1. Ch. 4: Soils, Nutrition etc.

2. Soil Definition: Natural body with layers (horizons)
Mineral + organic matter (OM)
Differs from parent material: substance from which soil derived

3. Weathering Factors
Mineral component: generated by weathering rock

4. Soil Texture
A: Sand & silt
Major particle sizes (know these)
B: clays

5. Textural triangle Distribution particles by size class: texture
Loam: mix sand, silt, clay
Texture important: fertility, water availability

6. Soil Structure Particles form peds (visible when soil dry)
Affect water + root penetration
How important??

7. Organic matter (OM) Humus: partly decomposed OM
Usually negatively charged:
carboxyl groups (-COOH)
phenols

8. Soil Fertility Defn: Ability soil hold & deliver nutrients
Determined by texture, organic matter, pH
Holding soil….

9. Soil Fertility Texture: clays important Huge surface area
Negatively charged: hold cations useful for plants (Ca++, K+, Mg++, Zn++)
Huge surface area
Surface of clay in top 10 cm 0.5 ha soil would cover continental U.S.!!

10. Soil Fertility
Humus has negative charge: clay & humus hold cations

11. Soil Fertility
Cation Exchange Capacity: amount negative charge per unit soil
Units? centimoles (+) charge/kg dry soil (cmolc/kg)
Represents “potential fertility”

12. Soil Fertility Examples: Great Plains prairie soil: 30 cmolc/kg
NE US conifer forest soil: 2 cmolc/kg

13. Soil Fertility H+ (and Al+++) also attracted to negative charge.
Take up space: not useful to plants.
Base saturation (BS): % exchange sites occupied by “good” cations (bases: Ca++, Mg++, K+) plus Na+

14. Soil Fertility BS, pH & CEC determine “actual fertility”
1) High CEC + high BS: more fertile
2) If BS low: pH low (lots H+)

15. Soil pH Measure H+ ion concentration
Most AL soils: (strongly acid)
Black Belt soils: (alkaline)

16. Soil pH pH effects: 1) H+ damages roots (@ extreme pH values)
2) soil microflora
Acid favors fungi (incl. mycorrhizal)
Alkaline favors bacteria
3) soil structure (sometimes)

17. Soil pH 4) nutrient availability. Major pH influence!
Nutrient deficiency:
Acid: N, P, Ca, Mg, K, S
Alkaline: Fe, Mn, Zn, Cu, Co, B

18. Soil pH 4) nutrient availability. Major pH influence!
Nutrient toxicity:
Acid: Fe, Mn, Zn, Cu, Co
Alkaline: Mo

19. Soil pH Plant sensitivity & nutrient needs matter Black Belt lab (#4):
Black Belt soil: no pines
Fe deficiency likely

20. Soil Horizons Caused by vertical gradients
Leaching: washing material from upper to lower layers
Weathering: greater at surface
Biotic effects: greater at surface

21. Soil Horizons Major horizons: O: organic matter at surface
A: surface soil. High organic matter
E: zone of eluviation (material washed out of layer), leaching strong

22. Soil Horizons Major horizons: B: subsurface soil.
Zone of illuviation (deposition of material from above).
Zone of alteration (chemical changes produce secondary soil minerals/clays)

23. Soil Horizons Major horizons: B: subsurface soil.
Hardpan: layer cemented soil grains
Claypan: layer dense clay
Both: interfere with water penetration, root growth
Humic layer: layer organic matter from E horizon

24. Soil Horizons Major horizons: C: weathered parent material
R: unweathered parent material

25. Soil Horizons
Layers subdivided using numbers
Fig. 4.5

26. Soil Forming Factors
Soil = function (climate, organisms, relief, parent material, time)
S = f (cl, o, r, p, t)
Zonal soils: Typical for climate & organisms
“Soil climax:” equilibrium with climate/organisms
Intrazonal soils: deviate due to relief, parent material
Azonal soils: too young (not enough time to mature)

27. Climate (cl) Moisture & temperature most important
More water & higher temperatures accelerate soil development
Higher temperature:
More weathering
Deeper soil, more clay

28. Climate (cl) More moisture: Deeper soil More clay More OM
Greater CEC (potential fertility)

29. Organisms (o: plants) Plants influence soil and vice versa
How plants influence soil?
1) Roots
Depth: record 120 m (394 ft) for fig tree (Echo Caves, South Africa)
Amount (biomass/unit volume/yr)
Size: woody (shrub/tree) vs. fibrous roots (grasses)

30. Organisms (plants) How plants influence soil? 2) Base cycling
Keeps nutrients “in play”

31. Organisms (plants)
Fertile island effect: under desert shrubs surface soil fertile
Ex, creosote bush: soil under shrubs--more nutrients

32. Organisms (plants) Fertile island effect due to: a) base cycling
b) stemflow (rainwater flowing down stem) concentrates nutrients in dust into soil below shrub
Result: annual plants bigger & healthier under shrubs

33. Organisms (plants) How plants influence soil? 3) Litter acidity
Ex: compare soils under spruce (conifer) vs hardwood

34. Relief (r) Topographic position affects soil Primary effects:
S = f (cl, o, r, p, t)
Topographic position affects soil
Primary effects:
Drainage: movement water through soil. Causes leaching
Runoff: movement water over soil surface. Causes leaching
Erosion: removal soil by water

35. Relief (r) Topographic position affects soil properties In general:
Hilltop soils shallow, less profile development
Basin soils deeper, more clay/salts, more profile development

36. Relief (r) Erosion not always harmful
Example: pygmy forest, Mendocino County, California

37. Relief (r) Each terrace 100,000 yr older than previous
At 300,000 yr, hardpan (layer of cemented iron) develops 1 foot below surface

38. Relief (r) No drainage when wet, little water holding when dry
Trees stunted

39. Relief (r)
Canyon sides: erosion prevents hardpan formation…. full sized forest

40. Parent Material (p)
Within climate area, parent material major influence on vegetation and soil
Ex, serpentine soil
High Mg, low Ca
Lots Ni, Cr
Shallow, stony

41. Parent Material (p)
Extreme cases, form serpentine “barrens”

42. Parent Material (p)
Within climate area, parent material major influence on vegetation and soil
Ex, granite outcrop soil
High silica, lots sand (coarse texture)
Soil dry (water drains fast)
Little nutrients (little clay: low CEC)
Forest on granite
in Australia

43. Time (t) General trends (as time increases): pH decreases
S = f (cl, o, r, p, t)
General trends (as time increases):
pH decreases
organic matter increases
clay content increases
soil depth increases