Lesson Outline |
Soils - Part 2: Physical Properties of Soil and Soil Water
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Pore SizeNext to soil texture, pore size is probably one of the most important physical features of a soil. It controls water and air movement and storage. Pores come in all sizes, although clays have predominantly small pores, and sands have large pores. Most soils are a mixture of sand, silt and clay particles, so there is a mixture of different sized soil pores (Figure 2.7).
Figure 2.7. Pore size arrange- ment in clay and sand. Small pores are connected to large pores that are connected to medium-sized pores. This complex connection of pores can be compared to a maze with wide, medium and narrow passageways. In addition, some passageways, and soil pores, may be dead ends. Within an aggregate, the pores are small. Between aggregates, pores are large. Small pores are usually called micropores, and large pores are called macropores. As organic matter is added, the number of macropores increases. These increases result from the increase in aggregation, decay of root channels and creation of earthworm channels. Macropores are crushed when a soil is compacted. Tillage tends to increase macropores in the short-term, but reduces the number of macropores in the long-term because of the loss of aggregation and severing of earthworm channels. |
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