Documentation:Aeration Porosity Table
Soil Aeration Porosity Measurement (Aeration Porosity Table):
Author: Teresa Porter, Dru Yates, Jason Lussier, Lewis Fausak
Last updated: April 6, 2021
What is Aeration Porosity?
Aeration porosity (fa) is defined as the relative pore volume occupied by aeration pores (i.e., the “large” pores or macropores).
The smallest aeration pore drains at a tension of 0.60 m of water (w). Thus, the aeration porosity of a soil is equal to the relative volume of water released by a fully saturated, undisturbed soil sample as the tension is increased from 0 to 0.60 m of w.
How is Aeration Porosity Measured?
Aeration porosity is determined with in several ways, including: hanging water columns, Hyprop (tensiometer system), ceramic pressure plates, and more commonly a tension table. To use a tension table: first, the soil core is slowly saturated from below in a basin, minimizing air entrapment. In a fully saturated soil (ft = qs), where ft is the total porosity and qs is the volumetric water content at saturation:
ft = Vf/Vt and qs = Vw/Vt
In a fully saturated soil Vw = Vf. Air entrapment causes qs < ft. Next, the fully saturated sample is weighed, drained on a tension table at a tension of 0.60 m of w, and weighed again.
Aeration porosity data provides information about aspects of soil structure that are related to the ease of gas exchange in the soil at the time of sampling. It says little about structure quality, or structure stability, although soils of low structural stability tend to have a low aeration porosity, except immediately after ploughing. In contrast to cultivated soils, in undisturbed soils the aeration porosity is more or less constant.
In some soils, earthworms holes make a negligible contribution to the aeration porosity, and a large contribution to the saturated hydraulic conductivity (Ks). Thus, it is possible for the traffic pan of a low aeration porosity (<0.10 cm3/cm3) to have a relatively high Ks (10-6 to 10-5 m/s).
Tension Table Method:
Sampling Supplies:
- 7.5cm x 7.5cm undisturbed soil samples, 7.5cm x 7.5cm cylinders and 3.75cm x 7.5cm spacer - Gloves
- Plastic bags
- Metal core rings
- Trowel
- Labels
- Coolers / fridge
- Shovel
- Drop hammer
- Knife or Butter knife
- Tape
Lab Supplies:
- Cheesecloth (grade 50)
- Basin
- Tin can discs, plastic lids (yogurt container type)
- Rubber bands
- Tension table (with a porous plate of air entry value 1.0m of w. Tension table is filled with carborundum (silicon carbide (SiC) powder)
- Electrical tape
- Silk/nylon cloth membrane
- Silicon carbide abrasive (400 grit, Lotone Inc brand) Carborundum powder
- Balance
- Spray bottle
- Vibrolator
- Metal stand, clamps, tubes
- Mixing bucket
- Bubble level
- Metallic mesh for tension plate
- Dust mask
Procedure
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Sampling
1. Sampling should be completed when soils are moist, at water contents below the plastic limit. Samples should be run as soon as possible after sampling.
2. Take soil samples from the field at the desired depth. Place metal core rings at the surface of the soil or desired depth after creating a level surface.
3. Use the drop hammer or metal hammer to pound the core into the soil.
4. Use a shovel to excavate the sample, and use a butter knife/knife to level the exposed surfaces so they are only the core volume.
5. Pack the sample in plastic bags and tape them so they are taught. This is to prevent compaction and loosening of soil in the core and store in a cooler until transporting back to a refrigerator (at 4⁰ Celcius). Mark which edge was the lower part in the soil.
Preparing Samples
1. Weigh plastic lid and elastic to be used with each sample and record (part of the tare weight.
2. Attach a cheesecloth ‘skirt’ using 4 layers of cheesecloth, to the top of the core with an elastic band (Fig 1). Cut the corners of the cheesecloth.
3.
Place 9 cores on a wire rack at the bottom of the saturation basin. [FL1] Saturate cores slowly from below, thereby allowing water to replace air and minimizing air entrapment. Make sure the water is added slowly (by dripping, not a continuous flow).
Preparing Tension Table
1. While wearing a dust mask, remove all the silicon carbide abrasive from the tension table into a bucket (Fig 2).
2. Remove the silk/nylon screen and wash clean.
3. Ensure the tension table base is dry
4. Saturate the silicon carbide abrasive with water in a mixing bucket.
5. Place metal mesh on the bottom of the tension plate (Fig 3). Cover the mesh with the cloth membrane (Fig 4). Tape the cloth membrane to the tension plate, completely covering all edges of the membrane with electrical tape (Fig 5).
6. Turn on the tap (outside the room) and raise the cup onto the wooden table so that water starts to fill the tension table from below.
7. Tap the tubes below the tension table to encourage air bubbles to rise. Remove air that is trapped beneath the membrane by lifting a corner and guiding bubbles out with your hand. Re-attach the cloth membrane (Fig 6)
8. When the tension table is filled with water, lower the cup back to the floor and adjust the outlet to be level with the table, at equilibrium.
9. Place the completely saturated carborundum powder (Fig 7) onto the table in an even layer (Fig 8).
10. After placing carborundum powder in the tension table, air entrapped in the powder’s pores needs to be eliminated by gently vibrating the bottom of the tension table with the Vibrolator set at 15 psi (repeat this until air bubbles stop appearing at the surface) (Fig 9).
11. No more than three batches should be run before cleaning and re-setting the table.
Running Your Samples
1. Quickly removed the saturated core from the basin. Do this by placing a metal lid below it before pulling it out of the water. Then place the core and metal lid on a plastic lid and weigh it.
2. Place 9 cores on the tension table with the top side up, and adjust the outlet at z=0 m (i.e., at the mid-point of the cores or 3.7 cm below the core’s surface), so that the water tension at the plate surface is equal to 0 m of w.
3. Cover the cores on the tension table to prevent evaporation.
4. Lower the outlet to z = -0.60 m. At equilibrium (i.e., when the outflow has stopped) the water tension halfway up the core will be 0.60 m of w (Fig 10 and 11).
5. After an equilibrium time of 24 hours, place samples on metal lids + plastic lids and weigh them.[FL2]
6. Remove elastic bands, and place samples + cloth + metal lids into the oven, and dry them at 105°C for 24 hours. Weigh again.
7. Remove the soil from the core with a minimum disturbance and obtain the second part of the tare weight (i.e., core + skirt + metal lid).
Calculations
Use the following equations to compute bulk density, total porosity, aeration porosity, and water-holding porosity:
*2.65 g cm-3 for mineral soils and 1.3 for organic soils.
References
1. Blake, G.R., Hartge, K.H., 1986. Bulk density. In: Klute, A. (Ed.), Methods of Soil Analysis. Part 1. 2nd ed., Agron. Monogr. 9. ASA-SSSA, Madison, WI, pp. 363-375.
2. Danielson, R.E., Sutherland, P.L., 1986. Porosity. In: Klute, A. (Ed.), Methods of Soil Analysis. Part 1. 2nd ed., Agron. Monogr. 9. ASA-SSSA, Madison, WI, pp. 443-461.
3. Hao, X., Ball, B.C., Culley, J.L.B., Carter, M.R., and Parkin, G.W. 2008. Soil density and porosity. In: Carter, M.R. and Gregorich, E.G. (Ed.), Soil Sampling and Methods of Analysis. 2nd ed. Canadian Society of Soil Science. CRC Press, Boca Raton, FL. pp. 743-759.