1. SOIL PHYSICAL PROPERTIES Chapter 4

4. Color

5. Thus, red = oxidized = aerated but gray = reduced = anoxic, poorly aerated. The oxidation state of Fe in Fe 2 O 3 and Fe 2 O 3 H 2 O is + 3.

6. Color This surface horizon is dark due to enrich- ment in organic matter from grass roots and surface residue. Common feature of prairie soils is high organic matter content to appreciable depth (see inches).

7. Color  

8. These colors, certainly, but also gray color generally indicate poor drainage and aeration. Colors of chroma 2 or 1 indicate gley.

9. Color These secondary colors modify one’s interpretation based on the main (matrix) color –like yellow or gray within an otherwise oxidized matrix say drainage and aeration are not uniformly good. And visa-versa –red mottles against gray matrix = some veins that are well drained / aerated.

11. Texture Illustration of how surface area increases with decreasing size. If you prefer algebra, for a cube, A = 6s 2 and V = s 3, so A / V = 6 / s. The smaller the cubes, the greater the surface area per volume (or per mass, since mass is proportional to volume through density).

12. Texture All these are surface phenomena, therefore, increase with increasing particle surface area. On the other hand, pulling an implement through a clayey soil is much harder than through a sandy soil, no?

14. Answer, sandy clay.

16. Texture

17. F UP = Bouyant + Drag F DOWN = Gravity F Bouyant = 1/6 π d 3 ρ L g F Drag = 3 π d μ V F Gravity = 1/6 π d 3 ρ S g At equilibrium, i.e., terminal velocity, forces balance

18. Texture Smaller d means smaller v, no? So comparing the largest clay (d = 0.002 mm) to largest sand (d = 2 mm), v clay / v sand = 0.002 2 / 2 2 = 1 / 1,000,000.

19. Texture Express velocity as distance / time, rearrange and find time need for smallest sand to settle a prescribed distance. So, aliquot taken above this depth contains no sand, only silt and clay. Do same calculation for smallest silt, take aliquot at that time and it contains only clay.

20. Texture Your times may vary depending on density and viscosity of the medium (see equation). Besides any solutes added to aid dispersion of particles, temperature affects these parameters.

21. Texture Depending on the nature of the soil, more extensive methods may be necessary to disperse particles.

22. Sure, erosion can change texture at a location but barring gross erosion, this is slow. Certainly, internal translocation of clay will make the topsoil more sandy but this is very, very slow. Unless you move a lot of dirt around, texture is effectively a fixed property.

29. Structure Munsell book give guide.

30. Probably blocky because the large pores between aggregates are more vertically oriented than in soil with platy structure. Obviously, the tiny pores in a massive clay conduct water very slowly.

31. The organics jointly adhere to surfaces of adjacent mineral particles. Cations like Ca 2+ tend to be closely associated with particle surfaces, bridging adjacent minerals. Shrink-swell clays tend to give persistent internal fissures.

33. X is better drained. You know this because the color indicates oxidized Fe, Fe 3+, which exists under a prevailing oxidized (oxic, aerated) environment. On the other hand, gray color is due to chemically reduced Fe, Fe 2+, that occurs under anoxic conditions.

34. The one with the greatest surface area, clay, of course.

35. The four types of geometric structure are spherical (all dimensions about the same with radial orientation, one dimension short compared to the other two, i.e., flat = platy, one dimension long compared to the other two, i.e., elongated = prism-like and all dimensions about the same but with x-y-z orientation, i.e., cubic = blocky.

36. It couldn’t hurt to remember these and the subtypes of spherical structure.

38. Structure Crust = dense, i.e., low porosity and pores are small so that infiltration is greatly reduced. Bad.

39. Structure Sure, there are large pores between clods but the large pores that previously existed throughout are gone. Bad.

40. Structure Protecting an otherwise bare surface from raindrop impact will preserve surface aggregation, thus, preserve good infiltration, soil water conservation and limit runoff, erosion and downhill degradation of surface water quality. Adding organic matter aids structure stability and redevelopment if degraded.

41. Besides color, texture and structure, this is another feature included in horizon descriptions.

43. PARTICLE DENSITY ρ s = mass soil solids / volume solids (g / cm 3 ) Depends on mineralogy to some extent

47. Well, it can vary depending on whether a very low density mineral predominates, however, this is usually not the case and common soil minerals have densities that are about 2.65 g cm -3.

48. BULK DENSITY ρ b = mass of soil solids / total volume occupied by solids (g / cm 3 ) Is ρ b greater than or less than ρ s ?

50. You are dealing with a solution. Recall that solutes elevate the boiling point.

52. This is the usual explanation –internal porosity of small aggregates that exist in finer texture soil compared with sand, which tends to lack much aggregation. Clayey soils are considered heavy by virtue of their typically higher water content. Strictly based on bulk density (oven-dry mass), they should be considered light.

53. Yes to all, no?

54. Well, the answer depends on your time of reference. Obviously, if you till a soil, you fluff it up, therefore, instantaneously reduce the bulk density. However, the fluff goes away and the soil reconsolidates. The long-term effect pertains to destruction of organic matter with consequent loss of porosity, especially the larger pores. So, in the long-term density increases. Heavy equipment tends to compact the soil. Plowing also tends to increase subsurface density (lifting the surface soil up, pushes the soil below it down).

55. This is the classic observation. Many studies, many places. However, this can be, to various extent, reversed by reducing tillage and adding organic matter.

57. Obviously, these are expressions for bulk density. Let’s see what density is implied on a g cm -3 basis. ρ B = 2,200 Mg x 1000 kg Mg -1 x 1000 g kg -1 / 10,000 m 2 ha -1 x 0.15 m x 1,000,000 cm 3 m -3 1.47 g cm -3 These density units are useful in production-scale calculations. By the way, what is the bulk density (g cm -3 ) assumed when saying an AFS weighs 1000 tons?

59. ρsρs ρbρb ρ b / ρ s

60. (ρ b / ρ s ) If you don’t think this is an intuitive approach, try algebra. The volume of solids V s = m s / ρ S (from definition of ρ S ) so V p = V t - ρ B x m s. The mass of solids, m s = ρ B x V t (from definition of ρ B ), so V p = V t – (ρ B / ρ S ) x V t. Now divide by V t.

65. macro- micro-

66. Native soil This is directly related to effect of tillage on bulk density. Tillage increases density in the long-term because of loss of organic matter that results in loss of inter-aggregate porosity. Again, the effect is primarily on macro- porosity.

67. Know these two definitions. By the way, which one of these can you always measure?

68. The basis for most soil chemical data is dry soil solid mass (i.e., oven-dry), however, you can’t maintain soil in an oven-dry state unless it’s in a dessicator and when you remove it to do something with it, it starts to immediately adsorb water from the atmosphere. So, you correct to oven-dry moisture. Here, gw is gravimetric water content.

69. What You Now Know About Soil Physical Properties And With Such Amazing Ease Color Munsell chart and parameters What soil colors usually indicate

70. Texture Definitions of separates How to use a textural triangle Basis and method of mechanical analysis Importance of texture to other soil properties

71. Structure Types Effects on aeration and water movement Formation and stability of surface structure Management ideas for preserving

72. Particle density Definition How to measure it Bulk density Definition How to measure it Influences of texture, structure and management on it

73. Porosity How to calculate it Influence of texture, structure and management on it

74. Water content Gravimetric Volumetric Air-dry soil contains adsorbed water and that’s a nasty little inconvenience