Properties of Clay Soil

Soil Aggregates and Structure

• Breakdown of aggregates creates crusts that close pores and other pathways for water and seedlings
• Optimum conditions for aggregates: large range of pore size distribution, large pores between aggregates, and small pores within aggregates
• Pore space provides zones of weakness through which plant roots can grow

Factors Affecting Aggregate Stability

• Soil texture and type of clay
• Extractable iron and cations
• Amount and type of organic matter (OM)
• Type and size of microbial population
• Expansion and contraction of clay particles can shift and crack the soil mass, creating or breaking apart aggregates
• Ca ions associated with clay generally promote aggregation, while Na ions promote dispersion
• Soils with > 5% iron oxides tend to have greater aggregate stability
• Soils with high OM content have greater aggregate stability
• Additions of OM increase aggregate stability after decomposition begins and microorganisms have produced chemical breakdown products/mycelia have formed

Soil Microorganisms and Aggregates

• Soil microorganisms produce organic compounds that help hold aggregates together
• Type and species of microorganisms are important
• Fungal mycelial growth binds soil particles together more effectively than bacteria
• Aggregate stability declines rapidly in soil under intensive cultivation

Classification of Soil Structure

• Classification of soil structure considers the grade, type, and size of peds
• Grade describes the distinctiveness of the peds
• Units are distinctly longer vertically (columnar), single grain (particles show little or no tendency to adhere to other particles), massive (show little or no tendency to break apart under light pressure), or have angular and subangular blocky, granular and crumb, platy, or prismatic and columnar shapes
• Size of peds varies from very fine (< 5 mm) to very coarse (> 50 mm)

Improving Soil Structure

• Organic matter management: regular additions feed the organisms that build soil structure
• Tillage practices: tillage increases decomposition of SOM and breaks up aggregates, but residues left at the surface protect aggregates from rain and encourage fungal growth
• Compaction prevention: compaction pushes aggregates together and eventually breaks them down
• Crops/cropping systems: dense roots of grasses, small grains, and pastures stabilize soil aggregates and improve structure

Cation Exchange Capacity (CEC)

• CEC is the measure of how many negatively-charged sites are available in the soil
• CEC determines the soil's fertility, nutrient retention capacity, and capacity to protect groundwater from cation contamination
• Soil properties that determine CEC are texture, clay mineral type, and OM content
• Sandy soils generally have lower CEC than clay soils
• CEC values vary by soil texture, with clay and clay loams having higher CEC than sands and loams

Clay Minerals

• Clay particles are colloids with large surface areas, high water-holding capacities, and slow movement of water and air
• Shrinking and swelling of clay particles affect buildings, roads, and walls
• Chemical adsorption is large in clay minerals
• Generalized influence of soil separates on soil properties and behavior
• Crystalline silicate clays have various types, including kandites, smectites, vermiculite, illite, and chlorite
• Each type of clay mineral has distinct properties and characteristics