What is soil?

1. What is soil?

2. discoveryschool.com • What’s the difference between soil and dirt? Dirt is what you find under your fingernails. Soil is what you find under your feet. Think of soil as a thin living skin that covers the land. It goes down into the ground just a short way. Even the most fertile topsoil is only a foot or so deep. Soil is more than rock particles. It includes all the living things and the materials they make or change.

3. discoveryschool.com, cont’d • There is no soil on Mars or Venus. How come? Those planets have plenty of rocks. Mars has windstorms that erode rocks into dust. Venus has an acid atmosphere that cooks rocks into new chemicals. But there's still something missing. Without life, there is no soil. Living things haven't just made a home in the soil on our planet. Life actually made the soil as we know it.

4. Definitions, Hillel, Introduction to Soil Physics, 1982 • Soil refers to the weathered and fragmented outer layer of the earth’s terrestrial surface. • The soil is a heterogeneous, polyphasic, particulate, disperse, and porous system, in which the interfacial area per unit volume can be very large. The disperse nature of the soil and its consequent interfacial activity give rise to such phenomena as adsorption of water and chemicals, ion exchange, adhesion, swelling and shrinking, dispersion and flocculation, and capillarity.

5. Definition, Soil Taxonomy, 2nd ed. • The lower boundary that separates soil from the nonsoil underneath is most difficult to define. Soil consists of horizons near the earth's surface that, in contrast to the underlying parent material, have been altered by the interactions of climate, relief, and living organisms over time. Commonly, soil grades at its lower boundary to hard rock or to earthy materials virtually devoid of animals, roots, or other marks of biological activity. For purposes of classification, the lower boundary of soil is arbitrarily set at 200 cm.

6. Unsaturated System Zone of Aeration Vadose Zone Unsaturated Zone Three phase system – air, water, rock

7. Infiltration & Percolation Rates • The rate that water enters (infiltrates) a soil and then moves through the soil profile (percolates) depends partly on soil structure. • Rapid infiltration with granular and loose, single grained structureless soil. • Moderate infiltration with block-like and prismatic structure. • Slow infiltration with platy and solid, massive structure-less structure.

8. Internal Drainage • If permeability is slow or very slow, water stays in the pore spaces and air cannot enter. • Soils with poor internal drainage and aeration are mostly gray with some red or yellow streaks. • Soils with fair to good internal drainage and aeration are yellow-brown or reddish-brown with some gray spots (mottlings)

9. Infiltration The process of water entry into the soil Partitions water at surface between storm runoff and recharge Plants depend on infiltrated water Infiltration rate volume (flux) of water per unit area that enters the soil, per unit time Infiltration capacity the maximum rate at which infiltration can occur under specific moisture conditions.

10. When rain hits a dry soil, surface effects between the soil and water exert a tension that draws moisture into the soil As the capillary forces diminish with increased soil-moisture content, the infiltration capacity decreases As more water infiltrates, the amount of water that can be infiltrated during the latter stages of a precipitation event is less than at the beginning

11. How Soils are Formed • Soil is formed from rock (Parent material) • Rock is slowly broken down or fragmented by • Weathering processes • Biological • Chemical • Physical • Topography helps control how fast parent material is broken down. • Steep slopes – very little or no soil • Moderate slope – deep soil formation

12. Physical weathering breaks rocks into small mineral particles.

13. Chemical weathering dissolves and changes minerals at the Earth’s surface.

14. Decomposing organic material from plants and animals mixes with accumulated soil minerals.

15. Soil Composition • Soil is composed of four major parts: • Mineral particles • Organic matter • Water • Air • When organisms die, bacteria and other soil organisms decompose the dead material, returning the nutrient minerals to the soil.

16. Mineral Portion • Comes from weathered rock • Constitutes most of what we call soil. • Mineral content is determined by the type of parent material. • Age of soil affects its mineral content. • Older soils are more weathered and have poorer mineral nutrient content. • Young soils formed in the area of volcanoes have many essential nutrient mineral ions available.

17. Parent material (bedrock) undergoes weathering to become regolith (soil + saprolite).

18. Soil is a mixture of mineral and organic matter lacking any inherited rock structure. Soil

19. Saprolite is weathered rock that retains remnant rock structure. Saprolite

20. Saprolite

21. Organic Matter • Organic material is composed of: • Litter • Dead leaves • Branches • Animal dung • Dead remains of plants, animals and microorganisms. • Microorganisms (bacteria and fungi) decompose the materials into basic nutrients in the soil • Organic matter increases the soil’s ability to retain moisture. • Humus is the black or dark brown organic material remains after much decomposition.

