Calculating total porosity, aeration porosity, and water retention of growing media or soil.
The procedure below was developed by Liegel and Venator, 1987 to determine total porosity, aeration porosity, and water retention of growing media. It can also be used to determine the same properties in soil.
- First you need a container with a drain hole or fine screen on the bottom. If a container with drain hole is used then you need a plug for the drain hole container. If the container has a fine screen, then you need a cap. I use a 2 Liter plastic pop bottle with the bottom cut out. You will also need a graduated cylinder or measuring container for measuring water volume. A marker will also be needed along with a piece of window screening.
- Cut bottom off pop bottle. Turn it upside down. Put screen over spout then put the cap on the pop bottle and fill it with water. Mark the side of the pop bottle where the water is when filled to 3 inches from opening cut from bottom. Measure volume of water in the container. The is equal to CONTAINER VOLUME.
- Empty container and obtain a sample of soil or potting mix that has been air dried. This soil or potting mix that has been totally died by placing in a oven at low temperature for at least 24 hours or left in a warm dry place for a couple of weeks. Soil or potting mix must be totally dry. Take the dry soil or potting mix and fill the pop bottle to the mark line. Make sure soil is firmly packed. You need it as firmly packed in the pop bottle as the soil is in the ground. Next slowly and thoroughly saturate by adding water at one edge of pop bottle. Measure all water added. Water added is the TOTAL PORE VOLUME.
- Remove the cap but leave the screen. Allow the water to drain through the screen into a container. Measure all water drained, this is the AERATION PORE VOLUME.
- You can calculate the POROSITY by dividing TOTAL PORE VOLUME (step 3) by CONTAINER VOLUME (step 2).
- AERATION POROSITY is obtained by dividing AERATION PORE VOLUME (step 4) by CONTAINER VOLUME (step 2).
- WATER RETENTION POROSITY is obtained by subtracting AERATION POROSITY (step 6) from POROSITY (step 5).
Percentage Porosity [step 5] = (CONTAINER SOIL PORE VOLUME / CONTAINER VOLUME) X 100
Percentage Aeration Porosity [step 6] = (AERATION PORE VOLUME / CONTAINER VOLUME) X 100
Percentage Water-retention Porosity [step 7] = POROSITY - AERATION POROSITY
This will be a good measure provided soil or growing media is packed as firmly in pop bottle as it is in the ground. If soil in ground is badly compacted then pore space will be higher in pop bottle.
