Presentation on theme: "SOIL ARCHITECTURAL AND PHYSICAL PROPERTIES"— Presentation transcript:
1 SOIL ARCHITECTURAL AND PHYSICAL PROPERTIES Soil ColourValuable clues to the nature of soil properties and conditions.Munsell Colour ChartsHue (colour)Chroma (intensity)Value (brightness)Value and chroma are assessed from each hue page (Plate 22; after Page 112).
3 Factors affecting soil colour: Organic content- darkness and masking of oxidation effects)Moisture level (darker when wet)3. Presence and oxidation state of Fe and Mn oxides- Oxidized - iron oxides - red- Reduced - greys and blues when iron reduced (gley)Well-drained soils have more oxidized conditions.Calcite gives whitish colour in semi-arid regions.
4 Soil Texture Based on sand, silt and clay fraction (see earlier notes) Effect of exposed surface area on other soil propertiesIncreases capacity to hold waterNutrients and chemicals retained more effectivelyRelease of nutrients from weatherable minerals faster4. Electromagnetic charges. Increases stickiness and aggregation.Not a lot of clay/organics are required to impart these featuresBest soils are usually those with relatively equal proportions of the different soil texture classes.
5 Review: surface area higher for smaller clasts 286 cm21,536 cm2
8 Particle-size analyses in the laboratoryPipette or hydrometer methodsTreat soil (eg. with H2O2) to remove organic matterPipette Method2. Separate out the coarse fragments (gravel, coarse sand and fine sand). Silt and clay fragments are washed into a sedimentation cylinder.3. Silt and clay suspension is stirred and allowed to settle4. Clay fraction assessed using pipette at given depth determined by Stokes Law (d is particle diameter)V= kd2t = h/(d2k)
9 Separating out thesand fragmentsSilt and claysuspensionWeight of eachsand fragment isdetermined
10 Hydrometer Method (Lab 2) Place measured quantity of soil in a stirring cup and mix with deionized water and a dispersing agent [eg.(NaPO3)6]Transfer to settling cylinder, add deionized water to ameasured level (eg. 1L) and record the temperature of thesuspension.Insert plunger and mix by pulling plunger up with short jerks. Record the start time with second accuracy.Gently insert the hydrometer and record its reading aftera set time (eg. 40 seconds). Correct for temperature.Repeat 4&5 three times or more to get a good average.After 3 hrs (less in our case), take another reading with the hydrometer.Calculate % sand, silt, and clay, and determine the soil textural class
12 Structure of Mineral Soils - aggregates or peds- affects water movement, heat transfer, aeration and porosityaffected by human action (logging, grazing, tillage, drainage, manuring, compaction and liming)1. Spheroidal (granular or crumb)- most common in A Horizons2. Plate-like- most common in E Horizons- due to compaction or inherited from parent material3. Block-like- common in B Horizons of humid regions4. Prism-like- common in B Horizons of arid and semi-arid regions
20 Analysis of structure in the field 1. Type of peds2. Relative size of peds (fine, medium, coarse)3. Distinctness or development of peds (weak, moderate, strong)*Difficult to assess when the soil is wet*Soil Particle DensityDp = Mass per unit volume of soil solidsMeasured in Mg/m3Particle density is not affected by pore space, because it does nottake them into account.Mineral soils mainly in the 2.60 to 2.75 Mg/m3 rangeUp to 3.00 Mg/m3 if minerals very dense (eg. magnetite, hornblende)Organic matter has a much lower particle density ( Mg/m3)
21 Soil Bulk DensityDb = Mass per unit volume of dry soilSoil corers used to obtain known volume without disturbanceSoils are then dried and weighed*Db includes both solids and pores*Bulk density is affected by soil porosityHighly porous soils have a low bulk densitySandy soils have a higher bulk density than clayey or silty soilsThe latter are organized into more porous granules (intraped micropores)
24 Well-sorted soils generally have lower bulk density Well-graded soils generally have higher bulk densityTightly-packed soils have higher bulk densityA typical, dry medium-textured soil weighs 1250 Kg/m3 or 1.25 Mg/m3Careful with your pick-up truck!
