1. Finish chapter 4: Structure, aggregation, consistence, etc.
21 Sep 2007
Finish chapter 4:
Structure, aggregation, consistence, etc.

2. Texture: proportion of Individual particles Ex: construction supplies
Photo courtesy of Lynn Betts, USDA NRCS
Structure: arrangement of
those individual particles
Ex: a house

3. Four Structural Types 1. 2.

4. 3. 4.

6. Spheroidal Structure (granular and crumb)
Rounded aggregates that lie loosely in soil
Non-porous spherical aggregates are called granular structure
Porous spherical aggregates are called crumb structure
Associated with surface soils having high organic matter content
Most desirable structure because this type of structure allows for the greatest fluid and gaseous exchange
Maintenance of this structural type in surface soils should be a goal in agriculture

7. Spheroidal Structure (granular and crumb)

8. Soil Aggregate models Candy or chocolate haystack
Computer-generated picture

9. Platelike (platy) – horizontal plate or lens shape
Generally a subsurface structural type that can be inherited from parent material or created by impact and traffic, particularly in clayey soils
Platelike structure is undesirable because fluid movement is slowed down

10. Blocklike (angular blocky and subangular blocky) - squarish aggregates with sharp corners (blocky) or round corners (angular)
Blocky is generally a subsurface structural type and more desirable (particularly subangular blocky) for fluid exchange than prismlike or platelike structure

11. Prismlike (columnar and prismatic)
Vertical columns or pillars in the subsurface
Columnar structure has rounded tops and prismatic structure has angular or flat tops
found in sodium affected soils (sodic soils)
Prismlike structure is common in heavier soils
Fluids tend to quickly move down quickly but have slow penetration laterally into the structure

13. Other models that demonstrate soil organic matter effects
Trail mix bars or any cookie that has a mixture of different size ingredients and preferably coated.
The cohesive nature of these desserts shows how soil can form stable peds.

14. roots and fungal hyphae networks,
Aggregation
Structural units form in response to physical, chemical, and biological processes of weathering:
Clumping,
Cementing
Carbonates, Fe oxides, organic secretions, gels,…
cation bridging,
roots and fungal hyphae networks,

16. Aggregate Stability
If aggregates are not stable, they quickly fall apart when disturbed by mechanical manipulation or water
If this happens, then soil loses aggregation and fluid movement is greatly reduced (soil seals up)
Stable aggregates resist disturbance or breakdown
Maintenance of structural stability is critical for fluid exchange properties of soil
Tillage, organic matter additions and the degradation of the organic matter are keys to maintenance or improvement of structural stability

17. A soil aggregate high in organic matter will stick together The water will be clearer (left)
A soil aggregate low in organic matter will slake rapidly. The water will soon be cloudy (right)

18. Aggregate stability is particularly important on the surface to prevent crusting
When aggregation is lost, pore spaces are filled with clay and silt particles and the soil forms a crust
This problem is commonly seen in the low organic matter soils in the Mesilla Valley

19. Soil
crusting

20. Soil Tilth
A poorly defined term meant to convey the general physical condition of the soil
A soil of good tilth is physically conducive to plant growth (good aeration, holds water, etc.)
A soil of poor tilth is not physically conducive to plant growth. It may be cloddy, crusty, hard to work, etc

21. Soil on the right has better tilth
Soil on the right has better tilth. Higher organic matter results in better and more stable aggregation that results in a more favorable root environment.

22. Soil Consistence
Resistance to deformation or rupture under stress by dry or moist soil clods or peds.
Loose, Friable, Firm and extremely firm when moist;
Loose, Soft, Hard, and up to extremely hard when dry.
Friable is generally the consistency sought after by farmers (it works easily, clods break apart, but still support roots).
Friability depends much on texture, structure, and ultimately, organic matter content.

23. Loose: You have trouble picking out a single ped and the structure falls apart before you handle it
Friable: The ped breaks with a small amount of pressure

24. Firm: The ped breaks when you apply a good amount of pressure and dents your fingers before it breaks.
Extremely Firm: The ped can't be crushed with your fingers (you need a hammer!).

25. Plasticity is the degree to which soil can be deformed without rupturing
Stickiness is the capacity of soil to adhere to other objects (like your shoes when walking through a wet field)
Swelling and shrinking is the change in volume with increases/decreases in soil moisture
How much
stress can we
take before
we break???