linked earlier.

Are you looking for a resource that tabulates the field capacity of garden loams or the container capacity of potting soils? A garden loam will have much higher water saturation in a container than in the field...

Scott

 

[jacob letoile]

I was thinking field capacity, i hadn't thought about it behaving differently in a container. I guess I'm going to have to do the experiments myself nothing wrong with that.

 

[markyscott]

I was thinking field capacity, i hadn't thought about it behaving differently in a container. I guess I'm going to have to do the experiments myself nothing wrong with that.

Plenty of stuff on field capacity of garden loams, but it's irrelevant to container culture. Let us know how it goes.

Scott

 

[Gary McCarthy]

I find this information on the water retention and oxygen retention properties of different soil ingredients very interesting and have started testing my own ingredients. Just curious how many other people have gone through the process of testing their own bonsai soil mixes.