Soil System & Composition

Soil as a Complex System

• There's exchange of matter and energy between system components and their environment
• The system has a large variety of interacting elements
• Interactions between elements create high heterogeneity in space and time
• Adequately predicting outcomes requires knowing initial conditions perfectly
• The system co-evolves through interactions and feedback at different scales
• The system's properties emerge from relationships among its elements
• Interactions can be from macroscopic to microscopic scales
• Unexpected behaviors arise of component interactions and not reproducible when considering elements separately

Soil Composition

• Mineral solids make up approximately 45% of soil.
• Pore space, which is filled with air and water make up about 40-60%
• Organic matter is about 5% of soil.
• Soils vary in space, along a catena, over time, and across multiple scales.
• A catena is a series of soils on a slope.

Soils Along a Catena

• Soil catena refers to the repeating pattern of soil types across a landscape
• Repeating pattern is due to combined effects of topography, parent material, climate, and other soil-forming factor

Soil Variability Over Time

• Annually between 1-10 mm of soil is formed
• The rate varies and it's dependent on other soil forming facts
• Soil erosion rates are much higher than soil formation rates

Soil Variability: Pore Distribution

• Soil pore distribution varies based on grazing management practices like no grazing(NG), grazing from May to September (CG), grazing in May and July (G57), grazing in June and August (G68), and grazing in July and September (G79).

Basic Classification: Rocks and the Rock Cycle

• Igneous rocks are rocks that cool from magma
• Sedimentary rocks are made from the build up of sediment
• Metamorphic rocks heated and pressed rocks, and are fundamentally changed
• Obsidian is glassy rock made from cooled lava
• Pumice is a light, pale colored volcanic rock which has lots of holes
• Basalt is made from solidified lava
• Granite is made from slowly cooled magma inside the earth
• Sandstone is made from worn down rocks into and cemented sand sized particles
• Shale is made from layers of pressed mud
• Fossil Limestone is made from pressed sea creature skeletons
• Conglomerate is made from cemented smoothed pebbles
• Quartzite is made when sandstone is heated and pressed inside the earth
• Gneiss is made when igneous or sedimentary rocks are heated and pressed
• Schist is made when shale is heated and pressed
• Marble is made when limestone is heated and pressed

Soils as Part of the Rock Cycle

• Sedimentary rocks undergo metamorphism to form metamorphic rocks
• Metamorphic rocks can experience anatexis to become magma.
• Magma undergoes crystallization to form igneous rocks.
• Igneous rocks undergo weathering and pedogenesis to form soils.
• Soils experience deposition and transport to become sediments.
• Sediments undergo diagenesis to become sedimentary rocks.
• Biogenic material can be added to sediments.

Exogenic and Endogenic Forces

• Exogenic processes are driven by solar radiation
  • Drivers: wind, water, glaciers
  • Processes: erosion, transport, deposition, weathering
• Endogenic processes driven by Earth's energy
  • earthquakes
  • volcanic eruptions
  • plate tectonics
• exogenic processes break down and transport parent material
• endogenic processes expose new parent material

Rock Type and Soil Properties

• Basalt
  • Primary minerals: Pyroxene, Plagioclase Feldspar, Olivine, higher in Fe and Mg
  • Texture: Fine-grained, darker (black or dark grey)
  • Fertility: More fertile and less acidic, soil color: Reddish or Brownish
  • Water-Holding Capacity: Higher
  • Suitability: diverse assortment of plants
• Granite
  • Primary minerals: Quartz, Alkali Feldspar, Plagioclase Feldspar, Higher in Silica (SiO2)
  • Texture: Coarse-grained, lighter (pink to grey)
  • Fertility: Less fertile and more acidic, soil color: Variable
  • Water-Holding Capacity: Lower
  • Suitability: For plants adapted to acidic conditions

Chemical Composition of Earth's Crust

• Most abundant elements in continental crust:
  • Oxygen (O): 45.20%
  • Silicon (Si): 27.20%
  • Aluminum (Al): 8.00%
  • Iron (Fe): 5.80%
  • Calcium (Ca): 5.06%
  • Magnesium (Mg): 2.77%
  • Sodium (Na): 2.32%
  • Potassium (K): 1.68%
• Macro-nutrients: N, P, K, Ca, Mg, S
• Micro-nutrients: Cl, Fe, Mn, B, Zn, Cu, Ni, Mo
• Useful elements: Si, Na, Al, Co