22. Water and Air • Soil has numerous pore spaces around and among the soil’s particles. • The pore spaces occupy roughly 50% of a soil’s volume and are filled with varying proportions of water (soil water) and air (soil air). • Both are necessary to produce a moist but aerated soil.

23. Soil Water • Soil water originates as precipitation which drains downward until it reaches the groundwater level. • Soil water contains low concentrations of dissolved nutrient mineral salts that enter the roots of plants. • Water not bound to soil particles or absorbed by plant roots, percolates through the soil taking the mineral nutrients with it.

24. Soil Water • Leaching is the removal of dissolved materials from the soil by water percolating down through it. • Illuviation is the deposition of leached material in the lower layers of the soil.

25. Soil Air • Soil air contains the same gases as atmospheric air, although the are usually present in different proportions. • As a result of cellular respiration by soil organisms, there is more carbon dioxide and less oxygen than in atmospheric air. • Nitrogen is used by nitrogen-fixing bacteria. • Carbon dioxide is turned into carbonic acid and is used in weathering of soil and bedrock.

26. 6 – 12 inches of water are added to the hole and the time per inch of decline is measured Time can’t be too long (>60 min) or too short (<3 min)

27. Infiltration Rates in Soils

30. Soil Horizons • A soil profile is a vertical section from the surface to parent material.

32. Gley soil in soil science is a type of hydric soil which exhibits a greenish-blue-grey soil color due to wetland conditions. On exposure to the air, gley colors are transformed to a mottled pattern of reddish, yellow or orange patches. During gley soil formation (a process known as Gleying), the oxygen supply in the soil profile is restricted due to soil moisture at saturation. Anaerobic micro-organisms support cellular respiration by using alternatives to free oxygen as electron acceptors. This is most often the case when the sesquioxide of iron, ferric oxide is reduced to ferrous oxide by the removal of oxygen. These reduced mineral compounds produce the gley soil color.

34. if more than 2-3 inches thick – probably been plowed

35. light colored, leached horizon typically present only in forests

36. : zone of accumulation Iron bearing leached from above and precipitated in B Yellowish brown to strong brown color

39. IDEAL SOIL HORIZON

40. Finding and Describing Horizons Soil Pit Technique Starting from top, observe profile to determine properties and differences between horizons. Place golf tee or marker at the top and bottom of each horizon to clearly identify it. Look for: different colors, shapes, roots, the size and amount of stones, small dark nodules (called concretions), worms, or other small animals and insects, worm channels, and anything else that is noticeable. Soil formed under very dry or arid conditions in New Mexico, USA

41. Finding and Describing Horizons Exposed Profile (Road Cut) Technique Obtain permission to take samples from the road cut, excavation, or other soil profile exposed by others. Obey any and all safety precautions requested. Expose a fresh soil face by scraping approximately 2cm off of the vertical surface of the soil profile. Follow Soil Pit Technique directions.

42. Recognizing Soil Texture & Structure

43. Terminology • Texture – physical property of soil referring to the relative amounts of sand, silt, and clay; how a soil feels. • Sand – largest individual soil particle • Silt – medium sized individual soil particle

44. Terminology • Clay – smallest individual soil particle • Peds – natural grouping of soil particles • Clods – artificial grouping of soil particles • Infiltration – movement of water into the soil

45. Terminology • Mechanical analysis – process of separating a soil into its various parts to permit study. • Mottlings – indication of internal drainage & aeration; soil exhibits spots of color. • Percolation – movement of water through the soil. • Permeability – characteristic of soil which permits variations in the speed of air & water movement.

46. Soil Texture • Soil texture = proportions of sand, silt and clay • Property of the soil controlled by the size of individual grains or particles • Soil is usually made up of particles of widely varying sizes. • Soil texture expresses the average or combined effect of all these grain sizes

47. Texture • At the most basic level, soil texture can be determined by feel & described as one of the following: • Coarse • Moderately Coarse • Medium • Moderately Fine • Fine

48. Specific Soil Textures (from Coarsest to Finest) determined by Mechanical Analysis • Sand (Coarse) • Loamy Sand (Moderately Coarse) • Sandy Loam (Moderately Coarse) • Loam (Medium) • Silt Loam (Medium) • Silt (Medium) • Sandy Clay Loam (Medium)

49. Specific Soil Textures continued (from Coarsest to Finest) • Clay Loam (Moderately Fine) • Silty Clay Loam (Moderately Fine) • Sandy Clay (Moderately Fine) • Silty Clay (Fine) • Clay (Fine)

50. Specific Soil Textures determined by Feel • Sand – Dry = no clods. Moist = easily crumbled ball, does not ribbon, does not stain fingers. • Loamy Sand – Dry = very weak clods. Moist = easily crumbled ball, does not ribbon or stain fingers.