Soil Physical Properties

1. Soil Physical Properties

2. Introduction Soil physical properties relate to: • the solid particles of the soil (Soil texture) • the manner in which the particles are aggregated together (soil structure) • creating some spaces within soil (for soil air and soil water).

3. Introduction Soil Physical Properties include the following: • Soil Color • Soil Texture • Soil Structure • Soil Consistence • Soil Density • Soil Porosity

4. Introduction • Soil physical properties influence how soils function in an ecosystem and how they are managed. • E.g. texture and structure determine how much water the soil can hold and how water can move around in soil. • Soil physical properties (e.g., soil color, soil texture and other physical properties) are used in soil classification.

7. Soil Color Why is soil color an important soil property? • Color directly affects the absorption of solar radiation and therefore is a factor in determining soil temperature. • Color is indirectly related to other soil properties and can provide information on subsoil drainage, organic matter content of surface horizons, and bulk density. • Distinguishing soil horizons and soil classification.

8. Causes of Soil Color • Organic matter or humus • imparts a brown to black color to soil. • The higher the organic content the darker the soil • Oxidation status of Iron (Fe) containing compounds • Fe[III] (ferric, oxidized form) imparts a red or yellow color to soil. • Fe[II] (ferrous, reduced form) imparts neutral gray colors to soils and allows the colors of other metal-containing compounds to predominate. • Other soil components (Soil Minerals): • Manganese oxide (black) • Glauconite (green)- not very common • Carbonates (whitish)- aridand semiarid soils

9. Determination of Soil Color • Soil color is determined by comparison of color of soil with standard colors on a soil color chart called the Munsell Soil Color Chart • Munsell soil color chart uses three variables (hue, chroma, and value) to determine the color of soil

10. Determination of Soil Color • Hue = the dominant spectral color of an object defined in terms of either pure (green, red, yellow, and blue) or mixed (yellowness or redness of soil). • Value= relative blackness or whiteness of soil. Value is made by mixing white and black. Therefore, a value of 5 is an equal mixture of white and black (10=white and 0=black) • Chroma= the purity of the color. The amount of gray of a particular value that is mixed with pure hue to obtain the actual soil color. As chroma increases, color is more brilliant.

11. Determination of Soil Color

12. Interpreting Soil Color • Color gives some indication of OM status of soils • Color gives some indication of soil mineralogy • Color gives insights into the drainage status of soils

13. Soil Texture • Soil texture refers to the relative abundance of sand, silt, and clay. The particles are defined by their sizes. •  The classes are based on the United States Dept. of Agriculture Textural Classification System

14. Many physical, chemical, and mineralogical properties of a soil particle are a function of size. E.g. • Surface Area (Specific Surface= surface area/g) • Water retention • Adsorption of gases and chemicals • Weathering of particles • Electromagnetic charges • Microbial reactions Therefore soil particle size distribution influences water retention, chemical adsorption, microbial reactions, weathering, etc

15. Soil Textural Analysis(2 methods of determination) 1. Feel Method (For quick field analysis) • High clay • Feel hard and cloddy when dry • sticky and plastic when wet ~ rolls into a ribbon • High silt • feel smooth and silky when wet • little stickiness or resistance to deformation • High sand • Rough and gritty ~ makes grinding noise • little or no plasticity or stickiness when wet

16. Feel Method(For quick field analysis) The smooth, dull appearance and crumbly ribbon of a silt loam. The gritty, non-cohesive appearance and short ribbon of a sandy loam The smooth, shiny appearance and long, flexible ribbon of a clay

17. Soil Textural Analysis 2. Laboratory Method (Sedimentation techniques based on Stokes Law) • Prepare oven dry sample (approx. 50-100 g) • Use mechanical dispersion to prepare a soil suspension • Determine sand, silt and clay fractions by settling method using a Buoycos Hydrometer (or by the pipette method).

18. Textural Triangle

19. Some Influences of Soil Separates on Some Properties and Behavior of Soils

20. Changes in Soil Texture • Texture of a given soil can only be changed by mixing with another soil of a different textural class. • This type of textural modification is not very common in the field. • However, over very long periods of time, pedologic processes such as erosion, deposition, illuviation, and weathering alter textures of soils.

