Do these values change between organics (such as Lowe's average run-of-the-mill potting soil) vs. inorganics (Akadama, Pumice, etc.)? And is this one of the many major distinctions between Development vs. Refinement? I've heard that soil such as Akadama has tubercles throughout the interior of each grain/particle: Does that play a role in the AFP/WHC?
Let’s define terms. “Macropores” or “mesopores” are the large spaces between the individual grains that make up the soil. “Micropores” are the tiny spaces within the grains themselves. All soils - whether organic or inorganic - have at least some degree of microporosity. Organic soils tend to have a lot. Inorganic soils have some and it’s pretty variable as to the amount. As the size of the pore decreases, the capillary and electrostatic forces binding the water to the soil surface increases dramatically. As a result, water contained in the macropores is generally plant available, but not all water contained in the micropores is, as the binding forces can exceed the plant’s ability to access it.
With that in mind, there are three saturation states that matter. “Saturation” happens when you are watering and all the available macro and microporosity is completely filled with water. The second you stop watering, the water drains away by gravity until the capillary forces holding onto the water are equal to or greater than the force of gravity. This state is called the “field capacity”. It’s the water available to the plant until the next time you water. Think of it like a battery. It’s fully charged at field capacity.
At field capacity, some of the pore space is filled with air, and some is filled with water. That’s the air-filled and water-filled porosity (AFP and WFP). How much of each is a function of the soil type, as discussed at length in this thread. Generally, the macropores are partially saturated and the micropores are fully saturated at field capacity.
Once you walk away, stuff happens to the remaining water. Some of it gets used by the plant. Some of it evaporates. The amount of water in the soil goes slowly and inexorably down over time just like draining the battery in your cell phone. At some point, the water saturation gets so low, the only water that’s left is bound to the soil by forces that exceeds the plant’s ability to access it. This point is called the “wilting point”. Most of the bound water at the wilting point is in the micropores.
All this is to say that yes. Micropores make a play a role in afp and wfp. All soils have micropores. Organic soils tend to have a lot of microporosity. Less of the water in micropores is plant available than that in mesopores.
S