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Evaluating Physical Data Objectives:

Evaluating Physical Data Objectives:. you should be able to: Locate the physical data on the data sheet Describe common physical data methods Give examples of how to apply physical data Determine if the physical data is internally consistent Calculate fundamental relationships.

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Evaluating Physical Data Objectives:

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  1. Evaluating Physical Data Objectives: • you should be able to: • Locate the physical data on the data sheet • Describe common physical data methods • Give examples of how to apply physical data • Determine if the physical data is internally consistent • Calculate fundamental relationships

  2. Exercise - find physical data • Complete exercise • SSL information manual (SSIR 45) pp 204-206

  3. Exercise - find physical data • Find and write the following (page 204): • Soil Survey Number ______ • Percent clay for C3 horizon _______ • Percent fine silt for BS horizon ______ • 1/3 bar bulk density for Ap2 horizon ______ • 15 bar on moist soil for Ap1 horizon ______ • Water dispersible clay for Ap1 horizon _____ • Which horizons have 15bar/clay greater than 0.6? ______ • What is the method code for WRD? _____

  4. Exercise - find physical data • Find and write the following (page 204): • Soil Survey Number - S88ME-003-001 • Percent clay for C3 horizon - 23.2 • Percent fine silt for BS horizon - 28.9 • 1/3 bar bulk density for Ap2 horizon - 1.24 • 15 bar on moist soil for Ap1 horizon - 9.9 • Water dispersible clay for Ap1 horizon - 7.7 • Which horizons have 15bar/clay greater than 0.6? Ap1, Ap2, BS, C1 • What is the method code for WRD? 4C1

  5. Physical Methods • Each data element has a method • Each method has a method code • The method is • the standard operating procedure • part of the quality assurance program • Lab methods are in SSIR 42: Laboratory Methods Manual • SSIR 42 was written for laboratory technicians • SSIR 51 – Field and Laboratory Methods

  6. Internal Consistency • Consistency of data – does it follow expected trends -with other analyses -within pedons -with genetically and geographically associated pedons Laboratory consistency (repeatability) is monitored by use of standard samples for each batch.

  7. Evaluating the consistency of physical data • Maximum, minimum, and representative values • Depth trends within a pedon : clay, Db … • Trends in physical, chemical and mineralogical data are compared. • Compare apples to apples. Know the methods used for the data you are comparing. Use the appropriate method.

  8. Inconsistency? • Rerun lab data? Representative site? Variability at sampling site? Sampling procedure. • Reevaluate your hypothesis

  9. Physical Analytical Methods • Particle-Size • Bulk Density • Water Retention • COLE • Aggregate Stability

  10. NASIS ‘Super 7’ • Coarse fragments • Sand • Silt • Clay • OM • Bulk density • Ksat

  11. Particle-size analysis • Method 3A1 describes measuring sand, silt, and clay fractions with the pipette and sieves. • The sedimentation theory is based on Stokes’ Law. • Most applicable for soils with crystalline clays.

  12. Particle-size analysis (pipette) • The method is precise within 1.5 percent, absolute. • Only method to obtain fine clay and CO3 clay. • Method is time consuming and labor intensive.

  13. Pipette PSDA analysis • Errors with pipette analysis are associated with sampling and measuring. • Uses pretreatments: OM, salts CO3 … • Generally considered to be superior to other sedimentation methods, particularly at low clay contents where the hydrometer has low sensitivity.

  14. Hydrometer PSDA analysis • Less equipment, quicker • Commonly does not remove OM and salts. • (Dispersion can be a problem.) • The major source of error is in the hydrometer reading. (Gee and Bauder, 1979) An error of +- 1 g/L in the hydrometer reading results in an error of about +- 2 wt % for clay size particles.

