The importance of the air/water cycle.
Woodland plant roots require Oxygen. Roots use Oxygen and give off Carbon Dioxide. So how does the Oxygen get into the soil and the Carbon Dioxide get out. First all gasses in the soil are in the pore spaces. There are two types of pores. Pore spaces larger than 0.03 mm are call Macro pores. Pore spaces smaller than 0.03 mm are called Micro pores. Macro pores are the most important for providing pathways for air and water to travel through the soil. I will use the drawings below to show the air/water cycle.
 | Dry soil with water in micro pores and on surface of particles. |
 | Water added to soil. Micro pores fill and Macro pores fill. Air is still found in Macro pores. Carbon Dioxide is being flushed as water fills the pores. |
 | Soil is saturated. Air is flushed from soil being replaced by water. |
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| As water drains Oxygen can enter as air is pulled into the soil. |
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| As soil dries more air can enter the soil. Water is still present in Micro pores and on the surfaces of particles. |
It is important that soil is allowed to become saturated and dry. If a soil is watered so only the first few inches are wet, Carbon Dioxide can not be flushed from the lower levels. If only the first few inches are allowed to dry then Oxygen can never enter the lower levels. So you can see the proper watering is very important not because the roots need water, but because the roots need Oxygen also.
One note of caution, clay and silt soils require longer periods of time to dry. Clay and silt have a lot of Micro pores. Capillary action of Micro pores causes them to draw in and hold water. Sandy and gravel soils can require frequent watering to maintain moisture levels.Sand and gravel soils have a lot of Macro pores.Macro pores can be very poor water holders.
