Soil Chemistry

SOIL CHEMISTRY BASIC CHEMISTRY  A chemical element is the building block of all matter.  Examples: nitrogen, oxygen, carbon, hydrogen The smallest indivisible unit of matter is an atom  Atomic structure is universal among all atoms – atoms are comprised of…  a negatively charged electron that orbit the inner nucleus  An inner nucleus consisting of  a positively charged proton  a neutrally charged neutron   When atoms combine, they form molecules MOLECULES, COMPOUNDS, ETC.    For example, water is a molecule consisting of two hydrogen atoms and one oxygen atom A collection of similar molecules is a compound Pure solid compounds found in the earth’s crust are called minerals   For example, the most abundant mineral in soil is quartz, a mineral compound consisting of silicon dioxide As you can see, quartz is a mineral compound because it is made of repeating units of the SiO2 molecule MINERALS, ROCKS, AND ORGANIC COMPOUNDS Minerals are pure compounds  Pure solid compounds are rare; usually minerals are found in a mixture  Rocks are mixtures of minerals   E.g. granite consists of feldspar, quarts, and others Minerals are inorganic  Organic compounds must contain carbon and hydrogen and make up the chemical structure of living tissues  Organic compounds are not necessarily alive; they may have been a part of something living at one time.  IONS A normal atom or molecule has an equal number of negative electrons and positive protons.  Imbalances can occur; when this does, the results atom/molecule is called an ion  For example, if we take salt (NaCl) and dissolve it in water, the molecule will break apart  The sodium ions will be short one positive charge, giving the ion a positive charge (Na+)  The chloride ions will have a negative charge (Cl-)   An ion is classified by its charge  Positive ions are called cations   Having a cat was a positive experience Negative ions are called anions  Smelling the onion was a negative experience CHEMICAL AND PHYSICAL REACTIONS  Chemical reactions involve the rearrangement of atoms to form new molecules and compounds   E.g. Na+ and Cl- combine to create salt, a molecule with entirely new properties Physical reactions cause the physical properties to change, but the chemical components remain the same. E.g. solid quartz must be broken down by weathering to create topsoil; the chemical components are the same  E.g. water freezes to become ice, but it is still H2O  OXIDATION-REDUCTION REACTIONS An oxidation occurs when an element loses an electron in a chemical reaction  A reduction occurs when an element gains an electron during a chemical reaction    The best-known Redox reaction is the formation of rust from iron and oxygen.   The Reds won last night, beating the Oxens by a narrow score of one electron 4 Fe + 3 O2  2Fe2O2 In soil, key oxidized elements include carbon dioxide, nitrogen, sulfur, and iron. ENERGY Energy is the capacity to do work; the greater the energy, the more work that can be done  Everything that happens in soil (growth, decomposition, aeration) cannot happen without energy  Energy has many forms – heat, light, motion  RULES OF ENERGY  Energy has some rules – 1. energy can change forms – radiant photons in light can be transformed into chemical energy in sugar through photosynthesis  2. matter tries to achieve the lowest possible energy state     e.g. water always runs downhill – it will never run uphill unless additional energy is added to its system 3. energy can neither be created nor destroyed, only changed These rules control all the physical and chemical processes of the soil. They are particularly important in regards to hydrological cycles and erosion. ORIGINS OF SOIL A SORT OF RENEWABLE RESOURCE Soil is constantly being made, but it is being made awfully slowly  Soil conservation is about ensuring that soil is not lost more quickly than it can be made  So where does soil come from?  Many people mistakenly assume that it came from the planet Zipdon in 3041 BC from the alien overlords controlled by Emperor Kronzike  This is simply not true  This is also a very good thing because if it did, we would have to rely on Emperor Kronzike to supply us with more soil each year (and he lives 213 million light years away!)  IF NOT FROM ZIPDON, THEN WHERE? So where does topsoil come from?  Picture a section of bare rock. Over time, freezing and thawing will cause the rock to break into chunks.  Physical weathering, wind, and rain will break apart each rock into smaller and smaller pieces  Over time, these fragments will become finer and finer and collect on top  Plants will further break up the rock with their roots, causing even more surface area on which weathering can act   Weathering and plants are the major agents responsible for forming soil from rock WEATHERING  Physical weathering refers to the effects of climatic factors on rock. As rocks heat during the day they expand; they then cool and shrink at night – this can cause fractures  When water goes into cracks and freezes, it expands, breaking apart rock  Windblown dust, running water, and rain also wear away at rock   Chemical weathering changes the chemical makeup of rock  E.g. some minerals dissolve in water; others react to form softer, more easily weathered compounds WEATHER (CONT.)  Biological weathering mostly occurs because of plants E.g. lichens form mild acids that react with the minerals to break down the rock; when the lichen dies, the dissolve rock and the organic debris add to topsoil  Plants will begin to grow in the crevices and their roots will widen cracks and fissures in the rock, enabling more chemical, physical, and biological weathering to occur  STEPS IN CREATING SOIL The formation of soil begins with rock in the earths crust  Rocks can be formed in 3 ways     Igneous – created by cooling and hardening of the molten mantle under the earth’s crust  Examples: granite, quartz  Examples: limestone; sandstone  Examples: marble (formerly sandstone that is heated to extreme temperatures under extraordinary pressure) Sedimentary – formed when loose materials like mud or sand become cemented by chemical agents and/or pressure Metamorphic – if igneous and sedimentary rocks undergo great changes in heat and pressure, they can become new kinds of rock PARENT MATERIAL OF SOILS  Soils can come from two sources –     Residual soils form very slowly because they must first be weathered from the existing rocks Transported soils grow from rock that has already been weathered and then carried somewhere else   1. Residual Soils: From currently existing bedrock (less common) 2. Transported Soils: From another area(more common) E.g. glacial ice carried parent materials from northern Canada and deposited them in the Midwest (it’s no accident that much of the world’s farming occurs in glacial regions) For one area to gain transported soils, another area has to lose. SOIL HORIZONS As soils develop and age, they form layers.  6 soil layers are identified by the USDA; 5 will be considered here – O, A, B, C, and D (or R)       O = organic layer; decayed plant and animal debris A = topsoil; mixture of mineral and organic matter B = subsoil; low in organic matter and high in minerals; area of most root growth C = parent material; weathered, fragmented rock D/R = Bedrock (limestone, sandstone, etc.)