Any1 no how to calculate % air in soil?

Question

Thanks!!

Answer 1

http://www.icrisat.org/OnlineResources/S...

Answer 2

It is very difficult to compute the percentage by weight of air in soil. And when we compute the percentage of air by volume, we are likely to include a large amount of space that includes water vapor, treating it as 'air'.

But we go ahead treating all spaces not filled with water as though they were filled with air.

Typical methods ignore clays, which expand with water inside the crystals. They basically assume that we have discrete volume for the sand or silt particles.

The next problem is, do we want the  percentage of space filled as a fraction of total space or as a percentage of available free space, that space that is either filled with water or air.

We really want to know what percentage of the available free space is air, what percentage is water. That way a soil that contains a lot of rock fragments  does not always have a low percentage of both air and water.

Answer 3

A basic way to measure would be to use a container of a known size and to gather a sample to start. A container might not be necessary if the sample can be managed and measured without falling apart but even with great care that is difficult. A container would be the way to initially approach this problem. To get an accurate measurement of how it is in its location, it needs to be as undisturbed as is possible when it is transferred to it's measuring container. Soil has both macro-pores and micro-pores that make up space around the actual soil components so in theory if one disturbs that structure one will not get a true reading. As with plowing, to "turn" or mix the soil breaks up it's structure and causes compaction of a sort; a settling by physical manipulation. Containers need to be large enough to give an accurate representation but small enough to work with. Larger amounts will give the better representation with the smallest amount of error but are harder to work and actually measure as sample size increases. Try a size of about a pint to quart. You need to figure out the exact cubic centimeters (inches can be used) of the container, as exact as possible. A round container has the ability to easily be turned upside down and pushed carefully into a soil that is to be sampled, then leveled off when turned upright without causing too great a disturbance. Keep in mind the actual measure will be slightly smaller as the manipulated sample will be compressed a bit.

The can I chose here is 11 cm X 8 cm which gives me a volume of pi X r squared X height or 3.1416 X 16 X 11 or 552.9 ccm, say 553. Next...

Now that you have a sample it will need a way to measure it. For this container, a can about the size of a soda can (a bit bigger) I need a well graduated liter sized container like a wide mouthed graduated cylinder. I want to put in enough water to submerge the sample completely, not just make mud. I would accurately take and record a sample of lets say 1000 ccm. The soil sample is measured and recorded as is the water quantity. Carefully add the soil to the water and see the difference. To make the example easy lets say the water rises to the 1513 ccm mark. That would mean the actual soil took up 513 ccm. Subtracted from 553 that would mean that 40 ccm was in macro-pores and micro-pores. 40/553 needs to be expressed as a ratio of 100. Soooooo....

40 is to 553  as "what number" is to 100

or by cross multiplying for ease...

40/553=a/100

or

40 X 100 = a X 553

or

7.233/100 or 7.23% space

While you have your sample in water you can do some fun stuff like, shake it up and measure the amount of material that settles immediately, measuring also different layers as the heavy coarse stuff falls first, light stuff after. Now you have sand, gravel on the bottom and humus products up top. Then give it some hours to fully settle and measure clay content. This is the real basics of it but gives a good idea. You can do measurements of different areas, plowed fieds and unplowed,  and different depths and compare and graph.

Answer 4

Typically, 50% of the soil, by volume, is pore space, which may contain either air or water. The amount of pore space which contains either water or air is very dynamic, and changes depending on how much or little rainfall has occured. After a heavy rain, all the pore space can be filled with water, and after a period of drought, almost all the pore space may be air.