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Soils & Soil Physical Properties

Unit 2.1 | Part 2 –19
Lecture 2: Soil Properties

Distance from furrow center (inches)

Soil texture not only affects how fast water moves through the soil, it also affects the
pattern of movement. Water will move almost straight down through a sandy soil
whereas it will have more lateral movement in a soil with higher clay content.
(See t Figure 2.5, Movement of Water through Sandy and Clay Soils)

Sandy loam Clay loam

Distance from bottom (inches)

t FIGURE 2.5 | MOVEMENT OF WATER THROUGH SANDY AND CLAY SOILS

ii. Structure

Soil structure has perhaps the greatest effect on permeability, especially as it
relates to pores between soil particles. As we saw earlier, these pores allow for the
movement of air and water through the soil.

Practices that improve soil structure also improve permeability. For example, heavy
overhead irrigation or flood irrigation breaks down soil structure, which can lead to a
sealing of the soil surface. This in turn makes it more difficult for any further water to
enter the soil. Tillage can help break up a soil that has become sealed, particularly if
it is done when the soil is not too wet (see Unit 1.2).

Other properties that relate to soil structure, permeability, and infiltration include:

• Salts: Sodium salts cause soil particles to disperse and clog pores, which has
  a negative effect on soil structure. Such soils tend to seal when wet, which
  drastically lowers both infiltration and permeability.


• Organic matter: As described above, organic matter improves soil structure,
  improving permeability and infiltration


• Compaction and pores: Fine-textured soils (soils with high clay content) contain
  more total pore space than coarse-textured soils (soils with high sand content),
  however the pore spaces are smaller. Because of this, water moves more slowly
  through a fine-textured soil, leading to lower permeability and infiltration.


• Calcium: Calcium improves soil structure by encouraging aggregation and
  increasing pore size. As a result it increases permeability and infiltration.


• Soil organisms: Microorganisms (e.g., bacteria and fungi) and macroorganisms
  (e.g., insects and earthworms) in the soil increase permeability and infiltration by
  encouraging the formation of soil aggregates and creating macropores in the soil

d) Additional properties influencing infiltration

i. Dryness: Frequently, dry soils will repel water until they become moistened to some
degree. This is especially true of soils that have high amounts of organic matter.
(See Peat Moss Demonstration in Supplemental Demonstrations and Examples.)

Illustration by José Miguel Mayo

m = minutes, hr = hours