Soil - Physical Properties of Soils - Structure

Structure

The clumping of the soil textural components of sand, silt and clay forms aggregates and the further association of those aggregates into larger units forms soil structures called peds. The adhesion of the soil textural components by organic substances, iron oxides, carbonates, clays, and silica, and the breakage of those aggregates due to expansion-contraction, freezing-thawing, and wetting-drying cycles form soil into distinct geometric forms. These peds evolve into units which may have various shapes, sizes and degrees of development. A soil clod, however, is not a ped but rather a mass of soil that results from mechanical disturbance. The soil structure affects aeration, water movement, conduction of heat, resistance to erosion and plant root growth. Water has the strongest effect on soil structure due to its solution and precipitation of minerals and its effect on plant growth.

Soil structure often gives clues to its texture, organic matter content, biological activity, past soil evolution, human use, and the chemical and mineralogical conditions under which the soil formed. While texture is defined by the mineral component of a soil and is an innate property of the soil that does not change with agricultural activities, soil structure can be improved or destroyed by our choice and timing of farming practices.

Soil Structural Classes:

1. Types: Shape and arrangement of peds

a. Platy: Peds are flattened one atop the other 1-10 mm thick.

Found in the A-horizon of forest soils and lake sedimentation.

b. Prismatic and Columnar: Prismlike peds are long in the

vertical dimension, 10-100 mm wide. Prismatic peds have flat

tops, columnar peds have rounded tops. Tend to form in the B-

horizon in high sodium soil where clay has accumulated.

c. Angular and subangular: Blocky peds are imperfect cubes,

5-50 mm, angular have sharp edges, subangular have rounded

edges. Tend to form in the B-horizon where clay has

accumulated and indicate poor water penetration.

d. Granular and Crumb: Spheroid peds of polyhedrons, 1-10 mm,

often found in the A-horizon in the presence of organic

material. Crumb peds are more porous and are considered ideal.

2.Classes: Size of peds whose ranges depend upon the above type

a. Very fine or very thin: <1 mm platy and spherical; <5 mm

blocky; <10 mm prismlike.

b. Fine or thin: 1-2 mm platy, and spherical; 5-10 mm blocky;

10-20 mm prismlike.

c. Medium: 2-5 mm platy, granular; 10-20 mm blocky; 20-50

prismlike.

d. Coarse or thick: 5-10 mm platy, granular; 20-50 mm blocky;

50-100 mm prismlike.

e. Very coarse or very thick: >10 mm platy, granular; >50 mm

blocky; >100 mm prismlike.

3. Grades: Is a measure of the degree of development or cementation within the

peds that results in their strength and stability.

a. Weak: Weak cementation allows peds to fall apart into the

three constituents of sand, silt and clay.

b. Moderate: Peds are not distinct in undisturbed soil but when

removed they break into aggregates, some broken aggregates and

little unaggregated material. This is considered ideal.

c. Strong:Peds are distinct before removed from the profile and

do not break apart easily.

d. Structureless: Soil is entirely cemented together in one

great mass such as slabs of clay or no cementation at all such

as with sand.

At the largest scale, the forces that shape a soil's structure result from swelling and shrinkage that initially tend to act horizontally, causing vertically oriented prismatic peds. Clayey soil, due to its differential drying rate with respect to the surface, will induce horizontal cracks, reducing columns to blocky peds. Roots, rodents, worms, and freezing-thawing further break the peds into a spherical shape.

At a smaller scale, plant roots extend into voids and remove water and cause the open spaces to increase, and further decrease physical aggregation size. At the same time roots, fungal hyphae and earthworms create microscopic tunnels that break up peds.

At an even smaller scale, soil aggregation continues as bacteria and fungi exude sticky polysaccharides which bind soil into small peds. The addition of the raw organic matter that bacteria and fungi feed upon encourages the formation of this desirable soil structure.

At the lowest scale, the soil chemistry affects the aggregation or dispersal of soil particles. The clay particles contain polyvalent cations which give faces of clay layers a net negative charge. At the same time the edges of the clay plates have a slight positive charge, thereby allowing the edges to adhere to the faces of other clay particles or to flocculate. On the other hand, when monovalent ions such as sodium invade and displace the polyvalent cations, they weaken the positive charges on the edges, while the negative surface charges are relatively strengthened. This leaves a net negative charge on the clay, causing the particles to push apart, and so prevents the flocculation of clay particles into larger assemblages. As a result, the clay disperses and settles into voids between peds, causing them to close. In this way the soil aggregation is destroyed and the soil made impenetrable to air and water. Such sodic soil tends to form columnar structures near the surface.