Soil science

• Soil


  A mixture of organic and inorganic materials which developed on the earth's surface through weathering process of rocks and minerals and whose properties are conditioned in various degrees by the influence of climate, living organisms, and topography acting on the parent material over a period of time
• Soil


  • Serves as a medium of plant growth (physical support for anchorage of plant roots; water and nutrient supplier)
  • Considered a non-renewable resource because it takes about a hundred years for natural processes to form an inch of soil
  • A natural body with dimensions of thickness and width with indistinct horizontal boundaries enabling it to blend with other soils and vertical boundaries of the air above it and the unweathered rocks below it
• Soil surface


  The upper limit of soil which is the boundary between soil and either air, shallow water, live plants, or plant materials that have not begun to decompose
• Pedon


  A hexagonal column of soil measuring from 1 to 10 m² in top surface area, the basic sampling unit used in soil surveys
• Polypedon


  An essential soil individual, comprising an identifiable series of soils in an area made up of multiple pedons and has distinctive characteristics that differentiate it from surrounding polypedons
• Approaches in the study of soil

  • Pedological approach (the study of soils with principal interest on characterization and differentiation of their properties and with only minor emphasis on their practical use)
  • Edaphological approach (the study of soils with emphasis on their practical use, particularly the relationship of soil properties to plant growth)
• Fields of study in Soil Science

  • Soil Fertility (quality of a soil to provide optimum level of nutrients for plant growth)
  • Soil Physics (characteristics, processes, or reactions of a soil caused by physical forces)
  • Soil Chemistry (interactions of solid, liquid, and gaseous phases or components of soil)
  • Soil Microbiology (soil biochemical reaction carried out primarily by microorganisms)
  • Soil Conservation and Management (protection of soil against physical loss by erosion or chemical deterioration; totality of all tillage operations, cropping practices, fertilization, liming etc. conducted on a soil for crop production)
  • Soil Survey and Classification (deals with the structural characteristics, mode of origin, and systematic arrangement of soils)
  • Soil mineralogy (structural chemistry of the solid components of soil)
  • Land use (deals with the allocation of lands for general or broad purposes such as agriculture, forestry, settlement and military reservations)
• Different components of soil


  • Soil solids (Mineral matter, 45%
  • Organic matter, 5%)
  • Pore spaces (Air, 20-30%
  • Water, 20-30%)
• Mineral matter


  Comes from the weathering of rocks and minerals, composed of sand, silt, and clay particles, major source of all nutrient elements (except nitrogen, carbon, and oxygen) needed for plant growth
• Organic matter


  Derived from the decayed and decaying remains of plants and animals intimately mixed with the mineral matter, the chief natural source of nitrogen, also contributes phosphorus, sulfur, and micronutrients but in smaller amounts compared to mineral sources, enables the soil to store cations, promotes the formation and stabilization of aggregates, giving the soil greater permeability and porosity
• Organic soils


  Contain more than 20% organic matter, most soils contain less than 20% organic matter, thus classified as mineral soils
• Air in pore spaces

  Occupies the pore spaces of the soil, composed largely of elemental nitrogen (78%), oxygen (20%), carbon dioxide (0.5%), and traces of other gases, provides oxygen for respiration of plant roots
• Water in pore spaces

  Also occupies the pore spaces of the soil, contains dissolved gases and salts or compounds as well as dissociated ions of various nutrient elements, carries the nutrients to the proximity of roots where they can be absorbed, also serves as a weathering agent of minerals to constantly renew the nutrient supply in the soil, needed by the plant in large amounts for their metabolic functions
• In paddy soils such as in lowland rice fields, the pore spaces are nearly filled with water
• The dissolved CO2 in soil water forms carbonic acid which is a solvent that can release nutrients from minerals
• 3 General classification of rocks

  • Igneous rocks
  • Sedimentary rocks
  • Metamorphic rocks
• Igneous rocks


  • Original rocks formed from the solidification of molten magma at temperatures of 900 °C to 1600 °C within or outside the earth's crust, the most common soil-forming igneous rocks are granite, diorite, gabbro, ryolite, andesite, basalt, and obsidian, granite and diorite are lighter in color due to the high proportion of light-colored minerals such as feldspar and muscovite, gabbro is dark in color due to the predominance of dark-colored minerals such as biotite, hornblende, and augite, granite is acidic while basalt is basic
• Sedimentary rocks


  • Formed from other existing rocks through transport (by water) and fragmentation, concentrated near the crust's surface, thus exert a dominant effect on many soils of the world, the most important soil-forming sedimentary rocks are limestone, dolomite, sandstone, and shale
• Limestone


  Made up chiefly of the carbonate mineral, calcite or CaCO3
• Dolomite


  A carbonated material composed of calcium magnesium carbonate CaMg(CO3)2
• Sandstone


  Composed of cemented sand grains
• Shale


  Made up of fine particles of clay which become consolidated after deposition in bodies of water
• Metamorphic rocks


  • Formed from other existing rocks through the processes or remelting and recrystallization under very high temperature and pressure, the most typical soil-forming metamorphic rocks are gneiss, schist, quartzite, slate, and marble
• Marble


