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Physical Properties of Soil
Permeability (the rate at which
water moves through the soil) and Water-Holding
Capacity (WHC; the ability of a soils micropores
to hold water for plant use) are affected by
- The amount, size and arrangement
of pores
- Macropores control a soil’s
permeability and aeration.
- Micropores are responsible for a
soil’s WHC
Porosity is in turn affected
by
- Soil texture
- Soil structure
- Compaction
- Organic matter
 Soil
texture (the relative proportions of sand, silt,
and clay) is important in determining the water-holding
capacity of soil:
- Fine-textured soils hold more water
than coarse-textured soils but may not be ideal
- Medium-textured soils (loam family)
are most suitable for plant growth
- Sands are the largest particles and
feel gritty
- Silts are medium-sized and feel soft, silky, or floury
- Clays are the smallest sized particles and feel sticky and are hard to
squeeze.
- Relative size perspective: Sand (house) > Silt > Clay (penny)
Four main types of soil structure (the
arrangement of aggregates in a soil):
- Platy - common with puddling
or ponding of soils
- Prismatic (columnar) – common
in subsoils in arid and semi-arid regions
- Blocky – common
in subsoils especially in humid regions
- Granular (crumb) – common
in surface soils with high organic matter content
Properties of soil particle size
| |
Sand |
Silt |
Clay |
| Porosity |
mostly large pores |
small pores predominate |
small pores predominate |
| Permeability |
rapid |
low to moderate |
slow |
| Water holding capacity |
limited |
medium |
very large |
| Soil particle surface |
small |
medium |
very large |
Soil Compaction destoys the quality
of the soil because it restricts rooting depth and
decreases pore size. The effects are more water-filled
pores less able to absorb water, increasing runoff
and erosion, and lower soil temperatures. To reduce
compaction:
- Add organic matter
- Make fewer trips across area
- Practice reduced-till or no-till
systems
- Harvest when soils are not wet
Next page: Soils, water, and plant growth
Updated
July 15, 2004
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