NRES 201 Lectures 12-13 (Fall 2014): Soil Physical Properties11NRES 201Soil Physical Properties2Today’s lecture topics- Review of previous coverage- Revisiting texture- Soil densitySoil Physical PropertiesNRES 201 Lectures 12-13 (Fall 2014): Soil Physical Properties23Review of previous coverageSoil’s basic physical properties- Color- Described by hue, value, and chroma- Texture- Size distribution of soil particles- Structure- Arrangement of soil particles to formaggregates or peds- Consistence- Cohesiveness holding soil particles together4Soil components- The soil solids- Inorganic minerals• Primary minerals present in the original rock• Secondary minerals formed by weathering- Organic matter• Living organisms (biomass)• Organismal remains (residues)• Decay products (humus)NRES 201 Lectures 12-13 (Fall 2014): Soil Physical Properties35- The soil solution- Consists of water + dissolved ions, molecules, and gases- Medium for chemical and biological processes- Vital to plant growth- Held within soil pores• Mobile in large pores• Immobile in small pores6- The soil atmosphere- Consists of gases that occupy soil pores not filled with water- Isolated, not continuous- Differs from ambient air by having:• Less O2• More CO2• More humidity- Critical to biological processesNRES 201 Lectures 12-13 (Fall 2014): Soil Physical Properties47Revisiting textureParticle-size analysis- Sieve soil to < 2 mm- Removes coarse particles- Disperse sample- To separate individual particles- Sedimentation by the hydrometermethod determines the percentagesof sand, silt, and clay- Sand settles first, then silt- Clay estimated by differenceSource: Singer and Munns (1987)8- Based on Stokes’ law, v = kd2,where:• v is the settling velocity• k is a constant that depends on water temperature and particle density• d is the particle diameter- Assumptions:• Soil particles are spherical Many are plate-like• Soil particles are identical in density Appropriate due to silicate mineralogy• Soil particles fall independently Achieved by soil dispersion/pretreatment• Liquid is uniform in density/viscosity Achieved by placing sedimentation cylindersin a water bathNRES 201 Lectures 12-13 (Fall 2014): Soil Physical Properties59Particle size and surface area- Subdivision increases surface area- Smaller area per surface-But many more surfacesSource: Brady and Weil (2008)Surface areas :(a) Single 8-mm cube6  8  8 = 384 mm2/1.3 g= 295 mm2/g(b) 64 2-mm cubes64  6  2  2 = 1536 mm2/1.3 g= 1182 mm2/g10Surface area and soil processes- Greater surface area enhances:- Water retention• In soil pores or as surface films- Adsorption• Surface adhesion of gases or dissolved ionsor molecules- Weathering• Exposure allows chemical or microbial attack- Aggregation• Surface cohesion of soil particles- Microbial activity• Microbes colonize surfacesNRES 201 Lectures 12-13 (Fall 2014): Soil Physical Properties611Textural effects on soil propertiesAdapted from Table 4.1, Brady and Weil (2010)12Why are the two units interchangeable?- Mg stands for megagram or metric ton- Defined as 1 million grams or 106g- A meter (m) is 100 or 102centimeters (cm)- And a cubic meter is 102 102 102= 106cm3- So Mg/m3= 106g/106cm3= g/cm3Soil densityWhat is density?- The weight or mass of an object divided by its volume- Expressed as g/cm3= Mg/m3NRES 201 Lectures 12-13 (Fall 2014): Soil Physical Properties713Soil densityWhat is density?- The weight or mass of an object divided by its volume- Expressed as g/cm3= Mg/m3Density, g/cm314Particle density (Dp)- The mass per unit volume of dry soil solids- Does not include the volume between particles- Unaffected by:- Pore space- Soil structure- Tillage- Compaction- Essentially constant for mineral soils at 2.65 g/cm3- Reflects the dominance of silicate minerals- Increased by iron and other heavy minerals- To 3.0 g/cm3or even higher- Decreased by organic matter- For which Dp is 0.9-1.4 g/cm3Source: http://soils.gsfc.nasa.govCompactionNRES 201 Lectures 12-13 (Fall 2014): Soil Physical Properties815Bulk density (Db)- The mass per unit volume of dry soilWhy are soil densities expressed on a dry weight basis?- To simplify data comparisons- Wet weight varies with gains or losses of water due to evaporation and rainfall16Bulk density (Db)- The mass per unit volume of drysoil- Includes pore space + soil solids- Method for measuring:- Collect an undisturbed soilcore with a core sampler- Trim the sample flush with theends of the sampling cylinder- Oven-dry and then weigh thecore- Always less than particle density(Dp)Source: http://wps.prenhall.comNRES 201 Lectures 12-13 (Fall 2014): Soil Physical Properties917Why is Dbalways less than Dp?- Because of a larger denominator that includesextra volume due to poresSource: Brady and Weil (2008)††Assume 2.65 g/cm3in this course.18- Much more variable than particle density- Ranges from < 0.7 g/cm3for Histosols to > 2.0g/cm3for fragipans or compacted subsoils- Root growth favored by lower bulk densities- Starts to be restricted at 1.45-1.55 g/cm3- Completely prevented above 1.8 g/cm3- Higher bulk densities for sands than for silt loams, clays, or clay loamsAdapted from Fig. 4.34, Brady and Weil (2008)HistosolsAridisolsCultivated clay and silt loamsUncultivated (forest and grassland) loamy A horizons and clayey Oxisol Ap horizonsCultivated sandy loams and sandsFragipansVertisols (dry)Compacted glacial tillParticle density of silicate mineralsNRES 201 Lectures 12-13 (Fall 2014): Soil Physical Properties1019Practice calculation- What would be the bulk densityfor an “ideal” soil with thecomposition shown?- Assume Dpvalues:- 2.65 g/cm3for minerals-1.0 g/cm3for organic matter- Calculate:- Db= (0.45  2.65) + (0.05  1)= 1.24 g/cm3Source: Hassett and Banwart (1992)20- Higher bulk densities for sandy soils than for silt loams, clays, or clay loams- Fine-textured soils form porous aggregates- Sand grains are nonporous and do not easily aggregateSource: Brady and Weil (2008)NRES 201 Lectures 12-13 (Fall 2014): Soil Physical Properties1121- Higher bulk densities for well-graded than well-sorted sand- Grain sizes vary with well-graded sand• Smaller grains fill spaces between larger grains- Grain size mostly uniform with well-sorted sand• Spaces more open between grainsSource: Brady and Weil (2008)22- Bulk densities usually increasewith profile depth- The increase is due to:• Lower