PHYSICAL PROPERTIES OF SOIL INTRO TO SOILS CH 4

SOIL TEXTURE Describes the proportion of soil particle sizes: Sand Silt Clay Soil Texture influences other traits such as: Water Holding Capacity Aeration

EFFECT OF PARTICLE SIZE Smaller particles – larger internal surface area

EFFECT OF PARTICLE SIZE More pores – micropores Larger pores – macropores

SOIL SEPARATES SAND 2.00 – 0.05 mm SILT 0.05 – 0.002 mm CLAY

TEXTURAL CLASSIFICATION Twelve Textural Classes Textural Triangle

SOIL DENSITY AND PERMEABILITY MASS per VOLUME D = M V

SOIL DENSITY AND PERMEABILITY Two densities in Soil: Particle Density – PD Bulk Density - BD

SOIL DENSITY AND PERMEABILITY PD average soils ~ 2.65 gm/cu cm BD average range from 1.0 – 1.8 gm/cu cm Depends on amount of pore space BD = wt. dry soil = _g_ vol. dry soil cu cm e.g. BD = 650 g = 1.3 g/cu cm 500 cu cm e.g.

SOIL POROSITY Usually expressed as a percentage; e.g. 50% Two means determining porosity: Calculate ratio water volume to total core volume Calculate from bulk density and particle density

SOIL POROSITY Examples Water Volume to Core Volume Porosity = wet weight (g) – dry weight (g) soil volume (cu cm) x 100%

SOIL POROSITY Examples An oven-dry soil core, volume 500 cu cm, weighs 650g. When wet, it weighs 900g. Find it’s % porosity. Porosity = 900g – 650g x 100% = 250g x100% = 50% 500 cu cm 500 cu cm Note: the unit cancellation is made possible by the metric system which defines 1 cu cm of water as weighing 1g.

SOIL POROSITY Examples Bulk Density to Particle Density Defines the percentage of the soil that is solid matter The percent solid matter is subtracted from 100% to give percent porosity: Porosity = 100% - (BD/PD x 100%)

SOIL POROSITY Examples An undisturbed oven-dry soil, BD of 1.3 g/cu cm, consists of average mineral composition (PD 2.65 g/cu cm). Find its % Porosity: Porosity = 100% - (1.3 g/cu cm/2.65 g/cu cm x 100%) Porosity = 100% - (0.49 x 100%) = 100% - 49% = 51%

SOIL POROSITY Question: Which has greater porosity, Sand or Clay? Answer: Clay at about 50%; Sand is lower at about 30% Why?

TEXTURE AND SOIL PORES

Why, then, does water move through sandy soil very rapidly, but moves s l o w l y into clay ?

This is explained by the physical property Permeability

PERMEABILITY Permeability – the ease with which air, water, and roots move through soil - Depends on number, size, and continuity of pores - Liken to a maze

Fine-textured soils would be impermeable if not for: Soil Structure

STRUCTURE Structure – the way soil particles clump together into large units called aggregates or peds

STRUCTURE Structure can alter the effects of texture e.g. a fine-textured silty clay with good structure can be permeable!

STRUCTURE Structure is classified by three groups of traits: Type – refers to shape of aggregates e.g. Granular, Platy, Blocky, Prismatic, Columnar Class – refers to size of peds e.g. very fine, fine, medium, coarse, very coarse Grade – refers to strength and distinction of peds e.g. weak/not visible vs. strong/easily distinguished

FORMATION OF SOIL STRUCTURE Two-step formation: Individual soil particles loosely aggregate Weak aggregates are cemented to strengthen - clay - iron oxides - organic matter - microorganism gums

ADDNL. SOIL PHYSICAL PROPERTIES Soil Consistence – the behavior of soil when pressure is applied; measured at three different moisture levels: Wet, Moist, Dry (fig 4-15, p. 59) Soil Tilth – ease of tillage, seedbed preparation, and seedling/root movement Compaction – results from pressure applied at the soil surface

ADDNL. SOIL PHYSICAL PROPERTIES Puddling, Clods, and Crusts: Puddling occurs when pressure is applied to very wet soils (esp. plowing) Crusts occur when bare soil is struck by raindrops; disperses soil then dries to a hardened crust

ADDNL. SOIL PHYSICAL PROPERTIES Improving Tilth: Best accomplished by improving structure - tilth relates to texture, structure, permeability, and consistence; however, texture and consistence cannot be altered easily therefore, improve tilth by improving structure and avoiding compaction

ADDNL. SOIL PHYSICAL PROPERTIES Soil Channels – continuous macropores leading from surface to deep subsoil Soil Pans – any layer of hardened soil; includes: - claypans - fragipans (clays) - plinthite (tropics) - caliche (Ca cemented)

ADDNL. SOIL PHYSICAL PROPERTIES Soil Temperature (varies w/color, texture, O.M.) Soil Color Munsell soil color chart Hue, Value, Chroma e.g. 10YR 3/6