soils and land use suitability - indiana university of
TRANSCRIPT
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Soils and Land‐use Suitability
•Composition
•Texture•Moisture and Drainage
•Landforms and Topography
Soil Composition
• Composition is the basic material that make up the soil
• Four basic things:1. Mineral particles
2. Organic material
3. Water (will discuss, but will return when we discuss groundwater)
4. Air
1. Mineral Particles – comprise about 50 – 80% of the volume of most soils and form the structural skeleton
a. The structure enables the soil to support its own weight, the weight of water, and the weight of surface structures (such as buildings)
• Sand and gravel – if packed solidly ‐ are generally the best at providing the best stability
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Soil Composition• Bearing Capacity – refers to the soil’s resistance to penetration from a weighted
object
– such as a structural foundation
– Determining bearing capacity is a geo‐technical exercise and should be determined by a geo‐technical expert or civil/structural engineer
2. Organic Matter ‐ decaying plants and animals / living organisms / varies greatly in soils / example ‐ top soil
– Positives of highly enriched organic soils:• Vital to fertility and hydrology of soils
• Create moisture reservoirs for wetland vegetation
• Points of entry for groundwater recharge
– Negatives of highly enriched organic soils:• Provide weak soil structural skeleton
• Compresses and settles differentially
• When dewatered, may experience volume loss, decomposition, and wind erosion
Soil Texture• An easy way to help determine what type of soil you have is to
simply feel it to determine texture and thus what the primary makeup of the soil is:
Grab a baseball size portion of the soil in your hands and wet the soil with water, working the moist soil with your hands
– The stickier it is, the more clay there is
– The soapier the soil feels the higher the silt content
– Grittiness is indicative of sand
The soil texture triangle shows the 12 major soil texture classes and what percent of each type soil makes them up
http://www.cmhc‐schl.gc.ca/en/co/maho/la/la_001.cfm
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USDA Textural Classification of Soils
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Increasing sand
USDA Textural Classification of Soilshttp://w
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sor.com/images/soiltriangle_large.jpg
Sticky Sticky
SoapyGritty
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Soil TextureFeel test – Thoroughly dry and crush a small amount of the soil by
rubbing it with the forefinger in the palm of your other hand
Then rub some of it between your thumb and fingers to measure the percentage of sand
The grainier it feels, the higher the sand content
Moist cast test – Compress moist soil by squeezing it in your hand*
When you open your hand, if the soil holds together (that is, forms a cast), pass it from hand to hand
The more durable the cast, the higher the percentage of clay
Ribbon test – Roll a handful of moist soil is into a cigarette shape and
squeeze it between your thumb and forefinger to form the longest and thinnest ribbon possible*
Soil with high silt content will form flakes or peel instead of forming a ribbon
The longer and thinner the ribbon, the higher the percentage of clay
Moist cast test Ribbon test
Testing Soil Texture Hand Field Test
*For these tests, the soil specimen should be gradually moistened and thoroughly reshaped and kneaded to bring it to its maximum "plasticity" and to remove dry lumps.
Do not add too much water, as the sample will lose its cohesion.
http://www.cmhc‐schl.gc.ca/en/co/maho/la/la_001.cfm
Moist cast test Ribbon test
Testing Soil Texture Hand Field Test
Field Tests for Soil Texture*
Texture Feel Test Moist Cast Test Ribbon Test
Sand Grainy, little floury material No cast Can't form a ribbon
Loamy sand Grainy with slight amount of
floury material Very weak cast, does not allow handling
Can't form a ribbon
Silty sand Some floury material does not allow handling Can't form a ribbon
Sandy loamGrainy with a moderate amount of floury material
Weak cast, allows careful handling
Barely forms a ribbon — 1.5 ‐2.5 cm (0.6 ‐1 in.)
Loam Fairly soft and smooth with
obvious graininess Good cast, easily
handled Thick and very short ‐ <2.5
cm (1 in.)
Silt loam Floury, slight graininess Weak cast, allows careful handling
Makes flakes rather than a ribbon
Silt Very floury Weak cast, allows careful handling
Makes flakes rather than a ribbon
Sandy clay loam Very substantial graininess Moderate cast Short and thick — 2.5 ‐ 5 cm
(1 ‐ 2 in.)
Clay loam Moderate graininess Strong cast clearly
evident
Fairly thin, breaks easily, barely supports its own
weight.
Silty clay loam Smooth, floury Strong cast Fairly thin, breaks easily, barely supports its own
weight.
Sandy clay Substantial graininess Strong cast Thin, fairly long, 5‐7.5 cm (2
‐ 3 in.). Holds its own weight.
Silty clay Smooth Very strong cast Thin and fairly long, 5 ‐ 7.5 cm (2 ‐ 3 in.). Holds its own
weight.
