Lecture 5 - Soils

Question 1

what are soils?

Answer 1

varies depending on the person considering it:
- civil engineer planning construction site = whatever unconsolidated material happens to be found at surface.
- miner = worthless material that is in the way and must be removed.
- farmer = medium that will nourish and supply water to crops.
soil scientists may also have different definitions depending on area of study.

Question 2

simplified composition of soil:

Answer 2

• 50% soil solids = mineral material and organic matter.
• 50% pore space = filled either with water, air, or a mixture of both.

Question 3

composition of soil - mineral:

Answer 3

• product of physical and chemical weathering of parent material (regolith).
• texture = size of individual soil particles.
• structure = formation of aggregates through the sticking together of textural separates.
  example: 21% clay, 16% sand, 66% silt.

Question 4

clay:

Answer 4

• larger specific surface area therefore greater opportunity especially for chemical processes.
• greater Cation Exchange Capacity (CEC) = ability to attract cations (+ charged particles) to surfaces.
• ability to retain K, Mg, Ca is important for plant roots and organisms (nutrients).

Question 5

examples of soil structure types:

Answer 5

• granular (soil aggregates).
• blocky (angular/subangular).
• platy.
• lenticular.
• wedge.
• prismatic.
• columnar.
• single grain (loose mineral/rock grains).
• massive (continuous, unconsolidated mass).

Question 6

composition of soil - air:

Answer 6

• proportionally exclusive with soil water, soil air occupies void spaces between textural separates and between aggregates (gravitational water, capillary water, hygroscopic water).
• soil air differs to atmospheric air in several respects (highlight variable, higher moisture content, higher CO2 content and lower O2 content)
• 2 processes = diffusion, mass flow.

Question 7

composition of soil - water:

Answer 7

• gravitational = fills all pore-spaces and leaves no room for oxygen, no gaseous exchange.
• capillary = water held with force of surface tension by soil particles, resistant to gravity force.
• hygroscopic = held tightly by surface tension, cannot be accessed by plant roots.

Question 8

composition of soil - organic matter:

Answer 8

• constituents of soil organic matter, ranging from total decomposition at the top, to fresh material (and incompletely decomposed) at bottom.
• humus = strictly humus substances + products of advanced decomposition of organic residue.
• pH determines organisms involved in decomposition (aerobic (pH>5.5) = bacteria and fungi and actinomycetes, aerobic (pH<5.5) = fungi, anaerobic = bacteria and actinomycetes).
• important for nitrogen cycle.

Question 9

pedogenic (soil forming) processes:

Answer 9

• Jenny’s (1961) soil forming equation: S = f(cl, or, r, p, t), cl = climate, or = organisms, r = relief, p = parent material, t = time.
• addition = inputs of material from outside sources.
• transformation = intensive in situ weathering involves chemical or physical modification or destruction of soil material.
• translocation = movement of organic and inorganic material laterally within a soil horizon, or vertically between horizons.
• loss = materials lost by leaching to the groundwater system, and/or erosion from the soil surface.

Question 10

soil pH:

Answer 10

• soil pH is measure of acidity or alkalinity of soil, related to hydrogen (H+) ion concentration.
• sources of acidity: CO2, Al3+, oxidation of Fe2+. CO2 + H2O x H2CO3+ + HCO3- x 2H+ + CO32-.
• sources of alkalinity: base cations Na2, Ca2+, Mg2+, K+. typically lost through leaching.
• schematic illustration of relation between plant nutrient availability and soil reaction (Truog, 1946).

Question 11

soil profile:

Answer 11

• O horizon = (organic) includes litter layer, and sometimes a humus layer (differentiated by the degree of decomposition of humification).
• A horizon = contains mixture of organic and mineral components, this layer most closely resembles ideal soil.
• E horizon = (eluvial) stripped of much of its clay and sometimes staining agents so is often lighter in colour than others.
• B horizon = zone of illuviation (accumulation of clay, organic matter, iron and aluminium compounds) leached or translocated from overlying horizons.
• C horizons = weathered parent material.

Question 12

USDA soil taxonomy:

Answer 12

• alfisol.
• andisol.
• aridisol.
• entisol.
• gelisol.
• histosol.
• inceptisol.
• mollisol.
• oxisol.
• spodosol.
• ultisol.
• vertisol.

Question 13

soils of Europe:

Answer 13

• double the categories.
• podzols = acid soil with bleached horizon underlain by accumulation of organic matter, aluminium and iron, cover 14% Europe, common under vegetation with acidic litter (e.g. conifer trees), typical contrasting leached and accumulation horizons of podzol (note formation of an ‘iron pan’).
• cambisols = soil that is moderately developed on account of limited age or rejuvenation of soil material, cover 12% Europe, can be very productive agriculturally (especially in loess areas), pedogenic processes evident in colour development or structure formation below the surface horizon.
• chernozems = soil with deep, dark surface horizon which is rich in organic matter and secondary calcium carbonate concentrations in deeper horizons, cover 9% Europe, high organic content comes from annual decay of grass, dark surface soil material mixed to significant depths by high biological activity.
• leptosols = shallow soil over hard rock, very gravelly material or highly calcerous material, cover 9% Europe, rocks often close to surface and many outcrops are visible, rendzinas (limestone) and rankers (granite).
• luvisols = show marked textural differences within profile, surface horizons depleted in clay while subsurface ‘argic’ horizon has accumulated them, cover 6% Europe, develop in well drained landscapes, marked textural differentiation in soil profile between surface and subsurface horizons.
• histosols = dark soil with high accumulation of partially decomposed organic matter generally developed in wet or cold conditions, cover 5% Europe (northern parts), contain recognizable remains of plants.

Question 14

soils of Britain:

Answer 14

• natural classifications = group soils by intrinsic property, behaviour, or genesis.
• technical classification = group soils by properties or functions that relate to use.
• brown Earths = one of most widespread soil in Britain and is ‘type’ soil for temperate climatic zone, characterised by A, B, C horizons, generally quite deep, well drained, fertile soils good for farming (most British agriculture depends on these soils).
• gley soils = most widespread soil in Britain, characteristic mottled appearance of B horizon due to periodical waterlogging and lack of oxygen (reduction of iron in parts of soil), can be productive agricultural soils but need to be drained artificially to remove surplus water.
• podzols = most acidic in (northern/upland) Britain, strongly leached and devoid of many nutrients (particularly in upper layer of soil), B horizon have layers of humans, iron and sometimes aluminium washed down from upper parts.