Basic Terminology: Rocks, Minerals, and Crystals

• Rocks build Earth's crust and consist of minerals in characteristic amounts/arrangements
• Minerals are solid inorganic materials with regular atomic arrangements, forming crystals and have a characteristic chemical composition such as CaCO3
• Freely growing minerals form shapes with regular planes, exhibiting properties like hardness, color, shape, and cleavage
• Igneous rocks form by cooling and crystallization of molten magma, with magma composition determining mineral development
• Sedimentary rocks form from weathered rocks transported and accumulated by wind/water in sedimentary basins
• Metamorphic rocks form when Ingenuous/sedimentary rocks undergo high pressure/temperature, causing mineral recrystallization and changed spatial arrangements from mechanical forces

Weathering Processes Overview

• Physical weathering (mechanical processes): Disaggregation and fragmentation of rocks without chemical alteration (fracturing, cryoclastism, aloclasticism, thermoclastism)
• Chemical weathering: Chemical reactions of minerals, involves water and air with chemical alteration of mineral (carbonation, oxidation-reduction, hydration, dissolution, hydrolysis, solution, leaching, corrosion)
• Biological weathering (physical-chemical action): Processes involving living things (bioclastism); no chemical alteration of mineral

Physical Weathering Mechanisms

• Frost Weathering (Cryoclasty/Cryoclastism): Expanding force. Water volume increases by ~9% upon freezing, pressure increase to 210 MPa
• Salt Crystallization Weathering (Haloclasty/Haloclastism): Expanding Force. Saline water seeps into rock cracks and water evaporates when heated, resulting in deposition of salt crystals, increasing pressure and eventually shattering the surrounding rock
• Insolation weathering thermal stress/temperature change: Consequence of expansion and/or contraction of rock at range of temperatures (e.g., Sahara Desert reaches 42 C/24h)

Chemical Weathering Mechanisms (Detailed)

• Dissolution: Water molecules separate/break ionic bonds of soluble minerals
• Carbonation: Dissolution when CO2 dissolves in water, forming carbonic acid (H2CO3)
• Hydrolysis/Protolysis: Water (hydrolysis) and acids (Protolysis) separate/break ionic bonds of soluble minerals
• Hydration: Water molecule forms shell of water on some compound surface
• Oxidation-Reduction (redox):
  • Oxidation: involves loss of electrons and an increase in oxidation state (OS)
  • Reduction: involves gain of electrons and decrease in oxidation state (OS)
• Complexation (Chelation): Formation of complex compound between metal ion and agents (e.g., humic and fulvic acids) and forming chelation of metal ions (complexes), mobility and availability further enhanced.
• Isomorphous substitution: One element is replaced by similar element, leading to the mineral composition properties changes.

Biological Weathering

• Biophysical Weathering is mechanical changes to rocks/minerals from living activities
• Biochemical Weathering is chemical changes to rocks/minerals driven by living metabolism

Factors in Weathering

• Physical weathering paves the way for chemical weathering processes.
• Cracks formed due to pressure release can act as pathways for water/oxygen to mineral surfaces
• The most important elements/compounds in chemical weathering are oxygen and water
• Increasing temperature increases chemical weathering rates
• Primary minerals decompose into their elements, which are released into soil solution to form secondary (pedogenic) minerals

Mineral Classification

• Pure Elements (e.g., Gold, Diamond, Platinum)
• Sulfides (e.g., Pyrite, Galena, Chalcopyrite)
• Halogenides (e.g., Fluoride, Chloride, Bromide, Iodide)
• Oxides and Hydroxides (e.g., Quartz, Goethite, Hematite, Gibbsite)
• Nitrates, Carbonates, Borates (e.g., Potassium nitrate, Calcium carbonate)
• Sulfates (e.g., Anhydrite, Gypsum)
• Chrom-, Molybd-, Wolframates (e.g., Wolframite)
• Phosphates, Arsenates, Vanadates (e.g., Apatite)
• Silicates (> 90% of Earth's crust): Olivine, Beryll, Pyroxene, Feldspar, Mica
• Organic Matter: Amber, Cole
• Note: Minerals important in soils are in red color

Primary and Secondary Minerals

• Primary minerals have not been altered chemically since initially formed
• Secondary minerals are recrystallized products of the chemical breakdown/alteration of primary minerals
• Chemical weathering involves the transformation of minerals to solutes (ions) and solid residues, which are clay minerals
• Weathering rates vary with temperature, precipitation, and mineral type
• Although a slow process, secondary minerals formed from weathering have a significant impact on soil properties, including the fate of nutrients and pollutants