SOILS notes
Soil Formation
Parent Material
– Quartz sand based rocks create nutrient depleted soil that is not good for farming
– Soil with calcium carbonate parent material will have plenty of calcium, a high pH and
be good for farming
Climate
–
need non-freezing temps to encourage decomposition plus climate determines
vegetation which provides the organic matter for soil
–
–
Steep slopes will constantly erode leading to poor soil
River deltas have seasonal flooding that deposit nutrients and silt which lead to good
soil
–
Organisms help churn soil mixing nutrients evenly plus they aid in decomposition and
nutrient cycling
–
It takes a long time for soil to form, so in general older soils are better and more
established, but it depends on the vegetation.
Desert soil might be old, the lack of vegetation means it does not improve much with
age
Topography
Organisms
Time
–
Horizons
O Horizon
• May or may not include the leaf litter layer
• Organic detritus (leaves, twigs, etc) on top of a layer of partially decomposed organic matter
(called humus)
• Soil is brown or black underneath litter
A Horizon
• Topsoil
• This consist of partially decomposed organic matter, inorganic minerals, and living organisms.
– dark brown or black usually = nutrient rich = good for farming
– gray, yellow or red = low organic matter = poor for farming
• Zone of biological activity
E Horizon
• Zone of leaching
• A zone present in acidic soils,
– either between the O and A horizon or below the A horizon
– always above the B horizon
• Nutrients and minerals move quickly through this layer are deposited in the B horizon
B Horizon
• Sub Soil
• Mostly inorganic – rich in Fe, Al, and humic compounds
• Nutrients leached down from A horizon
• Almost no organic matter
• Roots may extend into this layer
C Horizon
• Weathered parent material. Broken rock fragments
Soil Matrix
• Soil is a mixture of organic and inorganic materials such as air, water, plant and animal materials
• 50% = inorganic and organic particles
• 50% = open areas called pores
What’s in soils
 Organic
o Decomposing material
 Leaf litter, dead plants & animals provides nutrients
o Living Organisms
 Bacteria, invertebrates decomposers
 All require O2
 Inorganic
o Minerals
 Macronutrients
 Primary: N, P, K
 Secondary: Ca, Mg, S
 Micronutrients: B, Cu, Fe, Cl, Mn Mo, Zn
o Rock
 Pores: store o2 and N gas
 May fill with water and roots
Soil Texture: classified by particle size
o Sand (biggest)(0.05-2.0mm)
o Silt (0.002-0.05mm
o Clay (less thatn 0.002mm)
 Soil type determined by % comp of each size particle
o Gritty: sand
o Sticky: clay
o Smooth: silty
Importance of Soil Texture
• Impacts
– Too sandy = rapid infiltration and nutrient loss
– Too much clay = no infiltration, too hard
– Too silty = compact easily, crusty surface
• Loam (look at chart)
– a mixture of sand silt and clay
– BEST for farming!
– Promotes drainage while also retaining nutrients
Porosity and Permeability
• Porosity
– The percentage of open pore space in soil
– Silty soil holds water well
– Coarse soil holds air
• Permeability
– The rate at which water flows through soil
– Determined by porosity and structure
Soil pH
• The soil pH is a measurement of the acidity or alkalinity within the soil.
– Soil pH is the negative logarithm of the hydrogen ion concentration.
– The pH scale goes from zero to fourteen.
• < 7 = acidic
• 7 = nuetral
• > 7 = basic
• A pH of 6-7.5 is best for most crops
• Too acidic
• Add lime to neutralize
• Must be used with organic fertilizer
• Too Basic
• Add sulfur which is converted to sulfuric acid
• Very SLOW
Weathering:
1. Chemical
• Rock decomposes into sediment via chemical reactions. Usually a reaction of rock
material with carbon dioxide, oxygen and water
• Example: Limestone Caves carved by acidic groundwater percolation
2. Physical
• Large masses of rock are broken into smaller pieces through physical processes
• Example: Frost wedging
3. Biological
• Decomposition of rock material via biological activity.
• Example: Roots breaking through bedrock, Lichens producing acids that chemically
weather the rock.
Soil Issues:
Erosion
• Happening continuously (flowing water)
• Rates can be increased by human action
• Ex: changes in plant species, unnatural fires, over use of agricultural lands climate
change
Compaction
• Decreases pore space and causes water to run off landscape instead of being absorbed.
Salinization
• Occurs with irrigation
• Irrigation water=ground water=has elevated levels of salts
• Over accumulation of salts in soils.