Soil Chemistry - Sam Houston State University

Soil Chemistry - Sam Houston State University

SOIL CHEMISTRY By C. Kohn, Waterford, WI 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 earths 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

Reds won last night, beating the Oxens by a narrow score of one electron The best-known Redox reaction is the formation of rust from iron and oxygen. 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

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 earths crust Examples: granite, quartz

Sedimentary formed when loose materials like mud or sand become cemented by chemical agents and/or pressure Examples: limestone; sandstone Metamorphic if igneous and sedimentary rocks undergo great changes in heat and pressure, they can become new kinds of rock Examples: marble (formerly sandstone that is heated to extreme temperatures under extraordinary pressure)

PARENT MATERIAL OF SOILS Soils can come from two sources 1. Residual Soils: From currently existing bedrock (less common) 2. Transported Soils: From another area(more common) 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 E.g. glacial ice carried parent materials from northern

Canada and deposited them in the Midwest (its no accident that much of the worlds 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.) SOIL CHEMISTRY SOIL THE INTERFACE OF LIFE If texture is the most important physical property of soil, pH is the most important chemical property of soil pH is the most important determinant of the

growing capabilities of a soil sample pH indicates the acidity or alkalinity of soil The pH scale goes from 0-14, with 7 being neutral The further from 7 a pH sample is, the more reactive it is; below 7 is acidic and above 7 is basic, or alkaline For example, the acid in your stomach has a pH of 2; it could dissolve a metal razor blade because it is extremely acidic Lye has a pH of 13 and would dissolve the skin off your finger; it is extremely alkaline. EFFECTS OF PH Soil pH (or soil reaction) is most responsible for determining the availability of nutrients and minerals.

14 of 17 essential plant nutrients are obtained from the soil pH determines how available these nutrients are These nutrients are most available in a slightly acidic soil as compared to neutral or alkaline (basic) A pH between 6 and 7 is usually most ideal for plant growth (although some plants have adapted for different pHs) PH SIMPLIFIED The pH scale is based on the number of hydrogen ions. A hydrogen ion is a cation it has a positive charge

H+ The pH number is based on the hydrogen concentration When hydrogen cations are completely balanced by hydroxide (OH-) anions, you have a neutral, unreactive subtance (such as water) The more imbalanced the solution, the more reactive it is. SOIL PH Soil pH results from the interaction of soil minerals, ions, and cation exchange

A soil with a high, alkaline pH results due to the reaction of water with calcium, magnesium, and sodium These minerals steal hydrogen ions, leaving hydroxide (OH-) ions behind that make the soil alkaline. These kind of soils are very common in areas with large amounts of limestone (such as Wisconsin) This situation can also result around roadways, particularly in spring A soil with a low, acidic pH is most often caused by acidic rain, carbon dioxide from decomposition, and acidic fertilizers

EFFECT OF PH ON PLANTS Again, most plants do well at a pH range of 6.0-7.0 Blueberries and evergreens are among the few that do well in acidic soils Alfalfa is one of only a few that does well in basic soils The actual number of hydrogen cations does not actually affect plant growth that much. pH itself is usually does not have a direct effect

Rather, other soil conditions can lessen or increase the impact of soil pH on plant growth. They are Nutrient availability Aluminum toxicity Soil microbes PH AND NUTRIENTS A good example of soil pH and its impact on nutrients is phosphorus Phosphorus is one of three key nutrients for plant growth (NPK), and is a main ingredient of ATP

When soil pH falls below 5.8, phoshphorus reacts with iron to produce an insoluble iron compound. Plants obtain their nutrients through absorbing groundwater If a nutrient is insoluble, it is unobtainable by plants When pH rises above 6.0, the reaction reverses and phosphorus dissociates from iron At a high pH, phosphorus undergoes a similar reaction with calcium Phosphorus may be present in the soil and may provide a good test result, but will be unavailable because of the pH PH AND TOXICITY If the soil pH drops below 5.5, aluminum

begins to dissociate from the soil particles, especially in soils high in clay. Normally aluminum is held by soil particles. In clay-heavy soils, the clay and humus particles form a structure called a micelle. The micelle structure has a negative charge This negative charge attracts positively charged elements including metals such as aluminum as well as other elements such as hydrogen cations and sodium This can help negate the impacts of road salt and other environmental assaults. CATION EXCHANGE CAPACITY The ability of a soil to negate these oftenharmful assaults on the environment is called

the soil cation exchange capacity (or Soil CEC) cation exchange capacity (CEC): ability of a soil particle to attract positively charged ions If the pH drops too low, the strength of the chemical charge of the micelle is negated by the surrounding environmental conditions Aluminum becomes soluble; in this case, the solubility is bad because it can form a metal toxicity when absorbed by the roots of a plant Iron and manganese can have similar effects MICELLES AND CEC micelles: clay and humus form a complex

together known as the clay-humus micelle. A WORD ON SOIL CEC CEC can be thought of as the ability of soil to hold on to nutrients. Clay-based soils have a very high CEC Sand has a very low CEC The more cation exchange a soil has the more likely the soil will have a higher fertility level. Could CEC be too high? Why do you suppose manure pits must be clay-lined? PH AND SOIL ORGANISMS As we discussed last week, the biodiversity of

the soil is the most important biological property of the soil Again, soil pH is the most important chemical property and texture is the most important physical property Soil organisms grow best in near-neutral soil Acidic soil mostly inhibits the growth of organisms at the base of a soil food chain, particularly microbes and earthworms This reduces their crucial activities, including nitrogen fixation and decomposition SOIL ACIDIFICATION Soil Acidification is a natural process that is a part of all landscapes

Clearing land and replacing native vegetation with crops and pasture accelerates acidification The addition of lime can raise the pH to more productive levels The addition of sulfur can lower pH is the soil is too alkaline; ashes can also work

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