Chapter 15 Soil Resources

1. Chapter 15Soil Resources

2. Overview of Chapter 15 What is soil? Soil Properties Major Soil Orders Soil Problems Soil Conservation Soil Reclamation

3. Soil Uppermost layer of earth’s crust that supports plants, animals and microbes Soil Forming Factors Parent Material Rock that is slowing broken down Time Climate Organisms Carbon dioxide from plants helps form carbonic acid to break down rock. Lichens produce acids too. Topography Regions surface features

4. Soil Composition Mineral Particles (45%) Weathered rock Provides essential nutrients for plants Varies from soil to soil Older soil mineral deficient Younger soil mineral rich Organic Material (5%) Litter, animal dung, dead remains of plants and animals, humus (picture) Humus: black or dark brown material that remains after decomposition Helps to bind mineral ions Holds water in soil Water (25%) Air (25%)

5. Soil Composition Pore space 50% of soil Soil air- good for aeration Soil water- provides water to roots

6. Soil Composition Soil water Originates as precipitation Water not bound to soil particles or absorbed by roots leaches through various layers and carries minerals Deposition of leached material is illuviation Leached materials can reach groundwater

7. Soil Air Contains many of the same gases found in atmospheric air Gases present in different proportions More carbon dioxide than oxygen result of cellular respiration of soil organisms Carbon dioxide forms carbonic acid in water Important weathering processes Soil Composition

8. Soil Horizons (distinctive horizontal layers) Soil Profile is a vertical section showing horizon layers O-horizon Rich in organic material May be absent in deserts A-horizon Topsoil E-horizon Not present in all soils B-horizon Lighter colored subsoil nutrients C-horizon Weathered parent material Below extent of most roots Saturated with groundwater

9. Soil Organisms There are millions of microorganisms in 1 tsp of fertile agricultural soil Bacteria are most numerous

10. Soil Organisms Soil organisms provide ecosystem services Important environmental benefits that ecosystems provide Examples Decaying and cycling organic material Breaking down toxic materials Cleansing water Soil aeration (especially done by earthworms)

11. Soil Organisms Earthworms Transport soil components via castings (bits of soil passed through the gut) Tunnels aerate soil Ants Tunnels aerate soil too Some food collected decomposes increasing organic matter Carry seeds Fungi and roots of vascular plants Mycorrhizae Enhannce plant growth

12. Nutrient Cycling Nutrients are cycled between plants, organisms and soil Example Bacteria and fungi decompose plant and animal wastes They are transformed into CO2, soil nutrients and water

13. Soil Properties Soil Texture refers to relative proportion of different sized particles Particles bigger than 2 mm in diameter not considered (why?) Relative proportion of sand, silt and clay Sand: 2mm-0.05mm (can see with naked eye) Silt: 0.05mm-0.002mm (need EM to see) Clay: >0.002mm

14. Soil Properties Soil texture affects soil properties Coarse textured soil (sandy) Will not hold water well- flows through easily Fine textured soil (high in clay) Poor drainage Low oxygen levels in soil

15. Soil Properties Minerals Often present in charged forms (ions) Magnesium and potassium are positively charged ions needed for plant growth Clay has negative charge Attracts magnesium and potassium

17. Soil Properties

18. Soil Properties Soil Acidity Measured using pH scale 0-7 = acidic 7 = neutral 7-14 = basic pH of most soils range from 4-8 Affects solubility of certain plant nutrients Optimum soil pH is 6-7 This is the pH where plant nutrients are most available to plants Soil amendments (ex: lime) can be used to achieve this pH

19. Acidic Soil (low pH) Example: Pygmy Forest, in Mendocino County, CA (pH 2.8-3.9) Aluminum and manganese more soluble and may be toxic Potassium leaches more readily Basic Soil Calcium phosphate less available (needed for plants) Decomposition of organic matter generally decreases pH Soil Properties

20. Major Soil Groups Variations in soil forming factors cause variation in soils around globe Soil Taxonomy Separates soils into 12 orders Subdivided into more than 19,000 soil series that vary by locality Five common soil orders Spodosols, alfisols, mollisols, aridosols, oxisols

21. Major Soil Groups Spodosols Form under coniferous forests (cooler, good precip, good drainage) O-horizon composed of needles (acidic) E-horizon is ash-gray under A-horizon Not good farmland- too acidic

22. Major Soil Groups Alfisols Temperate deciduous forests Brown to gray-brown A-horizon Precipitation high enough to leach most organics and nutrients out of A-, E- and B-horizons Soil fertility maintained by leaf litter

