1. © 2011 Pearson Education, Inc. CHAPTER 11 Soil: The Foundation for Land Ecosystems

2. © 2011 Pearson Education, Inc. A rich soil is much more than dirt Soil: solid material of geological and biological origin Chemical, biological, and physical processes change soil Giving it the ability to support plant growth In productive soil, detritus feeders and decomposers constitute a biotic community Facilitating the transfer of nutrients Creating a soil environment favorable to root growth Productive topsoil involves dynamic interactions among organisms, detritus, and mineral particles of the soil

3. © 2011 Pearson Education, Inc. Topsoil formation

4. © 2011 Pearson Education, Inc. Soil formation Animation: Soil Formatioin

5. © 2011 Pearson Education, Inc. Soil characteristics Most soils are hundreds of years old They change very slowly Soil science is at the heart of agriculture and forestry Soil is classified by profile, structure, and type 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

6. © 2011 Pearson Education, Inc. Classification of soil Soil separates: small fragments smaller than stones Sand: particles from 2.0 to 0.063 mm Silt: particles range from 0.063 to 0.004 mm Clay: anything finer than 0.004 mm Gravel, cobbles, boulders: particles larger than sand You can see the individual rock particles in sand Clay particles become suspended in water Clay is “gooey” because particles slide around each other on a film of water

7. © 2011 Pearson Education, Inc. Soil makeup Animation: Soil Makeup

8. © 2011 Pearson Education, Inc. Proportions Sand, silt, and clay constitute the mineral part of soil If one type of particle predominates, the soil is sandy, silty, or clayey Loam: a soil with 40% sand, 40% silt, and 20% clay 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

9. © 2011 Pearson Education, Inc. The soil texture triangle

10. © 2011 Pearson Education, Inc. 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 properties Infiltration, nutrient- and water-holding capacity, aeration 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

11. © 2011 Pearson Education, Inc. 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 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

12. © 2011 Pearson Education, Inc. 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 E horizon: pale-colored layer below the A horizon Eluviation: process of leaching (dissolving) minerals due to downward movement of water

13. © 2011 Pearson Education, Inc. Subsurface layers B horizon (subsoil): below the E horizon Contains minerals leached from the A and E horizons High in iron, aluminum, calcium, other minerals, clay Reddish or yellow colored from oxidized metals C horizon: parent mineral material Weathered rock, glacial deposits, volcanic ash Reveals geologic process that created the landscape Not affected by biological or chemical processes

14. © 2011 Pearson Education, Inc. Soil profile

15. © 2011 Pearson Education, Inc. Soil and plant growth For best growth, plants need a root environment that supplies Mineral nutrients, water, oxygen The proper pH and salinity Soil fertility: the soil’s ability to support plant growth The presence of proper amounts of nutrients and all other needs Farmers refer to a soil’s ability to support plant growth as tilth

16. © 2011 Pearson Education, Inc. 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 Decreases soil fertility Contributes to water pollution Nutrient-holding capacity: the soil’s capacity to bind and hold nutrient ions until they are absorbed by roots

17. © 2011 Pearson Education, Inc. 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

18. © 2011 Pearson Education, Inc. 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 Wilting: a plant’s response to lack of water Conserves water Shuts off photosynthesis by closing stomata Severe or prolonged wilting can kill plants

19. © 2011 Pearson Education, Inc. Transpiration

20. © 2011 Pearson Education, Inc. 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 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

21. © 2011 Pearson Education, Inc. Plant-soil-water relationship

22. © 2011 Pearson Education, Inc. 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 Compaction: packing of the soil Due to excessive foot or vehicular traffic Reduces infiltration and runoff Strongly influenced by soil texture

23. © 2011 Pearson Education, Inc. 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 do best in acidic soils

24. © 2011 Pearson Education, Inc. 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 Only specially adapted plants grow in saline soils None of them are crops Irrigation can lead to salt buildup in soil (salinization)

25. © 2011 Pearson Education, Inc. 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

26. © 2011 Pearson Education, Inc. 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 Soil texture limitations are improved by the organic parts of the soil ecosystem Detritus Soil organisms

