1. Chapter 11
Soil

2. Soil
1. In productive soil, detritus feeders and decomposers constitute a biotic community
Facilitating the transfer of nutrients
Creating a soil environment favorable to root growth
2. Productive topsoil involves dynamic interactions among organisms, detritus, and mineral particles of the soil

3. Soil formation

4. Soil characteristics Most soils are hundreds of years old
They change very slowly
Soil texture: relative proportions of each soil type
Parent material: mineral material of the soil
Soil has its origin in the geological history of an area
Weathering: gradual physical and chemical breakdown of parent material
It may be impossible to tell what the parent material was

5. Classification of soil
Soil separates: small fragments smaller than stones
Sand: particles from 2.0 to mm
Silt: particles range from to mm
Clay: anything finer than mm
Clay particles become suspended in water
Clay is “gooey” because particles slide around each other on a film of water

6. Soil Formation Primary Succession
Rock
Lichens
Chemical and physical weathering
Detritus  humus; rock  minerals
Mosses  improving soil
Annuals
Perennials (grasses)
Shrubs + trees

7. Soil makeup

8. Proportions Sand, silt, and clay constitute the mineral part of soil
If one type of particle predominates, the soil is sandy, silty, or clay
Loam: a soil with 40% sand, 40% silt, and 20% clay

9. Soil Texture To determine a soil’s texture:
Add soil and water to a test tube and let the soil settle
Sand particles settle first, then silt, then clay
Scientists classify soil texture with a triangle
It shows relative proportions of sand, silt, and clay

10. The soil texture triangle

11. Properties Soil properties are influenced by its texture
Larger particles have larger spaces separating them
Small particles have more surface area relative to their volume
Nutrient ions and water molecules cling to surfaces
These properties profoundly affect soil fertility
Infiltration, nutrient- and water-holding capacity, aeration

12. Workability the ease with which soil can be cultivated
Clay soils are hard to work with: too sticky or too hard
Sandy soils are easy to work with

13. Soil profiles Horizons: horizontal layers of soil from soil formation
Can be quite distinct
Soil profile: a vertical slice through the soil horizons
Reveals the interacting factors in soil formation

14. Soil profiles O horizon: topmost layer of soil
Dead organic matter (detritus) deposited by plants
High in organic content
Primary source of energy for the soil community
Humus: decomposed dark material at the bottom of the O horizon

15. Subsurface layers A horizon (topsoil): below the O horizon
A mixture of mineral soil and humus
Permeated by fine roots
Usually dark
May be shallow or thick
Vital to plant growth
Grows an inch or two every hundred years

16. Soil profile

17. Mineral nutrients Initially become available through rock weathering
Phosphate, potassium, calcium, etc.
Much too slow to support normal plant growth
Breakdown and release (recycling) of detritus provides most nutrients
Leaching: nutrients are washed from the soil by water

18. Fertilizer Agriculture removes nutrients from the soil
Fertilizer: nutrients added to replace those that are lost
Organic fertilizer: plant or animal wastes or both
Manure, compost (rotted organic material)
Leguminous fallow crops (alfalfa, clover)
Food crops (lentils, peas)
Inorganic fertilizer: chemical formulations of nutrients
Lacks organic matter
Much more prone to leaching

19. Water is crucial for plants
Transpiration: water is absorbed by roots and exits as water vapor through pores (stomata; singular = stoma) in the leaves
Oxygen enters, and carbon dioxide exits, through stomata
Loss of water through stomata can be dramatic

20. Water is crucial for plants
Wilting: a plant’s response to lack of water
Conserves water
Shuts off photosynthesis by closing stomata
Severe or prolonged wilting can kill plants

21. Transpiration

22. Water and water-holding capacity
Water is resupplied to the soil by rainfall or irrigation
Infiltration: water soaks into the soil
Water runoff is useless to plants and may cause erosion
Water-holding capacity: soil’s ability to hold water after it infiltrates

23. Water and water-holding capacity
Poor holding capacity: water percolates below root level
Plants must depend on rains or irrigation
Sandy soils
Evaporative water loss depletes soil of water
The O horizon reduces water loss by covering the soil

24. Plant-soil-water relationship

25. Aeration Novice gardeners kill plants by overwatering (drowning)
Roots must breathe to obtain oxygen for energy
Land plants depend on loose, porous soil
Soil aeration: allows diffusion of oxygen into, and carbon dioxide out of, the soil
Overwatering fills air spaces

