NRES 201 Lectures 12 13 Fall 2014 Soil Physical Properties NRES 201 Soil Physical Properties 1 Soil Physical Properties Today s lecture topics Review of previous coverage Revisiting texture Soil density 2 1 NRES 201 Lectures 12 13 Fall 2014 Soil Physical Properties Review of previous coverage Soil s basic physical properties Color Described by hue value and chroma Texture Size distribution of soil particles Structure Arrangement of soil particles to form aggregates or peds Consistence Cohesiveness holding soil particles together 3 Soil 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 4 2 NRES 201 Lectures 12 13 Fall 2014 Soil Physical Properties 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 pores 5 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 processes 6 3 NRES 201 Lectures 12 13 Fall 2014 Soil Physical Properties Revisiting texture Particle size analysis Sieve soil to 2 mm Removes coarse particles Disperse sample To separate individual particles Sedimentation by the hydrometer method determines the percentages of sand silt and clay Sand settles first then silt Clay estimated by difference Source Singer and Munns 1987 7 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 cylinders 8 in a water bath 4 NRES 201 Lectures 12 13 Fall 2014 Soil Physical Properties Particle size and surface area Subdivision increases surface area Smaller area per surface But many more surfaces Surface areas a Single 8 mm cube 6 8 8 384 mm2 1 3 g 295 mm2 g b 64 2 mm cubes 64 6 2 2 1536 mm2 1 3 g 1182 mm2 g Source Brady and Weil 2008 9 Surface area and soil processes Greater surface area enhances Water retention In soil pores or as surface films Adsorption Surface adhesion of gases or dissolved ions or molecules Weathering Exposure allows chemical or microbial attack Aggregation Surface cohesion of soil particles Microbial activity Microbes colonize surfaces 10 5 NRES 201 Lectures 12 13 Fall 2014 Soil Physical Properties Textural effects on soil properties Adapted from Table 4 1 Brady and Weil 2010 11 Soil density What is density The weight or mass of an object divided by its volume Expressed as g cm3 Mg m3 Why are the two units interchangeable Mg stands for megagram or metric ton Defined as 1 million grams or 106 g A meter m is 100 or 102 centimeters cm And a cubic meter is 102 102 102 106 cm3 So Mg m3 106 g 106 cm3 g cm3 12 6 NRES 201 Lectures 12 13 Fall 2014 Soil Physical Properties Soil density Density g cm3 What is density The weight or mass of an object divided by its volume Expressed as g cm3 Mg m3 13 Particle density Dp The mass per unit volume of dry soil solids Does not include the volume between particles Unaffected by Compaction Pore space Soil structure Tillage Source http soils gsfc nasa gov 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 cm3 or even higher Decreased by organic matter For which Dp is 0 9 1 4 g cm3 14 7 NRES 201 Lectures 12 13 Fall 2014 Soil Physical Properties Bulk density Db The mass per unit volume of dry soil Why 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 rainfall 15 Bulk density Db The mass per unit volume of dry soil Includes pore space soil solids Method for measuring Collect an undisturbed soil core with a core sampler Trim the sample flush with the ends of the sampling cylinder Oven dry and then weigh the core Always less than particle density Dp Source http wps prenhall com 16 8 NRES 201 Lectures 12 13 Fall 2014 Soil Physical Properties Why is Db always less than Dp Because of a larger denominator that includes extra volume due to pores Source Brady and Weil 2008 Assume 2 65 g cm3 in this course 17 Much more variable than particle density Ranges from 0 7 g cm3 for Histosols to 2 0 g cm3 for 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 loams Uncultivated forest and grassland loamy A horizons and clayey Oxisol Ap horizons Vertisols dry Cultivated sandy loams and sands Compacted glacial till Aridisols Histosols Cultivated clay and silt loams Particle density of silicate minerals Fragipans Adapted from Fig 4 34 Brady and Weil 2008 18 9 NRES 201 Lectures 12 13 Fall 2014 Soil Physical Properties Practice calculation What would be the bulk density for an ideal soil with the composition shown Assume Dp values 2 65 g cm3 for minerals 1 0 g cm3 for organic matter Calculate Db 0 45 2 65 0 05 1 1 24 g cm3 Source Hassett and Banwart 1992 19 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 aggregate Source Brady and Weil 2008 20 10 NRES 201 Lectures 12 13 Fall 2014 Soil Physical Properties Higher bulk densities for well graded than wellsorted 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 grains Source Brady and Weil 2008 21 Bulk densities usually increase with profile depth The increase is due to Lower organic matter content Less aggregation Fewer roots and less soil disturbance by animal activity Compaction by the weight of overlying layers Subsoils can have bulk densities 2 0 g cm3 Highest for glacial till because of compaction under the enormous mass of ice Source Hassett and Banwart 1992 22 11 NRES