  15. Evaluation and Interpretation of PSDA Data • Lab vs. field texture • Clay distribution with depth and morphology; (argillic? ). • Clay illuviation - fine clay distribution (Higher ratio of fine clay to total clay than overlying and underlying horizons.) • Distribution of silt and sand fractions (PM discontinuities)

  16. Weighted average for control section

  17. Examples of Data Interpretation • Typical calculations - PSD • Weighted average for control section • SSIR 45 pp. 16-17 • PSA on clay-free basis • SSIR 45 pp. 188-199 • Changing basis • whole soil • <75 mm • with soil organic carbon

  18. PSA on clay-free basis Use to determine PM, lithologic discontinuities

  19. Changing Basis

  20. PSDA – comparison of standard vs. ‘gypsic’ method

  21. Bulk density • Moisture state when volume is measured should be specified. • 1/3 bar bulk density - oven-dry mass per unit bulk volume of soil equilibrated at 1/3 bar (<2 mm) tension. • a. Clod method (primary method used by SSL), provides ancillary data: COLE, water content at 1/3 bar. • b. Reconstituted (plowed surface layer)

  22. Bulk density • Field moist bulk density – oven-dry mass per unit of bulk volume of soil at field moisture • a. Compliant Cavity, frame, ring (excavation techniques) • b. Known-volume can, field-moist core • (a. & b. <2mm / not natural fabric) • Cores that are extracted without disturbing the natural fabric, and which remain in the core can be used for water retention. • 2. Oven-dry bulk density - oven-dry mass per unit bulk volume of soil at oven dryness (<2 mm)

  23. Other bulk density measurements • Oven dry bulk density - oven dry mass per oven-dry volume unit of soil • Rewet bulk density - used to determine irreversible shrinkage.

  24. Bulk density relationships • 1/3 bar bulk density is less than or equal to oven dry bulk density • There is less difference between clod and other methods in soils with less clay, more sand.

  25. Bulk density relationships • Average Db at 1/3 bar • Soils g/cc Organic 0.05-0.35 Sandy 1.50-1.70Silty 1.30-1.60Clayey 1.20-1.40 • Lab range for 1/3 bar / Db: • 0.03 g/cc - 2.53 g/cc • (>2.0 is unusual)

  26. Bulk density average db values

  27. Water Retention • Determined by desorption on pressure plate. • Determined on • 1)natural fabric clod /core or • 2) <2mm

  28. Water Retention • 15-bar (1500 kPa) --ground soil <2 mm • Approximate wilting point • 2-bar (200 kPa)--ground soil <2 mm • Approximate point at which plants begin to experience stress • 1/3 bar (33 kPa) --natural fabric • Approximate field capacity • 1/10 bar (10kPa)--natural fabric • Approximate field capacity in sandy soils

  29. Appropriate sample types for water retention • Low tension (1/3 bar , 1/10 bar, - 6, 10, 33, 100 kPa) water retention determinations require clods for textures finer than sandy loam. • Higher tension (2 bar /200 kPa, 15 bar / 1500 kPa) can be determined on <2mm sieved samples.

  30. 15 bar to clay ratio ( > 5-10 percent clay) • Provides an indication of dispersion • Rule of thumb-- percent clay equals 2.5 times 15-bar water content minus percent organic carbon. • This may be used to estimate clay content in hard-to-disperse soils.

  31. 15 bar : clay ratio • For a typical soil with well dispersed clays, the ratio is 0.4. (If you remember no more, remember point four.) • Higher ratios indicate poor dispersion of clays.

  32. 15 bar to clay ratiosOther factors: • Low activity clays lower ratio to < 0.35 • High activity clays increase the ratio • Variable surface charge and amorphous clay minerals • Organic soils – higher 15bar : clay • Other soils with dispersion problems, salts, gypsum

  33. Low activity clays, iron oxides, clay-sized carbonates produce 15 bar / clay of <0.4. • Ratios of less than 0.3 are common in some soils that contain large amounts • of gypsum.

  34. Isotic mineralogy criteria • The ratio of 1500 kPa water to measured clay is 0.6 or more. • (Also NaF pH > 8.4 and no carbonates.)

  35. 15 bar water = .12 • OC = 2% • % clay = ______?

  36. % clay = 2.5 x .12 +2 = 32%

  37. 15 bar water = .12 • clay = .50 • 15 bar water = .23 • clay = .27

  38. .12 / .50 = .24 gypsum? Salts? • .23 /.27 = .85 andic?

  39. Water relationships • Water is held at higher tension by smaller particles. • High tension water is held mostly in micropores (in interstitial spaces between soil particles): <2mm samples vs. clods can be used for 15 bar WR. There is no significant water in macropores at 15 bars.

  40. Water retention different (WRD) • The difference between the 15 bar water content and the 1/10 or 1/3 bar water content on a volume basis • weight percent X bulk density = volumetric water content

  41. Soil Water Retention Curve • Silty Clay Loam

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