27. © 2011 Pearson Education, Inc. 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 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

28. © 2011 Pearson Education, Inc. Soil as a detritus-based ecosystem

29. © 2011 Pearson Education, Inc. 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 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%

30. © 2011 Pearson Education, Inc. Humus and the development of soil structure

31. © 2011 Pearson Education, Inc. The results of removing topsoil

32. © 2011 Pearson Education, Inc. 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 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)

33. © 2011 Pearson Education, Inc. Predatory fungus

34. © 2011 Pearson Education, Inc. 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 The mutually supportive relationship between plants and soil is easily broken Keeping topsoil depends on addition of detritus

35. © 2011 Pearson Education, Inc. 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 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

36. © 2011 Pearson Education, Inc. The importance of humus to topsoil

37. © 2011 Pearson Education, Inc. Soil degradation Turnover of plant material produces detritus When humans cut forests, graze livestock, or plant crops, the soil is managed or mismanaged Soil degradation: occurs when key soil attributes required for plant growth or other ecosystem services deteriorate Some reports on soil degradation are incorrect or outdated 75% of the land in Burkina Faso was said to be degraded But agricultural yields have increased due to soil and water conservation

38. © 2011 Pearson Education, Inc. 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) 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

39. © 2011 Pearson Education, Inc. 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 Once started, erosion can turn into a vicious cycle Less vegetation exposes soil to more erosion

40. © 2011 Pearson Education, Inc. Erosion

41. © 2011 Pearson Education, Inc. Desert pavement 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 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)

42. © 2011 Pearson Education, Inc. Formation of desert pavement

43. © 2011 Pearson Education, Inc. 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 Desertification: a permanent reduction in the productivity of arid, semiarid, and seasonally dry areas (drylands) Does not mean advancing deserts

44. © 2011 Pearson Education, Inc. Desertification

45. © 2011 Pearson Education, Inc. 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 Tractors compact soil Reducing aeration and infiltration Increasing evaporative water loss and humus oxidation Rotating cash crops with hay and clover is sustainable

46. © 2011 Pearson Education, Inc. 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 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

47. © 2011 Pearson Education, Inc. Apparatus for no-till planting

48. © 2011 Pearson Education, Inc. Inorganic fertilizer Can provide optimal amounts of nutrients efficiently But it lacks organic matter to support organisms and build soil structure It can keep nutrient content high under intensive cultivation (two or more cash crops/year) But mineralization and soil degradation proceed Additional fertilizer leaches into waterways Chemical fertilizers have a valuable place in agriculture Organic fertilizers may not have enough nutrients Growers must use each fertilizer as necessary

49. © 2011 Pearson Education, Inc. 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 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

50. © 2011 Pearson Education, Inc. Contour farming and shelterbelts

51. © 2011 Pearson Education, Inc. Deforestation Porous, humus-rich forest soil efficiently holds and recycles nutrients Also absorbs and holds water Converting a forested hillside to grassland doubles the amount of runoff and increases nutrient leaching When forests are cut and soils are left exposed Topsoil becomes saturated with water and slides off the slope Subsoil continues to erode

52. © 2011 Pearson Education, Inc. Forests are cut at alarming rates 13 million hectares (32 million acres) are cut per year Mostly in developing countries Cutting tropical rain forests causes acute problems Heavy rains have leached soils of minerals Parent material is already maximally weathered So tropical soils (oxisols) lack nutrients Clearing rain forests washes away the thin layer of humus Leaving only the nutrient-poor subsoil Very poor for agriculture

53. © 2011 Pearson Education, Inc. The other end of the erosion problem Water that does not infiltrate enters streams and rivers Causing flooding Sediment: eroded soil carried into streams and rivers Clogs channels, intensifies floods, fills reservoirs Kills fish and coral reefs Damages streams, rivers, bays, estuaries Excess sediments and nutrients from erosion are the greatest pollution problem in many areas Groundwater is depleted Rainfall runs off and does not refill soil or ground water

54. © 2011 Pearson Education, Inc. 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 The U.S. Bureau of Reclamation is involved with supplying irrigation water to the western states Irrigating 4 million hectares (10 million acres) Worldwide irrigation is huge and is still rising