25 High bulk density indicates: Poor environment for root growthReduced aerationReduced water infiltration and drainageHuman Practices Increasing Bulk DensityVehicular traffic and frequent pedestrian trafficmajor impact on forest soils, which have low bulk densityTillageLoosens soil initially, but depletes organic matter, resulting inhigher bulk density
27 Effect of Soil Compaction on Root Growth 1. Resistance to penetration (roots must push the particlesaside and enlarge the pore to grow if pore is too small)Exacerbated by dryness due to increased soil strength.2. Poor aeration3. Slow movement of nutrients and water4. Build-up of toxic gases and root exudatesRoots penetrate moist sandy soils most easily for a given bulk density
30 Total PorosityParticle density approximately 2.65 Mg/m3 for silicate-dominated minerals.Total porosity (%) = [(Db/Dp) x 100]Porosity varies:25% in compacted subsoils60% or more in well-aggregated, undisturbed soils with high organic matter content80%+ in organic soils (peat)Cultivation reduces pore space, organic matter content and granulationCropping reduces macropore space.
31 Packing pores (between primary soil particles) Pore Type and ShapePacking pores (between primary soil particles)Interped pores (shape depends on ped/granules)Biopores (often long, narrow and branched; some are spherical)PACKING PORESBIOPORESINTERPED PORES
32 Macropores vs. micropores Macropores: 0.08mm to 0.5cm+Allow ready drainage of water and air movement.Penetrable by smallest roots and a multitude of organisms.Spaces between sand grains are macroporesThis is why sandy soils have low total porosity but rapid drainage (hydraulic conductivity)
33 Interped pores are macropores found between peds and granules. Biopores are macropores produced by roots, earthworms and other organismsBiopores are very important for root growth and infiltration in clayey soils.Vertical Pore-Size DistributionMacropores most prevalent near the surfaceMicropores usually dominate at depthWhy?1. Small aggregates are more stable than larger ones2. More organic material near surface
34 Vertical distribution of pore size in three distinct soils Sandy loamWell-structured silt loamPoorly-structured silt loam
36 Micropores <0.08 mmToo small to permit air movementWater movement slow (usually filled with water)A high porosity soil can still have slow gas and water movement if dominated by micropores.Water generally unavailable to plants (held too tightly)Reduces root growth and aerobic microbial activityDecomposition by bacteria very slow to near-zero in smallest pores.
37 Flocculation Factors Affecting Aggregate Formation and Stability Physical-chemical ProcessesBiological Processesof AggregationFlocculationclumps of clay develop, called flocculesTwo clay platelets come closetogether; the cations of the layerbetween them are attracted tothe negative charges on eachplatelet.
38 Clay floccules and charged organic colloids form bridges that bind to each other and to fine silt Clay domain: platelets are stuck together due to Ca2+, Fe2+, Al3+ and humus.This results in well-structured soils.Na+ has a weaker attraction to negative charges on clays, so clays repel one another and remain dispersed.This results in poorly-structured soils.
40 Shrinking and swelling Upon drying, water is removed fromwithin the clays, so the clay domainsmove closer togetherShrinkage results, with cracks along planes ofweakness (therefore, peds form)
41 Biological Proceses affecting Aggregation (1) Earthworms and termites (burrowing and moulding)Move soil, ingest it, and produce pellets or castsPlant roots also move soil particles(2) Roots and fungal hyphae (stickiness)Exude sticky polysaccharidesSoil particles and microaggregates bound into larger agglomerations called macroaggregatesMycorrhizae secrete a very gooey substance called glomalinN.B. Hyphae are tubular filaments making up the fungus(3) Organic glues produced by microoganismsBacteria also produce sticky polysaccharides in decomposed plant residuesThe glues resist dissolution by water
44 Effect of Tillage on Aggregation Short term:Improvement in aggregation if done on moderatelydry soilBreaks up large clods, loosening soil and increasing porosityIncorporates organic matter into the soilLong term:Loss of aggregationEnhanced oxidation of organic material reduces aggregationLoss of macroporosity occurs if tillage is carried out in a wet soil (puddled)Effect less pronounced where Fe & Al oxides plentiful