Ideal Soil


Ideal Soil

Soil Air and Composition

The diagram shows the relative components of the three phases of soil: solids, liquids, and gasses. The solids may be mineral or organic. The pores are filled with either air or water.

The mineral solids are composed of particles in the sand, silt, and clay size ranges.

I received two related questions about soil air: 

  • How much air is there in soil?
  • Can you explain about the air that present in the soil?


The answers are presented below.



Using the marbles, golf balls, beads, and water in a jar, ask students what they see? Most comment on the marbles, golf balls, and beads. Some will see the water. Say, the marbles, golf balls, and beads remind us that soil always has solid particles in it, and these particles are different sizes. The golf balls represent sand, the largest soil particles. Sand feels gritty when rubbed between the fingers. (Think sandpaper.) The marbles represent silt, the intermediate soil particles. The beads represent clay, the smallest soil particles.

The water is a reminder that soil always has some water in it. Ask what is in the jar that cannot be seen. If the students struggle, prompt them by taking a loud, deep breath. So the model demonstrates that soil always has three phases (solid, liquid, and gas) present at all times. Often, air bubbles can be seen below the water surface, and sometimes water can be seen in menisci above the water surface.

Water stored in soil can be used by plants, which transpire water back into the atmosphere. Some water stored by soil can be lost from bare soil surfaces by evaporation.

These are broad questions, that could have lengthy answers. A brief discussion follows. More specific questions are welcome.

Soil is a three-phase, porous media, composed of solids, liquids, and voids (empty spaces between the solids). The typical amount of total porosity (ratio of void volume to total volume) in a mineral soil ranges from about 40% to 60%. This means approximately 40 to 60% of the volume of a mineral soil is actually empty space between the solid particles (voids).

These voids are filled with air and/or water. The air in the soil is similar in composition to that in the atmosphere with the exception of oxygen, carbon dioxide, and water vapor. In soil air as in the atmosphere, nitrogen gas (dinitrogen) comprises about 78%. In the atmosphere, oxygen comprises about 21% and carbon dioxide comprises about 0.36%. However, in the soil air, oxygen usually is replaced by carbon dioxide, so both range from about 0.4% to 21%.Oxygen is used by plant roots and soil microbes during respiration, and carbon dioxide is released. Thus, in the soil, the oxygen levels are generally less than atmospheric levels and the carbon dioxide levels are generally greater than atmospheric levels. 

Some factors that determine the extent of the difference between atmospheric and soil air constituents include depth in the soil profile, soil pore size distribution, and soil water content. 


  • Depth: Oxygen levels generally decrease with depth in the soil profile due to slow diffusion rates of oxygen from the surface through the soil.
  • Pore size distribution: Soils with large pores promote more rapid oxygen diffusion into and through the soil, and carbon dioxide movement out of the soil. Soils with small pores have slower oxygen diffusion into the soil and carbon dioxide diffusion out of the soil. Sandy soils generally have low total porosity but large individual pores. Clay soils generally have high total porosity but small individual pores.
  • Aeration and drainage: Soils with large pores generally have good drainage (less water) and aeration, while soils with small pores generally have poor drainage and aeration. Thus, sands generally have good drainage, while clays have poor drainage and are more likely to become anaerobic (deprived of oxygen) as microbes use oxygen more rapidly than it is replenished through diffusion.
  • Soils with more pores filled with water have less space available for air, thus become anaerobic more rapidly than drier soils.
  • Water vapor: Soil air has a relative humidity very close to 100%. (Relative humidity is the amount of water vapor actually in the air relative to the amount the air could hold at that temperature.) This is much different than atmospheric air, which may vary in relative humidity between 5% and 100%, sometimes within 24 hours in semi-arid and arid regions. A Boy Scout Survival Kit applies this concept by providing a shovel, piece of plastic, and a cup. Dig a hole in the morning, place the cup in the bottom of the hole. Anchor the plastic with soil around the rim of the hole. Place a rock on the plastic above the cup. During the day, the plastic allows the soil to heat, evaporating water. At night, the plastic allows radiational cooling. As the air cools, water vapor condenses on the plastic and drips into the cup. This provides enough water for subsistence. If cacti or other succulents are available, place some of their vegetation in the hole to enhance the water provision.