21. Soil Structure Soil Structure is the grouping or arrangement of primary soil particles into secondary units called aggregates or peds. Soil structure defines the pattern of pores which in turn influences other physical properties of soil.

22. Importance of Soil Structure • Aeration - to allow oxygen in and carbon dioxide out. • Drainage - water infiltration and its relationship to runoff • Formation of a good seedbed for plants • Affects the degree of resistance to erosion.

23. Types of Soil Structure • Soil structure is characterized in terms of the shape, size, and distinctness. • The four principal shapes (Types) of soil structure are: • Spheroidal • Platy • Blocky • Prismatic

24. Spheroidal Structure • Spheroidal structures are characteristic of many surface soils (A horizons). • Two types of spheroidal structures are: • Granular- when spheroidal peds are separated from one another in a loosely packed arrangement. • Crumb- when spheroidal peds have very abundant pores

25. Platy Structure • Characterized by thin horizontal peds or aggregates. • The peds develop due to processes related to soil formation: • inherited from the initial arrangement of the parent material or • due to disturbance (compaction). • May be found in foot-paths or in cattle feedlots.

26. Blocky Structure Blocky aggregates, as the name implies, look square. If the intersections are sharp the aggregates would be angular blocky = angular blocky structure. If the intersections have some rounded and some sharp corners = subangular blocky structure.

27. Prism-Like Structures Prism-like structures are vertically oriented prisms or columns that vary in height in soils. The aggregates are longer than they are wide. There are 2 subtypes of prism-like structures ~Prismatic or Columnar subtypes. Prismatic Prismatic structure have pillars that are relatively angular and flat horizontally. They are typically found in subsoil horizons with abundant clay.

28. Columnar structures are also longer than they are wide and they have rounded tops. Prism-Like Structure- Columnar Columnar • Both prismatic and columnar often occur where large amounts of sodium are present in arid soils, or in poorly drained areas in humid regions.

29. Structure Grades (Distinctness) • Weak- poorly formed, barely observable. • Moderate- well formed, moderately durable. • Strong- very evident, durable peds.

30. Structure Classes (size) Structure size class depends on the type e.g. Angular and subangular have the following classes: • very fine <5mm • fine 5 - 10 mm • medium 10 – 20 mm • coarse 20 – 50 mm • very coarse > 50 mm

31. Some soils are structureless: • Single Grain - each soil particle is independent of all others like a sandy beach. • Massive - The entire soil mass clings together with no definite signs of weakness. Usually indicative of large amounts of clay. • Some soils have had their structure destroyed, these soils are called puddled. When soils are plowed when too wet, trampled, or otherwise mashed, the large pores collapse and the soil is left in an undesirable condition.

32. Soil Consistence • Soil Consistence describes the relative resistance of soil to mechanical stress and its ability to be molded or changed in shape. Consistence is usually described when the soil is dry or when it is moist. • When dry, soil consistence could be: • Loose, soft, hard or rigid • When moist, soil consistence could be: • Loose, friable, firm, or rigid

33. Soil Density Particle Density (PD) • Particle density is the weight per unit volume of individual soil particles (Mg/m3 = g/cm3), calculated as PD = Ws / Vs • Particle density is not affected by particle size and pore space (texture and structure). • In general the mean particle density (PD) for mineral soil varies in a narrow range from 2.6 to 2.7 g/cm3. (Average = 2.65 g/cm3).

34. Bulk Density (BD) Bulk Density (BD) - weight of soil per unit volume in its natural state (Mg/m3 = g/cm3), calculated as BD = Ws / (Vs + Vp) Where: Ws = weight of soil solids Vs = volume of soil solids Vp = volume of soil pores • Bulk density is highly variable and can range from 1.6 or higher for sandy soils to less than 1.0 g/cm3 for soils high in organic matter.

35. Porosity • Air and water flow through soil are functions of: • Total pore space • Pore size distribution (macro and mesopores) • Continuity of macropores • There is a relationship between BD, PD, and Pore space in soil. That relationship is shown with a calculation (see Brady text) • That relationship is important because pore space in soil is a difficult property to measure. % Pore space = 100 –(BD/PD x 100)