Chapter 8

1. Chapter 8 Soil: Foundation for Land Ecosystems

2. 8.1 Soil and plants

3. Soil vs. Dirt • Soil is much more than just “dirt”

4. Top Soil Formation Productive topsoil involves dynamic interactions among the organisms, detritus, and mineral particles of the soil

5. Soil Characteristics - Texture • Parent material • Mineral material of the soil, has its origin in the geological history of the area • Parent material could be rock, or sediments deposited by wind, water, or ice • Eventually parent material is broken down by natural weathering

6. Weathering • Gradual physical and chemical breakdown • Physical • Ex – ice wedging, erosion • Chemical • Formation of H2CO3 of organisms, or other chemical erosion • As rock weathers, it breaks down into small stones (soil separates)

7. Soil Separates • Sand • Particles from 2.0 – 0.02 mm in size • Silt • Particles from 0.02 down to 0.002 mm • Clay • Particles finer than 0.002 mm

8. Proportions • Sand, silt, and clay particles constitute the mineral portion of soil • Soil texture refers to the relative proportions of each type of the particle in a given soil • Loam • 40% sand, 40% silt, 20% clay • Best type of agricultural soil

9. Major classes of soil are indicated on the triangle. For clay, read across horizontally; for silt, read diagonally downwards; for sand, read diagonally upwards to the left. The texture content of any soil should total 100% if the triangle is read properly.

10. Properties • Larger Particles • Like sand • Have larger spaces separating them than smaller particles • Visualize difference between packing softballs and golf balls in the same size container

11. Properties • Smaller Particles • Such as silt and clay • Have more surface area relative to their volume than larger particles • (Visualize cutting a block in half again and again. Each time you cut it, you create two new surfaces, but the total volume of the block remains the same)

12. Properties • Nutrient ions and water molecules tend to cling to surfaces • Smaller particles will have a larger surface area, therefore tend to hold on to these ions and molecules

13. Workability • Soil texture affects workability • Ease with which a soil can be cultivated (typically for agriculture)

15. Soil Profiles • Soil formation creates a vertical gradient of layers that are often quite distinct • Layers are known as horizons • Vertical slice through a horizon is called soil profile • 5 horizons

16. O-Horizon • Consists of dead organic matter (detritus) deposited by plants, leaves, stems, fruits, seeds, etc. • High in organic content • Primary source of energy for the soil community

17. Humus • Towards the bottom of the O-horizon, the decomposition process is well advanced and the original materials may be unrecognizable • At this point the material is dark and is called HUMUS • Not to be confused with the chick-pea dip, hummus.

18. A-Horizon • Below O-Horizon • Mixture of mineral soil from below and humus above • AKA Topsoil. • Fine roots from O-horizon can permeate this layer • A-Horizon is usually dark because of the humus, and may be shallow or thick depending on the ecosystem

19. E-Horizon • E stands for eluviation • The process of leaching (dissolving away) many minerals due to the downward movement of water • This layer is often paler in color than the two layers above it

20. B Horizon • Characterized by the deposition of minerals that have leached from the A and E horizons, • often high in FE, AL, CA, and other minerals. • Referred to as the SUBSOIL • Often high in clay and is reddish or yellow in color

21. C-Horizon • Parent material originally occupying the site • Weather rock, glacial deposits, volcanic ash • Affected little by the biological and chemical processes that go on in the overlying layers

22. Study Aide • O • A • E • B • C

25. Soil Classes • Soil scientists have crated a taxonomy of soils • Order, suborder, group, subgroup, families • Literally hundreds of soil classes • There are 12 major orders • We will investigate 4 orders

26. Mollisols • Fertile, dark soils found in temperate grassland biomes • World’s best agricultural soil • Deep A-horizon and are rich in humus and minerals • Precipitation is insufficient to leach the minerals downward • Found in • Midwestern US, temperate Ukraine, Russia, Mongolia, Argentina

27. Oxisols • Tropical and subtropical rainforests • Layer of iron and aluminum oxides in the B-horizon • Little or no O-horizon • Due to rapid decomposition of the plant matter • Most minerals are in living plant matter • Most oxisols are of limited fertility for agriculture

