soil horizons

Question 1

organic matter

Answer 1

little layer of plants undecomposed form

Question 2

surface soil

Answer 2

most organic matter accumulation and soil life. layer elevates iron, clay, aluminium, organic compounds and other soluble consistituents

Question 3

subsoil

Answer 3

accumulates iron, clay, aluminium and organic compounds. process referred to as illuviation

Question 4

parent rock

Answer 4

large unbroken rocks. this layer accumulates the more soluble compounds

Question 5

bedrock

Answer 5

partially weathered bedrock. unlike above layers, bedrock largely comprise continuous masses of hard rock that cannot be excavated by hand.

Question 6

soil organic matter contents

Answer 6

• prairie grassland soil contains 5-6% SOM (by weight)
• sandy desert soil may have <1% of SOM,
• Organic soils contain >30% SOM.

Question 7

primary and secondary SOM components

Answer 7

• 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,

Question 8

O horizon

Answer 8

can be classified into L, F and H layers at the surface

- depends on the different degrees of decomposition of organic matter

Question 9

L layer

Answer 9

“litter layer” has recognisable plant and soil animal remains,

Question 10

F layer

Answer 10

“fermentation layer” has a mixture of organic matter in different stages of decomposition,

Question 11

H layer

Answer 11

• H layer “humose layer” has largely humified material with little or no plant structure visible.

Question 12

humid acid

Answer 12

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

Question 13

what 3 things are humic substances divided into

Answer 13

humic acids
fulvic acids
humin

Question 14

humic acids

Answer 14

insoluble in water under acidic conditions (pH < 2) but is soluble at higher pH values.

Question 15

fulvic acids

Answer 15

soluble in water under all pH conditions. They remain in solution after removal of humic acid by acidification.

Question 16

humin

Answer 16

not soluble in water at any pH value

Question 17

humic acid/fulvic acid ratio

Answer 17

The humic acid/fulvic acid ratio usually, but not always decreases with increasing depth.

Question 18

humus holds what percentage of its own weight in water

Answer 18

• Humus holds 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

Question 19

flocculation

Answer 19

first step in aggregate formation
• RCOOH + OH- = RCOO- + H20
• Floculation of humic substances can also occur with the addition of electrolytes.
• Cations:
- trivalent > divalent > monovalent
• Anions:
- sulphate > nitrate > chloride

Question 20

aerobic conditions

Answer 20

oxygen is available. Dry soils have adequate oxygen supply for aerobic respiration.

Question 21

anaerobic conditions

Answer 21

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.

Question 22

absorption of metal cations to soil

Answer 22

• Metal cations can adsorb to both the humic substances and clay constituents of soil “ion-exchange”

Question 23

cation exchange capacity of soil

Answer 23

milliequivalent = molecular weight of cation/valency x1000§

Question 24

cation exchange capacities (at pH 7) that are typical of a variety of soils

Answer 24

• 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

Question 25

sheet silicates

Answer 25

• 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

Question 26

anion exchange capacity

Answer 26

• Clay also has sites of positive charge under acid conditions and contributes to the soils anion exchange capacity (AEC).
• 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

Question 27

isomorphous substitution

Answer 27

substitution of one element for another in a mineral without a significant change in the crystal structure
• Elements that can substitute for each other usually have similar ionic radii and valence state.
• Fe and Mg commonly substitute for each other, as do Na and K

Question 28

source of H+

Answer 28

• 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.

Question 29

soil pH influences soil chemistry

Answer 29

• availability of nutrients and toxic substances,
• activities and nature of microbial populations,
• solubility of heavy metals,
• and activities of certain pesticides.

Question 30

soils tend to become acidic as a result of

Answer 30

• rainwater leaching away basic ions (calcium, magnesium, potassium and sodium);
• carbon dioxide from decomposing organic matter and root respiration dissolving in soil water to form a weak organic acid;
• uptake of positive ions by plant roots and the resulting release of H+ by the root to balance internal charge;
• 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.

Question 31

the munsell colour system

Answer 31

• 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.

Question 32

hue - colour

Answer 32

• 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.

Question 33

value - lightness

Answer 33

• varies vertically from black (value 0) at the bottom, to white (value 10) at the top.
• Neutral greys lie along the vertical axis between black and white.

Question 34

chroma - colour purity or colour-fullness

Answer 34

• measured radially from the center of each slice, represents the “purity” of a color
• lower chroma being less pure (more washed out, as in pastels).
• Vivid soil colours are in the range of approximately 8

Question 35

specifying a colour

Answer 35

List the three numbers for hue, value, and chroma in that order.
e.g. a purple of medium lightness and fairly saturated would be 5P 5/10
· 5P meaning the colour in the middle of the purple hue band,
· 5/ meaning medium value (lightness)
· a chroma of 10

Question 36

colour of soil

Answer 36

• differentiate soil horizons
• estimate the amount of soil organic matter
• determine soil drainage
• Common pages in Munsell Colour book for soil are the following hues:
- 10R - dark red –orange
- 5YR - strong brown
- 10YR - medium brown
- 2.5Y - light olive brown
• R –represents red soil and arises from iron oxides
• As the iron oxide becomes less abundant the soil becomes more yellow
• Strongly flooded soils (no free oxygen) are dark blue, dark greenish, dull bluish grey or black

Question 37

• Scaling the standard patterns to the same peak heights can offer some estimation of the mineral composition

Answer 37

• cautious about the quantitative analysis since the standards in those programs were recorded on specific X-ray diffractometers, which may have different configurations and/or operating conditions from the system with which the unknown sample is recorded

Question 38

forensic characteristics of soil

Answer 38

• Soil also includes glass, paint, chips, asphalt, brick fragments, cinders  uniqueness
• Value of soil evidence rests on transferability between scene and crime i.e. comparatively, however expert geologists could determine crime scene directly.
• Gross appearance used for identification e.g. Colour (darker when wet) therefore performed on dried samples (1, 100 distinguishable soil colours
• pH
• Particle size (density gradient tube – 6 – 10 layers in each tube, 6-10mm width x 25-40cm length. Particles remain suspended in liquid of same density as particle. Not definitive as soils from different locations can give similar results.
- CBr4 d = 2.96 g/mL C2H5OH d = 0.789g/mL
• Low power microscopy (plants, animals, pollen)
• High power microscopy especially polarised light microscope (characterises (using RI pleochromism) minerals (2,200 known but only 40 are common) + rocks
• Quantitative X – ray diffraction (new)
• Chemical composition? AA, ICP, flame photometer
- NH4+  Nesslers Reagent
- NO3-  Ion chromatography
- Ce-  AgNO3

Question 39

collection and preservation of evidence

Answer 39

• Collect samples at intervals within 100 yard radius of crime scene and at crime site. Also, all alibi locations  Usually only top-surface soil.
• Soil on suspect clothing – each object should be individually wrapped  lab
• Cars collect + build up layers of soil under car body – Unique layering  could link car to crime site if lumps dislodged