26. Compaction packing of the soil
Due to excessive foot or vehicular traffic
Reduces infiltration and runoff
Strongly influenced by soil texture

27. Relative acidity (pH)
pH refers to the acidity or alkalinity of any solution
The pH scale runs from 1 to 14
7 is neutral (neither acidic or alkaline)
Different plants are adapted to different pH ranges
Most do best with a pH near neutral
Many plants do better with acidic or alkaline soils
Blueberries, azaleas do best in acidic soils

28. Salt and water uptake
Buildup of salt in the soil makes it impossible for roots to take in water
High enough salt levels can draw water out of a plant
By osmosis
Dehydrates and kills plants

29. Salt and water uptake
Only specially adapted plants grow in saline soils
None of them are crops
Irrigation can lead to salt buildup in soil (salinization)

30. The soil community To support plants, soils must
Have nutrients and good nutrient-holding capacity
Allow infiltration and have good water-holding capacity
Resist evaporative water loss
Have a porous structure that allows aeration
Have a near-neutral pH
Have low salt content
According to the principle of limiting factors, the poorest attribute is the limiting factor

31. Limiting factors in plant growth
Sandy soils dry out too quickly to be good for agriculture
They have poor water-holding capacity
Clay soils do not allow infiltration or aeration
The best soils are silts and loams
They moderate limiting factors

32. Limiting factors in plant growth
Soil texture limitations are improved by the organic parts of the soil ecosystem
Detritus
Soil organisms

33. Organisms and organic matter in the soil
Dead leaves, roots, other detritus on and in the soil
Support a complex food web
Bacteria, fungi, mites, insects, millipedes, spiders, earthworms, snails, slugs, moles, etc.
Millions of bacteria are in a gram of soil

34. Organisms and organic matter in the soil
Humus: residue of partly decomposed organic matter
In high concentrations at the bottom of the O layer
Extraordinary capacity for holding water and nutrients
Composting: fosters decay of organic wastes
Is essentially humus

35. Soil as a detritus-based ecosystem

36. Soil bacteria

37. Soil structure and topsoil
Animals feeding on detritus also ingest mineral soil particles
Castings: earthworm excrement of stable clumps of “glued” inorganic particles plus humus
Burrowing of animals keeps clumps loose

38. Soil structure and topsoil
Soil structure: refers to the arrangement of soil particles
Soil texture: refers to the size of soil particles
A loose soil structure: best for infiltration, aeration, and workability
Topsoil: clumpy, loose, humus-rich soil
Loss of topsoil reduces crop yield by 85–90%

39. Humus and the development of soil structure

40. The results of removing topsoil

41. Interactions between plants and soil biota
Mycorrhizae: a symbiotic relationship between the roots of some plants and certain fungi
Fungi draw nourishment from the roots
Fungi penetrate the detritus, absorb nutrients, and pass them to the plant
Nutrients are not lost to leaching

42. Interactions between plants and soil biota
Bacteria add nitrogen to the soil
Nematodes: small worms that feed on roots
Detrimental to plants
May be controlled by other soil organisms (e.g., fungi)

43. Predatory fungus

44. Soil enrichment Most detritus comes from green plants
So green plants support soil organisms
Soil organisms create the chemical and physical soil environment beneficial to plants
Green plants further protect the soil by reducing erosion and evaporative water loss
So keep an organic mulch around garden vegetables

45. Mineralization If detritus is lost, soil organisms starve
Soil will no longer be kept loose and nutrient-rich
Humus decomposes, breaking down the clumpy aggregate structure of glued soil particles
Water- and nutrient-holding capacities, infiltration, and aeration decline

46. Mineralization Mineralization: loss of humus and collapse of topsoil
All that remains are the minerals (sand, silt, clay)
Topsoil results from balancing detritus and humus additions and breakdown

47. The importance of humus to topsoil

48. Erosion
Erosion: the process of soil and humus particles being picked up and carried away by water and wind
Occurs any time soil is bared and exposed
Soil removal may be slow and gradual (e.g., by wind) or dramatic (e.g., gullies formed by a single storm)

49. Erosion Vegetative cover prevents erosion from water
Reducing the energy of raindrops
Allowing slow infiltration
Grass is excellent for erosion control
Vegetation also slows wind velocity

50. Splash, sheet, and gully erosion
Splash erosion: begins the process of erosion
Raindrops break up the clumpy structure of topsoil
Dislodged particles wash between other aggregates
Decreases infiltration and aeration
Sheet erosion: the result of decreased infiltration
More water runs off, carrying away fine particles
Gully erosion: water converges into rivulets and streams
Water’s greater volume, velocity, energy remove soil

