Slide 1 Soil Structure LHAP 1 Urban Soils Slide 2 Texture: LOAMS - the best but...... wide range of composition wider range of properties (air and water) some set like concrete, some fine like powder, some form surface crusts ...... Slide 3 Soil Structure Soil structure may be more important in determining how a soil reacts towards water movement and plant roots and YES! we can improve soil structure Slide 4 Aggregates Freezing & thawing, swelling & shrinking, will push these particles together to form aggregates sand and silt particles COHERE together as clay particles coat them and hold them together usda.com Slide 5 Aggregates the result of this “coming together” or flocculation, of soil particles is the formation of aggregates Aggregates are porous with voids between the grains (micropores) usda.com Slide 6 Flocculation particles of various sizes come together to form aggregates besides this physical action, there is also help from Cations in the soil - Calcium in particular Ca++ Ca++ Ca++ Ca++ Slide 7 Flocculation Calcium (two hands!) draws clay particles together to help keep aggregates together Ca Clay Clay Slide 8 sciencedaily.com Slide 9 Aggregate Stability unstable aggregates break up easily these are susceptible to compaction these are said to have “weak” structure + WATER Slide 10 Time to introduce:Organic Matter as organic matter breaks down (leaves, roots, etc.) bacteria and fungi release gummy, mucilaginous materials Slide 11 Organic Matter: Cementation these act as cementing agents - they coat the sand and silts Slide 12 Aggregate Stability Stable aggregates do not break up easily these are said to have “strong” structure + WATER Slide 13 NOOO... Even STABLE aggregation (strong structure) will be destroyed under strong compactive forces! Slide 14 Aggregates larger pores, macropores, exist BETWEEN the aggregates. Most of the water movement and root growth occurs in these macropores usda.com Slide 15 Deflocculation Sodium however keeps particles apart particles are pushed away from each other Sodium, Na+, destroys soil structure a Clay Clay Na Na Slide 16 Slide 17 Naturally-formed Structures Slide 18 Naturally formed Structures over a couple of thousand years! Slide 19 Granular (Topsoil) rounded aggregates usually less than 1 cm allows good water movement and air flow resists soil erosion usda.com Slide 20 Prismatic (B horizon) provide good conditions for air and water to move into the subsoil usda.com Slide 21 Blocky (B horizon) provide good conditions for air and water to move into the subsoil usda.com 3 cm Slide 22 Columnar ( B horizon- Solonetzic soil •Provides poor infiltration, percolation and root penetration Slide 23 Structureless ( amorphous) Slide 24 Massives soils that are moved or worked when they are wet will tend to lose their structure. after soil is dug up, transported, spread, graded and compacted, it may fail to drain or support plant growth. Slide 25 Massive water infiltrates VERY slowly does not drain through fast enough to allow roots to have oxygen Slide 26 Massive prevents the growth of roots plants “drown out” with high rainfall and “dry out” under dry conditions Why? Roots cannot penetrate to grow deep in the soil Slide 27 D9 Cat scrapes away the soil elite-excavation.com Slide 28 Stock Piling of Topsoil on site or may be hauled in from elsewhere COD.EDU. Slide 29 Removal of the surface layer note the sod pieces spitfirespares.com. Slide 30 Scraping, Piling, compacting, mixing.... several layers get mixed, may “average” out the textures- destroys structure farm4static.flicks.com Slide 31 Grading with heavy equipment upodc.govrt.nz.com Slide 32 Sandra Allen Cut and Fill dumping & spreading soil to raise it to a higher grade, etc. Slide 33 Soil Interface abrupt boundary roots and water have difficulty penetrating the interface. soilsofcanada.ca Slide 34 Soil Interface Slide 35 Perched Water Table if water sits on top of the interface Slide 36 Urban Soils: General Characteristics great vertical and spatial variability modified soil structure leading to compaction presence of soil crusting on bare soil - usually hydrophobic pH usually elevated sandy loam fill weathred rock silty loam fill clay fill sandy fill concrete refuse organic layer deep clay original topsoil bedrock Slide 37 Urban Soils: General Characteristics restricted aeration and drainage modified (lack of) soil organism populations and activity presence of man-made materials and other contaminants highly modified soil temperature regimes sandy loam fill weathred rock silty loam fill clay fill sandy fill concrete refuse organic layer deep clay original topsoil bedrock Slide 38 Natural vs Urban Slide 39 End of Structure Lecture