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Soils & Soil Physical Properties

Part 2 – 12 | Unit 2.1

i. There are 12 soil textures
(see u Table 2.5, 12 Soil Textures
Names and their Abbreviations),
varying in percentages of sand,
silt, and clay

2. Structure

Structure is the arrangement of soil
particles into aggregates, and the pore
space around them

a) Aggregates.

i. Aggregates can be natural or
made by people (e.g., by tillage
in wet soils; these aggregates are
called clods)

Lecture 2: Soil Properties

ii. Types (shape) (See t Figure 2.3, Soil Structure and Its Effects on Permeability)

• Granular


• Blocky (angular and sub-angular)


• Platy


• Columnar and prismatic


• Single grain (non-structure)


• Massive (non-structure)

iii. Size: Very fine, fine, medium, coarse, very coarse, thick, thin (see u Table 2.6, Size
Classes of Soil Structural Units)

iv. Aggregate stability is the ability to withstand wetting and drying, wind, and actions
such as tillage. This is key for water infiltration, gas exchange, root growth, and long-
term resistance to wind and water erosion, and is an indicator of soil health.

b) What causes soil aggregates to form?

i. Biological factors help bind soil particles together

• Bacterial exudates


• Root activity and exudates (sugars that act as glue)


• Fungal hyphae


• Macrofauna (especially earthworm) activity and waste


• High organic matter content

ii. Soils high in sand and silt do not form aggregates well. The type and quantity of clay
particles greatly affects how well aggregates form and how they persist: Some types
of clay form very stable aggregates, while other form weak aggregates.

iii. Calcium can help stabilize soils, although growers need to be aware of the type of
calcium to apply depending on soil pH and the possibility of raising salinity. Overall,
gypsum is an inexpensive and non-toxic source of calcium, although it should be
used with care. See Resources and Unit 1.11, Reading and Interpreting Soil Test
Reports for more specific information.

iv. Climate—especially the temperature and precipitation of an area—can affect soil
aggregate formation. The physical action of freezing and thawing increases the
likelihood of particles sticking together. Drying of soils can pull particles apart, as can
the impact of raindrops.

u TABLE 2.5 | 12 SOIL TEXTURES NAMES AND THEIR ABBREVIATIONS

clay C sandy loam SL

sandy clay SC loam L

silty clay SIC silt loam SIL

clay loam CL loamy sand LS

sandy clay loam SCL sand S

silty clay loam SICL silt Sl