28. Alfisols • Widespread • Moderately weathered forest soil • Not deep, but have well developed O, A, E, and B horizons • Typically of the moist, temperate forest biome • Suitable for agriculture if they are supplemented with organic matter or mineral fertilizers

29. Aridisols • Widespread soils of dry-lands and deserts • Relatively unstructured in soil horizons • Thin, lightly colored • Irrigation used on these soils usually leads to salinization • High evaporation rates draw salt to the surface

30. Soil and Plant Growth • Plants need a root environment that supplies optimal amounts of mineral nutrients, water, and air • The pH and salinity of the soil are also critically important • Soil Fertility • Soils ability to support plant growth • Refers to the presence of proper amounts of nutrients

31. Mineral Nutrient and Nutrient-Holding Capacity • Phosphate, Potassium, Calcium and other ions are present in rocks and become available to roots through weathering • Processes is usually too slow for plant growth • Nutrients that support plant growth are supplies mostly though breakdown of detritus

32. Leaching • Nutrients may be washed from the soil as water moves though it • Lessens soil fertility • Contributes to pollution • Soils capacity to hold nutrient ions until they are absorbed by roots becomes very important

33. Fertilizer • Unavoidable removal of nutrients from soil with each crop • Nutrients absorbed by plant are contained in harvested material • Therefore agricultural systems require input of fertilizers

34. Fertilizer Organic Inorganic Chemical formulations of required nutrients, with out any organic matter included Much more prone to leaching than organic fertilizers • Includes plant or animals wastes or both • Manure and compost • Rotten organic material • Includes nitrogen fixing plants • Alfalfa, soy (legumes), peas, lentils

35. Water/Water Holding Capacity • Transpiration • Water is absorbed by the roots of plants, passed up through plant, and exit as water vapor through microscopic pores in leaves (stomata)

37. Infiltration • Water is resupplied to soil by rainfall or irrigation • Soils ability to allow water to soak in is important

38. Water-Holding Capacity • Poor water holding capacity implies that most of the infiltrating water percolates on down below the reach of the roots

39. Evaporation • Evaporative water loss from the soil surface • Depletes soil’s water reservoir without serving the needs of plants

40. Plant-Soil-Water relationships Water lost from the plant by transpiration must be replaced from a reservoir of water held in the soil. In addition to the amount and frequency of precipitation, the size of this reservoir depends on the soil’s ability to allow water to infiltrate, to hold water, and to minimize direct evaporation

41. Aeration • Roots need to “breathe” • Living organs need a constant supply of oxygen for energy via metabolism (respiration) • Overwater fills air spaces preventing aeration • Compaction • Or packing of the soil due to large amounts of traffic (foot or vehicular) reduces aeration

42. Relative Acidity (pH) • Refers to the acidity or alkalinity of any solution • Most plants do best with a pH near neutral

43. REVIEW • To support good crop soil must • Have good supply of nutrients and a good nutrient holding capacity • Allow infiltration and have a good water holding capacity/resist evaporative water loss • Have a porous structure that permits good aeration • Have a pH near neutral • Have a low salt content

44. Detritus • Detritus accumulated on and in the soil supports a complex food web • Including bacteria, fungi, protozoans, mites, insects, millipedes, spiders, centipedes, earthworms, snails, slugs, moles, and other burrowing animals • Most numerous and important – BACTERIA

47. Humus • As organisms feed, bulk of detritus is consumed through cellular respiration • REVIEW • 6CO2 + C6H12O66CO2 + 6H20 + ENERGY • However, each organism leaves a part of the detritus undigested • This is humus

48. Humus and the development of soil structure On the left is a humus-poor sample of loam. It is relatively uniform, dense “clod”. On right is a sample of the same loam, but rich in humus. Note that is has a very loose structure, composed of numerous aggregates of various sizes

49. Soil Structure • Refers to soil particle arrangement (whereas texture refers to size) • Ex a loose soil structure is ideal for infiltration, aeration, and workability

50. Soil Interactions • Between soil and biota • Roots of some plants and a fungi called mycorrhizae • Mycorrhizae go deep into the detritus, absorbs nutrients, and transfers them directly to the plant • No loss of nutrients due to leaching • Example of a mutualistic relationship!