UCANR

Soil Mixes Part 2: Water and Air Porosity

  • February 13, 2019
  • By Steven A Tjosvold

Container soils must have the right balance of air-filled and water-filled pore space. This is especially important right after irrigation. I showed in the previous blog that field soils shouldn't be used in containers because they don't drain well and create a saturated root environment.  Roots need air (oxygen) for respiration, so you can't have the roots drowning for very long. You need a good balance of air and water. This blog discusses what is a good balance and explains why container size affects it.

Simply, container soils consist of solids that form the intricate pore space that holds water and air. After full drainage (container capacity), the pore space on the bottom of the pot is saturated with water. Closer to the surface, there are more open pores that hold air.  This is seen in Figure 1.

Fig 1 soil, water and air
Diagram showing soil composition as a mix of solid particles, water, and air. On the left, packed circles of different sizes represent soil particles with blue spaces indicating water and air between them. On the right, a vertical chart labeled “% by volume” shows proportions of air at the top, water in the middle, and solids at the bottom after full drainage.

Fig 1 soil, water and air

The balance between air and water filled pores in the container can be illustrated more specifically. In Fig 2, water volume (ml. / cc.) in vertically sliced sections are shown on the container with the corresponding air volume in the table. Note that total porosity is constant if the soil components are uniform, and that Air + Water = Total Porosity. 

Fig 2 Water in sections
Photo of a plant pot marked with horizontal lines indicating water volume at different depths after full drainage (151 ml to 261 ml). Next to it, a table lists total porosity (261 ml) and corresponding water and air volumes at each depth, showing water increasing and air decreasing with depth.

Fig 2 Water in sections

It is useful to define terms to describe the amounts of air and water and total porosity. Total porosity is the percent of the soil volume occupied by pore spaces, water- holding capacity is the percent of the soil volume that contains water, and air-filled porosity is the percent of the soil volume that is filled with air.  These parameters can be measured for any soil mix for a designated container size. With standardized parameters, researchers can study how they affect plant growth and root diseases. Soil manufacturers can develop, standardize and describe soil mixes.  See figure 3.

Fig 3 Total, water, and air porosity
Text slide defining soil physical properties: total porosity (percent pore space by volume), air-filled porosity (percent air content), and water-holding capacity (percent water content). A note explains “container capacity” as the point just after full drainage.

Fig 3 Total, water, and air porosity

These parameters would of course be affected by the particle sizes of the soil components. (See previous blog, Fig 5). In addition, the size of the container-- actually the height of the container-- affects these parameters too.  For any given container mix, as the container height decreases, the water- holding capacity increases. As container height increases, the water-holding capacity decreases.  This is seen in a very simple way using the pot example given earlier. I have visually "sliced off" the top of the container to show the water that would be held in shorter containers. See figure 4.

Fig 4 Water-holding capacity vs depth
Three black plastic plant containers of different heights are shown side by side, each marked with horizontal purple lines and labeled water volumes (from about 151 ml near the top to 261 ml near the bottom). The tallest container shows more air space above the soil line, while the shortest container is filled closer to the top. A caption explains that for a given soil mix, shorter containers hold a greater proportion of water and therefore less air. A label at the bottom reads “Water volume (ml) versus depth.”

Fig 4 Water-holding capacity vs depth

And seen more specifically with this example using air-filled porosity. Remember, more water, less air! Fig 5.

Fig 5 Air filled porosity for certain soils
Chart titled “Percent air volume of various growing containers.” Table shows air-filled porosity values: 648 tray—0.5%; 288 cell—2.8%; 4-inch pot (3.25 inches tall)—13%; 6-inch pot (4.5 inches tall)—20%. Note at bottom: “1:1 peat:vermiculite soil.”

Fig 5 Air filled porosity for certain soils.

So a container soil that works well in a tall container may have too high of a water holding capacity and too little air-filled porosity in a short container.  The opposite is true for a short container too. The container height should be considered when selecting the container mix. You probably have seen this. The container media for a propagation flat for vegetative cuttings often is composed of components with large particles (e.g. perlite, scoria, and vermiculite) to minimize the overall water holding capacity and maximize the overall air-filled porosity.

 

Next:  What are favorable levels for water-holding capacity, air-filled porosity and total porosity?

 

Figures and tables adapted from:  Management of Container Media by Richard Evans, UC Davis, for the class, ENH 120. Photo of pot/water relationship by James Altland

Source URL: https://ucanr.edu/blog/nursery-and-flower-grower/article/soil-mixes-part-2-water-and-air-porosity