Clay Smooth Very strong cast Very thin and very long —
>7.5 cm (3 in.)
*Table adapted from Denhom, K.A. and L.W. Schut, 1993.
Field Manual for Describing Soils in Ontario. Centre for Soil Resource Evaluation. Guelph, Ont.http://www.cmhc‐schl.gc.ca/en/co/maho/la/la_001.cfm
Soil Texture
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Soil Moisture and Drainage
‐ Water content varies with particle size, drainage and topography
‐ Two principal forms of water occur in both mineral and organic soils:1. Capillary water – a form of molecular water; called so because it is held in soil by
the force of cohesion among water molecules
• Under this force water molecules are mobile and move from moist to dry
• During summer, most capillary water transfer is upward toward soil surface as water is lost to evaporation and transpiration – (more on these later too…)
2. Gravity water – liquid water that moves in response to the gravitational force
• Movement is usually downward
• Tends to accumulate in subsoil and bedrock to form groundwater
• Groundwater completely fills inter‐particle spaces– Thus, below the water table, soil is largely devoid of air
Angle of Repose ‐
Generally, for dry materials the angle of repose increases
with increasing grain size, but usually lies between about 30 and 37o
Slightly wet unconsolidated materials exhibit a very high angle of repose because surface tension between the water and the grains tends to hold the grains in place
Material saturated with water reduces the angle of repose to a small value and the material tends to flow like
a fluid
The angle of repose is the steepest angle at which a pile of unconsolidated grains remains stable, and is controlled by the frictional contact between the grains
Thank you, http://www.tulane.edu/~sanelson
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Soil Particles Soil Water
Soil Moisture Concepts
total solid liquid vaporV V V V
Moisture Content ( ) = 100 water
total
V
V
Porosity ( ) 100 100 water vaporvoids
total total
V VVn
V V
(or soil-water content)
Porosity
• Soil Porosity – is the percentage of total soil volume not occupied by soil particles and indicates its water‐holding capacity
Porosity (n)
Porosity ( ) 100 100 water vaporvoids
total total
V VVn
V V
Or simply
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Specific Yield (Sy)
• Specific yield – the amount of water draining from the soil due to gravity
(gravity water)
Soil Moisture and Drainage
• Drainage – refers to gravity water and a soil’s ability to transfer water downward
• Three terms are used to describe this process:1. Infiltration capacity – the rate that water penetrates the soil surface
2. Permeability – the rate that water within the soil moves through a given volume of material
3. Percolation – the rate that water in a soil pit or pipe within the soil is taken up by the soil
Each of these will get more attention when we get to groundwater
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Unsaturated (or Vadose) Zone
‘Vadose’ is Latinfor ‘shallow’
Water table
Movement of water within the vadose zone is studied within soil physics and hydrology, particularly hydrogeology, and is of importance to agriculture, contaminant transport, and flood control
Soil Landforms and Topography
– Soil horizon ‐ a specific layer in the soil which measures parallel to the soil surface and possesses physical characteristics which differ from the layers above and beneath
• Solum ‐ consists of the surface and subsoil layers that have undergone the same soil forming conditions
• Parent material – the underlying geologic material in which soil horizons form Soil Horizon
‐‐ The base is the relatively unweathered parent material (or substratum)
• Toposequence ‐ Predictable trends in soil makeup related to topographic gradients on individual landforms
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Common sources and settings of deposits in which soils form
wetland ‐ slope ‐ floodplain ‐ upland deposits
Relationship between soils and landforms in mountainous terrain
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Example of soil variation associated with toposequence on hillslope
Soil Landforms and Topography –Unified Soil‐Classification System
First Letter
Definition CriterionSecond Letter
Definition
G gravel Texture W well graded
(diversified particle sizes)
S sand Texture Cwell graded w/ clay fraction; binds soil
together
M silt (inorganic)
Texture / composition
P poorly graded
(uniform particle sizes)
C clay (inorganic)
Texture / composition
L low compressibility and
low plasticity
O organic silts and clays
Texture / composition
H high compressibility and
high plasticity
P peat Composition
http://soilmap.psu.edu/code/mapindex.asp
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Soil ConservationSoil conservation is a set of management strategies for prevention of soil being eroded
from the earth’s surface or becoming chemically altered by overuse, salinization, acidification, or other chemical contamination
• The principal approaches these strategies take are:
• choice of vegetative cover
• erosion prevention
• salinity management
• acidity control
• encouraging health of beneficial soil organisms
• prevention and remediation of soil contamination
• mineralization
other ways include
• no till farming
• contour plowing
• wind rows
• crop rotation
• the use of natural and man‐made fertilizer
• resting the land