  The metamorphic form of limestone
• 2 Modes of rock formation

  • Extrusive (volcanic) formation (happens when the magma is ejected out of the earth's crust through volcanic eruption and solidifies on the surface)
  • Intrusive (plutonic) formation (happens when magma solidifies within the earth's crust)
• Most abundant elements in the earth's crust

  • Oxygen (46.6%)
  • Silicon (27.7%)
  • Aluminum (8.13%)
  • Iron (5.0%)
  • Magnesium (2.09%)
  • Calcium (3.63%)
  • Sodium (2.83%)
  • Potassium (8.13%)
• Mineral


  A naturally occurring inorganic substance with more or less definite chemical composition and specific physical properties, serves as building block of rocks
• 2 General classifications of minerals

  • Primary minerals (persist in the soil in their original state due to high resistance to decomposition, chief sources of the sand and silt fractions of soils)
  • Secondary minerals (arise from the chemical breakdown of the least resistant primary minerals, contributes the clay fraction to the soil)
• Some of the most important primary minerals

  • Quartz, SiO2 (microline)
  • Orthoclase, K AISI 30s (anorthite)
  • Gypsum
  • Limonite
  • Hematite
  • Gibbsite
  • Kaolinite
  • Montmorillonite
  • Illite
  • Sodium-plagioclase, Na AISI30 (albite)
  • Calcium-plagioclase, Ca Al₂Si208
  • Muscovite, K Al Si3010 (OH)2 (white mica)
  • Biotite (black mica)
  • Hornblende
  • Augite
  • Apatite
• The more commonly occurring secondary minerals

  • Calcite
  • Dolomite
• Physical weathering


  Breaking up or rocks and minerals into smaller pieces without any drastic alteration of their chemical composition, examples include unloading and freezing water in narrow cracks
• Processes involved in chemical weathering

  • Hydrolysis (the reaction of water with the mineral resulting in the destruction of the original chemical structure and the formation of an acid and base)
  • Hydration (involves the reaction of water with the mineral but not leading to the destruction of the chemical structure, the result is a rigid attachment or association with a water molecule)
  • Oxidation (changes the iron in the mineral from ferrous (reduced) to ferric (oxidized) form, the reduction in size and increase in valency of Fe results in the weakening and instability of the mineral's structure)
  • Carbonation (the reaction of carbonic acid (H2CO3) with a mineral to produce a more soluble product)
  • Solution (the dissolution of minerals through the solvent action of H2CO3 or H+ ions which results into the separation or dissociation of component cations)
• Stages of soil formation

  1. Physical weathering: reducing the size of the parent material (rocks and minerals) particles

  2. Rearranging the mineral particles

  3. Adding of organic matter

  4. Chemical weathering: changing the composition and structure of minerals including clay formation

  5. Formation of soil horizons
• 5 factors of soil formation (CLORPT)

  • Climate
  • Living organism
  • Relief or Topography
  • Parent material
  • Time
• Climate


  Affects the amount of leaching that takes place in the soil and the speed with which soil horizons develop, temperature (as the mean annual temperature increases, the weathering of rocks and minerals in the soil will be faster, for every 10 °C rise in temperature, the rate of biochemical reactions doubles, tropical soils will weather faster because of faster chemical reactions which can occur throughout the year), rainfall (areas with more rainfall will have greater weathering and greater leaching, leaching occurs when water moves through the soil and removes the soluble constituents, soluble silica and bases are leached out to give rise to soils high in kaolinite and sesquioxides, water is an agent of erosion and deposition of soil materials, at optimum levels, water facilitates decomposition of organic matter, cool and wet areas will have more leaching than hot and wet areas, in areas with low rainfall, silica and basic cations like Ca, Mg, Na and K accumulate and form monmorillonitic soils, hot and dry areas have slower soil formation resulting to a shallow solum)
• Living organism


  Affects soil formation through their effect on the amount and kind of organic material decomposed and accumulated, vegetation affects the thickness and color of the surface horizons, bioturbation: the mixing of the soil by organisms, soils of forested areas (dipterocarps) have thin surface horizon, leached, light colored zone below the surface and an accumulation zone that is often brown or red in color, soils of grassland areas have thick, black surface horizon
• Relief or Topography


  The shape or contour of the land surface affects the movement and accumulation of water which can modify the effect of the climate factor, steep slopes: less water to soak the soil but more runoff to erode the surface thus preventing the formation of a thicker soil profile, flat lowlands: develop thicker solum due to accumulation of moisture and deposition of soil materials from the uplands, soils at the summit and shoulder will develop horizons the fastest, soils on the backslope will develop slower, soils at the footslope will collect sediments from the upslope
• Parent material


  The partly weathered mineral or organic debris from which true soil (solum) is formed, the starting point of soil formation
• Soils of forested areas (dipterocarps)

  • Thin surface horizon
  • Leached, light colored zone below the surface
  • Accumulation zone that is often brown or red in color
• Soils of grassland areas


  • Thick, black surface horizon