23. Major Soil Groups Mollisols Found in temperate, semi-arid grassland Very fertile soil Thick, dark brown/ black A-horizon (humus) Soluble nutrients stay in A-horizon due to low leaching (rain does not wash out) Grain crops are grown

24. Major Soil Groups Aridosols Found in arid regions of all continents Low precipitation preclude leaching and growth of lush vegetation Development of salty horizon possible Need water to grow crops

25. Major Soil Groups Oxisols Found in tropical and subtropical areas with high precipitation Very little organic material accumulation due to fast decay rate B-horizon is highly leached and nutrient poor As plants and animals decay nutrients are taken up quickly

26. Soil Problems Sustainable soil use: wise use of soil resources, without reduction in fertility, so that it is useful for future generations Soil Erosion Defined as wearing away or removal of soil from the land Caused primarily by water and wind Why a problem? Causes a loss in soil fertility as organic material and nutrients are eroded More fertilizers must be used to replace nutrients lost to erosion Sediments can cause problems (How?) Plant cover can reduce erosion Accelerated by poor soil management practices

27. Soil Problems Problems in USA and world Every 5 years Natural Resources Conservation Service measure soil erosion in USA Soil erosion of croplands has declined (between 1982-1992) Most serious in S. Iowa, N. Missouri, Texas, E. Tennessee Water erosion is bad in midwestern grain belt and parts of California Worldwide annual loss of as much as 75 billion metric tons of topsoil

28. Case in Point: American Dust Bowl Great Plains have low precipitation and subject to drought 1930-1937 severe drought Area was overused (farming/grazing) No natural vegetation roots to hold soil in place (removed) Replaced by annual crops Winds blew soil as far east as NYC and DC. Rain caused severe erosion Farmers went bankrupt US Soil Conservation Service addressed the issues (how?)

29. Soil Problems Nutrient Mineral Depletion When organisms die and microorganisms decompose them, the essential nutrient minerals are released into the soil Farming and clearcutting disrupt cycle

30. Soil Problems Mineral depletion in tropical rainforest soils Soils are somewhat nutrient-poor because the nutrient minerals are stored primarily in vegetation When the forest is cleared, its efficient nutrient cycling is disrupted Can recover but takes a long time

31. Soil Problems Soil Salinization Def: gradual accumulation of salt in the soil, usually due to improper irrigation techniques Often in arid and semi-arid areas (natural mineral salt conc. high) The little precipitation that falls is quickly evaporated Leaves behind salts Salt concentrations get to levels toxic to plants

32. Soil Problems Desertification Def: degradation of once-fertile rangeland, agricultural land, or tropical dry forest into nonproductive desert Typically a human-induced condition Change in vegetation changes climate, further decreasing precipitation levels Big problem in Asia and Africa (soil is overexploited)

33. Soil Conservation Conservation Tillage Residues from previous year’s crops are left in place to prevent soil erosion Includes no tillage Leave soil undisturbed

34. Soil Conservation Contour Plowing Plowing around hill instead of up-down Decreases soil erosion Strip Cropping (type of Contour Plowing) Alternating strips of different crops along natural contours Terracing Creating terraces on steep slopes to prevent erosion

35. Preserving Soil Fertility Organic fertilizers Animal manure, crop residue, bone meal and compost Nutrient available to plants only as material decomposes Slow acting and long lasting Inorganic fertilizers Manufactured from chemical compounds Soluble Fast acting, short lasting Environmentally sound to limit use Mobile- easily leach and pollute groundwater No water holding capacity (like organic)

36. Compare Organic Fertilizers to Inorganic Fertilizers What are the pros and cons of both? Which is better for the environment and why?

37. Soil Reclamation It is possible to fix soil erosion EX: Dust Bowl in USA in 1930s and badly eroded land in N. China Two steps Stabilize land to prevent further erosion (planting of native grasses during the dustbowl) Restoring soil to former fertility (use fertilizers) Best way to reduce effects of wind on soil is shelterbelts Row of trees planted to reduce wind erosion of soil

38. Soil Conservation Policies in US Soil Conservation Act 1935 Authorized formation of Soil Conservation Service, now called Natural Resource Conservation Service (NRCS) Assess soil damage and develop policies to improve soil Food Security Act (Farm Bill) 1985 Required farmers with highly erodible soil had to change their farming practices Instituted Conservation Reserve Program (CRP) Pays farmers to stop farming highly erodible land