soil horizon Flashcards
(34 cards)
soil horizons
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composition of soil organic matter (SOM)
Soils vary greatly in their organic matter contents:
- prairie grassland soil contains 5-6% SOM (by weight)
- sandy desert soil may have <1% of SOM,
- Organic soils contain >30% SOM.
SOM includes primary components
(non-humic substances) from plant and animal residues. E.g cellulose,l.ignin, lipids, proteins, carbohydrates
• easily decomposed by microorganisms and they persist in soil for a brief time (e.g. several months or years). They make about 20-30% of total SOM.
SOM also includes secondary compounds
- with broken down organic structures e.g. humic substances,
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Soil hummus
O Horizon in soils can be classified into L, F and H layers at the surface
depends on the different degrees of decomposition of organic matter
L layer “litter layer” has recognisable plant and soil animal remains,
F layer “fermentation layer” has a mixture of organic matter in different stages of decomposition,
H layer “humose layer” has largely humified material with little or no plant structure visible.
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Structure of humic acid
- Humic acid is a principal component of the major organic constituents of soil (humus), peat, coal. Produced by biodegradation of dead organic matter e.g. from plants.
- Complex mixture of many different acids containing carboxyl and phenol groups
- Behaves as a dibasic cid with a pK1 value around 4 for protonation of carboxyl groups and around 8 for protonation of phenolate groups.
- The carboxylate and phenolate groups complex with ions such as Mg2+, Ca2+, Fe2+ and Fe3+.
- Many humic acids have the metal ion complexed by a number of carboxyl and phenolic groups
Humic substances can be divided into three main fractions
- Humic acids - insoluble in water under acidic conditions (pH < 2) but is soluble at higher pH values.
- Fulvic acids - soluble in water under all pH conditions. They remains in solution after removal of humic acid by acidification.
- Humin - not soluble in water at any pH value
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How much does humus hold of its own weight in water
- 80-90% of its own weight in water – helps soil withstand drought, buffers soil against excess acid or alkali. Dark colour of humus helps warm up a cold soil in spring
Flocculation is the first step in aggregate formation
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RCOOH + OH- = RCOO- + H20
Floculation of humic substances can also occur with the addition of electrolytes.
Cations:
• trivalent > divalent > monovalent
Anions:
- sulphate > nitrate > chloride
under aerobic conditions
• -oxygen is available. Dry soils have adequate oxygen supply for aerobic respiration.
Under anaerobic conditions
- oxygen is not available agents, such as Fe3+, Mn4+, NO3- or SO42-, are used. Wet soils are oxygen limited and anaerobic (sometimes called anoxic) respiration will take place.
Absorption of metal cations to soil
- Metal cations can adsorb to both the humic substances and clay constituents of soil “ion-exchange” e.g. look at ppt
Units
- The units are centimoles of ions per kilogram of exchanger (soil) cmolec/kg
- Other units are: Milliequivalents per 100g of dry soil
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Cation exchange capacities (at pH 7) that are typical of a variety of soils
- Humus: contributes over 50% of the total CEC of a soil
- Histosol - mostly organic material e.g. peat
- Vermiculite and smectite are high surface area clays
- Vertisols - mostly clay with medium surface areas e.g. montmorillonite
- Micas and chlorites low surface area
- Kaolinite very low surface area
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Sheet silicates
- rings containing 6 silica tetrahedra with 3 bridging oxygen atoms in a flat layer and one non bridging oxygen atom out of plane of paper
- Silicate layers usually stacked AB-BA-AB-BA
- Exception is kaolin clays – which are stacked AB-AB-AB-AB
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- Layers cemented together at A-A faces by either
- i) Brucite - Mg2+ and HO- ions, or
- ii-) Hydrargillite - Al3+ and HO- ions.
- Way B-B faces held together is responsible for difference in physical properties of the different layer silicates.
SHEET SILICATES - Kaolin (china clay)
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Anion exchange capacity (AEC)
- Clay also has sites of positive charge under acid conditions and contributes to the soils anion exchange capacity (AEC).
- Another example is Gibbsite:
look at ppt - AEC is very low (2-5cmolc/kg) - Anions in soil are generally in soil water and are easily washed away
- Exception is phosphate anion which forms an insoluble compound with aluminium oxide or iron oxide sites or Al-O sites on clay.
- In general AEC increases as soil pH decreases
Structure of clays
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isomorphous substitution-
- Isomorphous substitution is the substitution of one element for another in a mineral without a significant change in the crystal structure
- Elements that can be substitute for eachother usually have similar ionic radii and valence state. (goldschmidts rules of ionic substitution)
- Fe and Mg commonly substitute for eachother, as fo Na and K
- In mineral formulae elements that can be substituted for each other are often placed in parentheses for example olivine in which fe and mg can resided on either the m1 of m2 sites
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Coupled substitutions
- Al can also replace Si in silica tetrahedra within silicate structures, although this substitution required charge balancing due to the different valences of these elements. Isomorphous substitution allows mineral composition to vary and forms solid solutions
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- Exchangeable hydrogen is the principal source of H+ until the pH of the soil goes below 6.
- Below pH6, exchangeable aluminium becomes the source of hydrogen ions, due to the dissociation of Al from clay minerals.
Soil pH influences soil chemistry:
- availability of nutrients and toxic substances,
- activities and nature of microbial populations,
- solubility of heavy metals,
- and activities of certain pesticides.
Soils tend to become acidic as a result of:
(1) rainwater leaching away basic ions (calcium, magnesium, potassium and sodium);
(2) carbon dioxide from decomposing organic matter and root respiration dissolving in soil water to form a weak organic acid;
(3) uptake of positive ions by plant roots and the resulting release of H+ by the root to balance internal charge;
(4) formation of strong organic and inorganic acids, such as nitric and sulfuric acid, from decaying organic matter and oxidation of ammonium and sulfur fertilizers. Strongly acid soils are usually the result of the action of these strong organic and inorganic acids.
Munsell color system
- a circle of hues at value 5 and chroma 6;
- the neutral values from 0 to 10;
- and the chromas of purple-blue (5PB) at value 5.
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Hue ——–Colour
Five principal hues: Red, Yellow, Green, Blue, and Purple,
5 intermediate hues (e.g., YR) halfway between adjacent principal hues.
Each of these 10 steps, is then broken into 10 sub-steps, to give 100 hues
In practice, colour charts have 40 hues, in increments of 2.5, progressing as for example 10R to 2.5YR.
Two colours of equal value and chroma, on opposite sides of a hue circle, are complimentary colours, and mix additively to the neutral grey of the same value.