51. Erosion

52. Desert
Another devastating feature of wind and water erosion: differential removal of soil particles
Lighter humus and clay are the first to be carried away
Rocks, stones, coarse sand remain
The remaining soil becomes coarser
Deserts are sandy because wind removes fine material

53. Desert pavement Desert pavement: occurs in some deserts
Removal of fine material leaves a thin surface layer of stones and gravel
This protective layer is easily damaged (e.g., by vehicles)

54. Formation of desert pavement

55. Drylands and desertification
Clay and humus are the most important parts of soil
For nutrient- and water-holding capacity
Their removal results in nutrients being removed
Regions with sparse rainfall or long dry seasons support grasses, scrub trees, and crops only if soils have good water- and nutrient-holding capacity
Erosion causes these areas to become deserts

56. Desertification
Desertification: a permanent reduction in the productivity of arid, semiarid, and seasonally dry areas (drylands)
Does not mean advancing deserts

57. Desertification

58. Causes of erosion: overcultivation
Plowing to grow crops exposes soil to wind and water erosion
Soil remains bare before planting and after harvest
Plowing causes splash erosion
Destroying soil’s aggregate structure
Decreasing aeration and infiltration

59. Causes of erosion: overcultivation
Tractors compact soil
Reducing aeration and infiltration
Increasing evaporative water loss and humus oxidation
Rotating cash crops with hay and clover is sustainable

60. Apparatus for no-till planting

61. No-till planting
No-till agriculture: a technique allowing continuous cropping while minimizing erosion
Routinely practiced in the U.S.
After spraying a field with herbicide to kill weeds
A planting apparatus cuts a furrow through the mulch
Drops seeds and fertilizer
Closes the furrow

62. No-till planting The waste from the previous crop becomes detritus
So the soil is never exposed
Low-till farming uses one pass (not 6–12) over a field

63. Reducing soil erosion
Contour strip cropping: plowing and cultivating at right angles to contour slopes
Shelterbelts: protective belts of trees and shrubs planted along plowed fields

64. Reducing soil erosion
The U.S. Natural Resource Conservation Service (NRCS)
Established in response to the Dust Bowl
Regional offices provide information to farmers and others regarding soil and water conservation practices
U.S. soil erosion has decreased through conservation
Windbreaks, grassed waterways, vegetation to filter runoff

65. Contour farming

66. Shelterbelts

67. Irrigation Irrigation: supplying water to croplands artificially
Dramatically increases production
Is a major contributor to land degradation
Flood irrigation: river water flows into canals to flood fields
Center-pivot irrigation: water is pumped from a well into a giant pivoting sprinkler

68. Flood irrigation

69. Review Question-1
The process of soil formation creates a vertical gradient of layers that are known as
a. loam.
b. aeration.
c. infiltration.
d. horizons.

70. Review Question-1
The process of soil formation creates a vertical gradient of layers that are known as
a. loam.
b. aeration.
c. infiltration.
d. horizons.

71. Review Question-2
The residue of partly decomposed organic matter is called ______ and is found in high concentrations at the bottom of the O horizon.
a. desertification
b. decomposition
c. humus
d. topsoil

72. Review Question-2 Answer
The residue of partly decomposed organic matter is called ______ and is found in high concentrations at the bottom of the O horizon.
a. desertification
b. decomposition
c. humus
d. topsoil

73. Mineralized soils can be revitalized through the addition of
Review Question-3
Mineralized soils can be revitalized through the addition of
a. compost and other organic matter.
b. materials from the C horizon.
c. topsoil.
d. all of the above.

74. Review Question-3 Answer
Mineralized soils can be revitalized through the addition of
a. compost and other organic matter.
b. materials from the C horizon.
c. topsoil.
d. all of the above.

75. a. The tragedy of the commons; overgrazing b. Deforestation; logging
Review Question-5
______ occurs when there is an accumulation of salts in soil as a result of ______.
a. The tragedy of the commons; overgrazing
b. Deforestation; logging
c. Salinization; irrigation
d. Overcultivation; no-till farming

76. Review Question-5 Answer
______ occurs when there is an accumulation of salts in soil as a result of ______.
a. The tragedy of the commons; overgrazing
b. Deforestation; logging
c. Salinization; irrigation
d. Overcultivation; no-till farming