Chapter 4: Soil Architecture and Physical Properties

Munsell Chart

1) Hue - Redness, yellowness 2) Value - lightness, darkness 3) Chroma - intensity, brightness

How do physical properties influence how soils function in an ecosystem?

1) Success or failure of engineering/agriculture 2) Growth of plants 'movement of water and its dissolved nutrients/chemical pollutants

Silt

0.05-0.002 mm • Comp of weathered material • Large SA => allows weathering rapid enough to release plant nutrient • Pores b/w particles are small => retains more H2O • Does not exhibit stickiness/plasticity

Major factors that influence soil colour

1. Organic matter content 2. Water content 3. Presence/oxidization states of Mg, Fe

4 Principles of Soil Peds

1. Spherodial 2. Platy 3. Prismlike 4. Blocklike

Sand

2-0.05mm • Ex: quartz • Large pores can't hold water against the pull of gravity => quick drainage => allows entry of air and water • Low specific surface area Infertile

Formation and Stabilization of Soil aggregates

Granular aggregation of surface soil: highly dynamic while small aggregates: more stable

Prismlike

B hor, located: arid, semi-arid Ex: columnar, prismatic

Blocklike

B/C hor, located: humid • Ex: angular blocky, subangular blocky

Soil color

Compare small piece of soil to standard color chips in MUNSELL CHART

Macroaggregates

Composed of microaggregates bound together mainly by a kind of sticky network formed from fungal hyphae an fine roots

Cods

Compressed, cohesive chunks of soil, can form artificially when wet soil is excavated/plowed

Bulk Density

Db', mass of a unit volume of dry soil, includes both solids and pores • Soils with high proportion of pore space to solids have lower bulk density • Factors that affect ~ ○ Effect of soil texture § Silt, loams, clays, clay loams have lower bulk density - due to soils being organized in porous granules, pores exist b/w and within granules = high total pore space § Sandy - organic contents are low, solid particles are less aggregated and bulk densities are high, have few fine, within ped pores = less porosity ○ Depth in soil § Deeper - higher ~, due to result of lower organic contents, less aggregation, fewer roots and soil dwelling organisms and compaction due to weight of overlaying layers § Compact subsoils = 2.0 Mg/m3

Particle density

Dp', mass per unit volume of soil solids in contrast to the volume of soil, which includes spaces b/w particles • Same as specific gravity of a soil substance • Determined by a soils chemical composition and crystal structure • Vary b/w 2.6 - 2.75 Mg/m3 because quartz, feldspar, micas and colloidal silicates usually make up major portions of mineral soils and all have densities within this range ○ Examples: § Arable mineral surface soils - 2.65 Mg/m3 □ Can be adjusted to 3.0 Mg/m3 or higher, depending on large amounts of high density mineral (garnet, magnetite) § High organic matters - 0.9 - 1.4 Mg/m3

Platy

E hor, caused by compaction, inherited by Pm

Microaggregates

Fine sand grains, smaller clumps of silt, clay and organic debris bound together by root hairs, fungal hyphae, microbial gums

Submicroaggregates

Fine silt particles encrusted with organic debris and tiny bits of plant and microbial debris (particulate organic matter) encrusted with smaller packets of clay, humus, and Fe/Al oxides

Pseudosand

In highly weathered clayey soils, the cementing action of iron oxides and other inorganic compounds produce very stable small aggregates

Management practices Affecting Bulk Density:

Increase in bulk density = poorer environment for root growth, reduced aeration, undesired changes in hydrologic function (reduced water infiltration)

Macroaggregates

Large (0.25-5mm) comprised of microaggregates (2-250um, tiny packets of clay and o matter)

Massive:

Large, cohesive mass of material

Biopore:

Pores created by worms, roots, other organisms

Soil compaction

Process by which sediment progressively loses its porosity due?

Gleyed

Reduced Iron

Specific Surface Area

SA for a given mass particle

Clay

Smaller than 0.002mm • Large specific SA => Increase absorption of H2O • Causes clay particles to cohere in a hard mass after drying • Behave like colloids • Shape: flake, flat palettes • Spaces b/w are so small => slow H2O and air movement • Properties: shrink/swell, plasticity, H2O hold capacity, soil strength, chemical absorption

Affect of water on soil color

Water makes soils darker => influences level of O2 in soil ○ Results: § Affects oxidization of Fe and Mg (bright reds/browns) § Rate of organic matter accumulation § Reduced Fe (grey, bluish) § Anaerobic conditions (grey)

Primary particles

building blocks from which the house is constructed

Loam

defined as mixture of sand, silt, clay particles that exhibit the properties of those separates in equal proportions

Soil structure

describes the manner in which soil particles are aggregated, defined the nature of the system of pores/channels in soil, arrangement of sand, silt, clay and organic particles in soil ,different forces to form peds/aggregates

Soil texture

describes the sizes of soil particles • Larger mineral particles are embedded in, coated with clay and colloidal size materials • Organic matter/ other substances act as cement b/w individual particles => encouraging the formation of clumps/aggregates of soil

Both soil texture/structure

determines ability of soil to hold and conduct the water and air necessary for life

Macropores

larger than 0.08m ○ Allow the ready movement of air and the drainage of water ○ Large enough o accommodate plant roots and wide range of tiny animals that inhibit the soil ○ Sand grains, coarse soils ○ In well structured spores, located in peds Interped pores occur as spaced b/w loosely packed granules or as the planar cracks b/w tight fitting blocky and prismatic peds larger than 0.08m ○ Allow the ready movement of air and the drainage of water ○ Large enough o accommodate plant roots and wide range of tiny animals that inhibit the soil ○ Sand grains, coarse soils ○ In well structured spores, located in peds Interped pores occur as spaced b/w loosely packed granules or as the planar cracks b/w tight fitting blocky and prismatic peds

