Two types of pores occur in soil termed macropores and micropores. Macropores are associated with soil aeration porosity, and are known as noncapillary pores. Micropores are capillary pores and associated with moisture retention.
Macropores are extremely important in water movement (infiltration, percolation, drainage), gas exchange (oxygen and carbon dioxide), and provide root channels. Macro pores are larger than 0.10 mm in diameter (some soil scientists use 0.075 mm), while micropores are less than 0.10 mm.
If a soil is completely saturated with water it is the macropores that drain within about an hour. If the macropores fail to drain for various reasons including a lack of macropores, and poor drainage the soil can change from an aerobic soil (adequate oxygen) to anaerobic (limiting oxygen). The impact of changing from an aerobic to anaerobic situation is shown in Table 1. In addition a reduction in iron, sulfur and manganese forms which are black layer compounds.
Under hot humid weather the challenge is to create, protect and prevent waterlogging of macropores. Creating macropores can occur by natural soil structure formation, cultivation, and soil modification (sandcapping, or complete modification). Preventing water logging of macropores or keeping them empty of water can be accomplished by surface and subsurface drainage, cultivation of layers impeding water percolation, and controlling the water table. Protecting current macropores consists of providing macropore continuity to the surface would include topdressing, controlling traffic (compaction), and surface sealing (ex. algae, organic matter).
Cultivation creates macropores. Hollow tine core cultivation with 13 mm diameter tines or larger with the cores removed and holes filled with topdressing will last 5 to 8 weeks. Venting operations (link) such as small solid tines (6 to 7 mm in diameter) into the rootzone mix will last 2 to 3 weeks. Light frequent sand topdressing will help dilute the organic matter.
Table 1. Gas levels when a soil changes from aerobic to an anaerobic.
|Change in gas||Aerobic||Anaerobic|
|Carbon Dioxide||0.032- 2.%||2 to 10 %|
|Methane||0||0.1 to 1.0 %|
|Nitrous oxide||0||<.65 %|
|Hydrogen Sulfide||0||< 0.1 %|