55. © 2011 Pearson Education, Inc. Flood irrigation

56. © 2011 Pearson Education, Inc. Salinization Salinization: the accumulation of salts in and on the soil Suppresses plant growth Even the freshest irrigation water has some salt Watering dryland soils dissolves minerals in the soil Evaporation or transpiration leaves salts behind Salinization is considered a form of desertification 1.5 million hectares (3.7 million acres) are lost each year to salinization and waterlogging 160,000 hectares (400,000 acres) in California are unproductive, costing $30 million/year

57. © 2011 Pearson Education, Inc. Salinization

58. © 2011 Pearson Education, Inc. CHAPTER 11 Soil: The Foundation for Land Ecosystems Active Lecture Questions

59. © 2011 Pearson Education, Inc. The process of soil formation creates a vertical gradient of layers that are known as a.loam. b.aeration. c.infiltration. d.horizons. Review Question-1

60. © 2011 Pearson Education, Inc. The process of soil formation creates a vertical gradient of layers that are known as a.loam. b.aeration. c.infiltration. d.horizons. Review Question-1 Answer

61. © 2011 Pearson Education, Inc. 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 Review Question-2

62. © 2011 Pearson Education, Inc. 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 Review Question-2 Answer

63. © 2011 Pearson Education, Inc. 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. Review Question-3

64. © 2011 Pearson Education, Inc. 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. Review Question-3 Answer

65. © 2011 Pearson Education, Inc. All of the following lead to the loss of soil except a.splash erosion. b.horizon erosion. c.sheet erosion. d.gully erosion. Review Question-4

66. © 2011 Pearson Education, Inc. All of the following lead to the loss of soil except a.splash erosion. b.horizon erosion. c.sheet erosion. d.gully erosion. Review Question-4 Answer

67. © 2011 Pearson Education, Inc. ______ 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 Review Question-5

68. © 2011 Pearson Education, Inc. ______ 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 Review Question-5 Answer

69. © 2011 Pearson Education, Inc. According to Fig. 11-3, soil with roughly 40% sand, 40% silt, and 20% clay is called a.loam. b.silt loam. c.sandy clay loam. d. loamy sand. Interpreting Graphs and Data-1

70. © 2011 Pearson Education, Inc. According to Fig. 11-3, soil with roughly 40% sand, 40% silt, and 20% clay is called a.loam. b.silt loam. c.sandy clay loam. d. loamy sand. Interpreting Graphs and Data-1 Answer

71. © 2011 Pearson Education, Inc. According to Fig. 11-2, when detritus, mineral particles, and the detritus food web interact, they form a.leaves, stems, flowers, and seeds. b.leached minerals. c.nitrogen fixation. d.topsoil. Interpreting Graphs and Data-2

72. © 2011 Pearson Education, Inc. According to Fig. 11-2, when detritus, mineral particles, and the detritus food web interact, they form a.leaves, stems, flowers, and seeds. b.leached minerals. c.nitrogen fixation. d.topsoil. Interpreting Graphs and Data-2 Answer

73. © 2011 Pearson Education, Inc. The human activities that lead to erosion and desertification are a.overcultivation, overgrazing, and deforestation. b.sustainability, stewardship, and sound science. c.pollution, overcultivation, and sustainability. d.overgrazing, sound science, and hypotheses. Thinking Environmentally-1

74. © 2011 Pearson Education, Inc. The human activities that lead to erosion and desertification are a.overcultivation, overgrazing, and deforestation. b.sustainability, stewardship, and sound science. c.pollution, overcultivation, and sustainability. d.overgrazing, sound science, and hypotheses. Thinking Environmentally-1 Answer

75. © 2011 Pearson Education, Inc. True or False: If soil conservation is to be successful, it must be practiced on the levels of both the individual landowner and public policy. a.True b.False Thinking Environmentally-2

76. © 2011 Pearson Education, Inc. True or False: If soil conservation is to be successful, it must be practiced on the levels of both the individual landowner and public policy. a.True b.False Thinking Environmentally-2 Answer