Hierarchy of soil aggregates

macro, micro,submicro,primary particles

Single grained

particles not aggregated

1. Spherodial

surface A hor, wide/rapid changes Ex: Granular, crumb

Ped

used to described large scale structure

Formation of soil aggregate

• Biological processes ○ Large, sandy soils • Physical-chemical process ○ Flocculation - mutual attraction among clay and organic mol ○ Swelling and shrinking of clay Smaller scale, clay

Primary Particles

• Cluster of parallel and random clay platelets interacting with Al and Fe oxides, organic polymers and smallest scare, these organoclay clusters or domains bind to the surface of humus particles and the smallest of mineral grains

Useful Density Figures:

• Engineers moving soil during construction - useful in estimating the weight of soil to be moved

Influence of Bulk Density on soil strength and root growth:

• High ~ may be natural soil profile feature or indicative of human induced soil compaction • Root growth is inhibited by excessively dense soils b/c soil's resistance to penetration, poor aeration, slow movement of nutrients/water, toxic gases buildup ○ Roots penetrate soils by pushing through pores, if pores are too small, root must push soil particles aside and enlarge soil § Thus, density restricts root growth, if pores are too small • Soil Strength: ○ Root penetration is limited due to ~ ○ Defined as the property of soil that causes it to resist deformation ○ Increases with bulk density, decreasing with water content Thus, root growth is most restricted when compacted soils are DRY

Agricultural Land

• Intense tillage increase Db b/c it depletes soil of organic matter and weakens soil structure ○ Add crop residues, manure and rotating crop with grass sod • Traffic pans: dense zones below plowed layer ○ Years to restore back to og porosity • Use large chisel type plows can be used in subsoiling to breakup dense subsoil layers • Must minimize trips of large equipment and tillage on wet soil Controlled traffic: restrict all wheel traffic to specific lanes

Green Roofs:

• Mass of soil must be minimized ○ Use sedums (shallow root) Soil with low Db - need netting system to prevent wind from blowing them off roof

Alteration of soil textural class

• Over long period of pedologic processes such as illuviation, erosion, mineral weathering => alter texture Mixing of different soil materials of different soil texture required

Urban Soils:

• Practices to help plant: Plant large hole, thick layer of mulch to drip line = enhance root growth , series of narrow trenches • Create artificial soils - skeleton of coarse angular gravel, mixture of loam topsoil = nutrient and water holding capacity

Forest Lands

• Surface of normal forest soil = low Db • Tree growth and forest ecosystem is sensitive to increase in Dd • Timber harvest compacts 20-40% of forest floor: Dragging of timber across floor, log piles, ○ Increasing Db = diminishing capacity of the soil to take in water = increase lost to surface run off • Damage from hikers Soil used as roads to access campsite

Pore space of Mineral Soils:

• We measure bulk density is that the value can be used to calculate pore space The lower the bulk density, the higher percent of pore space

Influence of organic matter

○ @ temperate zone soils: formation of granular aggregates is influenced by soil om ○ OM: provides energy substrate possible for biological processes ○ During aggregation soil mineral particles become coated and encrusted with bit of decomposing plant residue ○ Organic polymers resulting from decay chemically interact with part of silicate clay and fe, al oxides to form bridges b/w individual soil particles

Activities of soil organism

○ Biological processes of aggregation: burrowing and molding activities of soil animals, enmeshment of particles by sticky networks of roots and fungal hyphae, production of organic glues by microorganisms (bacteria, fungi) ○Ex: Earthworms => pellets/casts, plant roots exude sugar like polysaccharides (sticky networks), thread like fungi produce sticky sugar protein, glomalin

Flocculation

○ Flocculation of clay particles into microscopic clumps 'floccules' ○ If 2 clay platelets come close enough to each other, (+) charged ions compressed in a layer b/w them will attract (-) charged on both platelets, serves as bridges holding platelets together ○ Forms small stack of platelets aka clay domain ○ Domains with random orientation are formed by polyvalent cations (Ca2+, Fe 2+) with complex hydrophobic humus mol, allowing them to bind to clay surfaces ○ Clay/humus domains form bridges that bind to each other and to fine silt particles, creating the smallest size grouping of soil aggregates ○ Stability for microaggregates are through these domains + flocculation of polyvalent cations and humus ○ Monovalent cations attractive forces, esp Na+, in some arid and semiarid soils, can not overcome the natural repulsion of clay platelet, so they remain in gel-like condition § Soil becomes structureless, impervious (not allowing water/air to pass), infertile

Volume changes in clayey materials

○ Soil dries out = water is withdrawn = platelets in clay domains move closer together = soil mass shrink in volume § Cracks will open, along with zones of weakness § Over many cycles, cracks become defined ○ Plant roots aids in drying ○ Freezing and thawing cycles have similar effects = creates fissures/pressures that break apart large soil masses

5. Influence of tillage

○ Too wet or dry = tillage can break large clods into natural aggregates, creating loose, porous condition conducive to plant growth ○ However, greatly hastens (rapid) the oxidative loss of soil organic matter from surface horizons, thus weakening soil aggregates ○Tillage when wet = crush/smear soil agg = lost of macroporosity and creation of puddled condition

Micropores

○ Usually filled with water § When not: too small to permit much air movement ○ Water movement = slow, not available to plants ○ Fire texture soils ○ Too small to permit entrance of organism Act as hiding places for some absorbed organic compounds, protecting them from breakdown for years