Soil Components and Formation Factors in Geotechnical Science

1. An introduction to soils Andrew Biggs

2. What’s in a soil? • Mineral particles (inorganic fraction) – small particles of rock and other minerals, produced from weathering of rocks • Organic materials – humus and the dead and decaying parts of plants and soil animals • Water – the ‘soil solution’ in which nutrients for plants are dissolved • Air – which fills the spaces between the soil particles not filled by soil solution • Living organisms – ranging in size from small animals to viruses

3. Air 25% Air 25% Mineral 45% Mineral 64% Water 10% Water 25% Organic matter Soil components Clay soil Sandy soil + living organisms

4. Why do soils differ? • Different proportions of the main components • Components are grouped together in different ways • Many different types of minerals in the inorganic fraction, and each soil has different proportions of them

5. Different disciplines • Morphology – description of the soil • Chemistry (a key aspect) • Physics (includes geomechanics) • Biology (the latest rage)

6. How do you describe these things? • Depends on who you are • AS1726 – Geotechnical site investigations • AS1289 – Testing for engineering properties of soils • AS4119 – Soils for landscaping and garden use • AS4439, 4479, 4482 – Contaminated land • AS4454 – Compost/Manufactured soils

8. How did the soil get there? • Know your geology • Topography • Is the soil unrelated to the underlying rock/material? • Understanding the formation of a soil helps understand how it will behave

9. Soil forming factors • Parent material (geology, rock type) • granite = often sandy, infertile • basalt = clayey, fertile • Climate (rainfall, temperature, wind) • influences rate of weathering, plant growth • Topography (shape, length, grade of slope, aspect) • Organisms • plants, bacteria, fungi, animals, worms, insects • Time

10. Morphology • Colour (including mottles) • Field texture • Structure • Presence/absence of segregations, coarse fragments, pans • Field tests (pH, EC, dispersion, carbonates, peroxide etc) • Horizons • And the landscape in which the profile sits (landform, geology, vegetation, etc)

11. Texture • The proportion of sand, silt and clay sized particles that make up the mineral matter of the soil • How a soil ‘feels’ Sand: 2.0 – 0.02 mm Silt: 0.02 – 0.002 mm Clay: <0.002 mm

14. Texture influences: • Amount of water that can be stored in the soil (water holding capacity) • The rate of water and air movement through the soil (drainage, permeability, aeration) • The soil’s nutrient supply (amount and availability) • Ease of root growth • Workability, trafficability (potential for compaction) • Resistance to erosion • Ability of a soil to maintain a stable pH

17. Sand Clay

18. Texture

19. Structure • Soil particles (sand, silt, clay) are usually arranged into larger units (called aggregates or peds) • Soil structure refers to the size and arrangement of the aggregates, and the pore space between them

21. Structure influences • Water entry into the soil • Runoff of water • Permeability (ease of movement) of water and air in the soil • Root penetration • Seedling emergence • Resistance to erosion • Workability • Drainage

23. Colour • Colour may be due to soil forming processes, or inherited from the parent material • In general, soil colour is determined by the amount and state of organic matter and iron oxides

24. Colour gives an indirect measure of other soil attributes: • Presence/amount of organic matter • Drainage/waterlogging potential • Degree of weathering/leaching

26. Red Grey/blue-grey Yellow

27. Colour

28. Horizons • Layers with differing properties • Why describe them? • A common language for communication – an A2e has known characteristics • O – organic material • A – surface • A1 is zone of organic matter accumulation • A2 may/may not be present. Often bleached

29. B is zone of maximum accumulation (colour, texture, chem) • Usually a B2 (B21, B22) • Various suffixes (g, h, k, y, s etc) e.g B22k • C – parent material • D – unrelated to the overlying horizons – common in alluvial soils • R - Rock

30. Understanding horizons demonstrates an understanding of how the soil got there and what its properties are • You don’t have to use all the letters!!

31. Medium blocky peds A1 40% clay B21k Coarse prismatic peds 50% clay B22y Coarse lenticular peds 70% clay

32. Loamy sand - massive A1 5% clay Texturecontrast soil Loamy sand - bleached A2e Hard clay - columnar B21t 35% clay Hard clay - massive B22t

33. Organic Matter • Biological origin (dead plant and animal material) • Has a strong influence on soil properties - reservoir of nutrients (esp N, P, S)- contributes to cation exchange capacity of the soil- improves water holding capacity - improves structural stability of the soil • Measured as ‘organic carbon’

34. Organic matter pools

35. Soil water (and air) • Total porosity = all the air spaces • A proportion is generally filled with water • The amount of water varies • Plants can extract varying amounts of water • Crop lower limit (Permanent wilting point, 15 bars suction) • Saturation = full of water • After drainage has reached “zero’ • Drained upper limit (Field capacity, 1/3 bar suction)

36. Soil water content Lower limit Saturation DUL Depth Air dry (45o C) Oven dry (105o C)

37. Soil water movement • Deep drainage = water moving out of the profile (downwards) • Lateral flow = water moving out of the profile (sideways) • Proportion of each varies with soil type and landform • There is a lot more to it!!

38. Soil classification • Description is different to classification • Different classification schemes for different purposes • Northcote factual key: Ug5.15 • Stace et al. (1968) Great Soil Groups: Black Earth • Australian Soil Classification: Self-mulching Black Vertosol • Unified soil classification: CH • Classify what you have described!

39. Available from CSIRO Publications