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.
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
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.
