1. Understanding Soil

2. What is soil? Top layer of the Earth’s surface
Created when rocks, dead animals and plants break down into tiny pieces
Rocks, animals & plants in an area determine the composition of the soil
Weathering turns mineral particles into topsoil

3. Soils Perform Vital Functions
Sustaining plant and animal life below and above the surface
Rain
Runoff
Soil
Infiltration
Regulating and partitioning water and solute flow
Filtering, buffering, degrading, immobilizing, and detoxifying
Storing and cycling nutrients
Providing support to structures

4. Soil is the Basis of the Ecosystem
The living systems occurring above and below the ground surface are determined by the properties of the soil. We often ignore the soil because it is hard to observe.

5. Soils Support Life Organism Types Roles & Benefits bacteria
fungi
protozoa
nematodes
arthropods
earthworms
Roles & Benefits
decomposition
release nutrients
create pores
stabilize soils

6. Pedology, the study of soil, is a unique discipline.
Soils Have Unique Physical, Chemical, and Biological Properties Important to Their Use
color texture
structure
consistence
roots
pores
other features
Soil is a natural body of solids, liquid, and gases, with either horizons, or layers or the ability to support rooted plants.
Pedology, the study of soil, is a unique discipline.

7. Soil-Forming Factors Determine the Location and Kind of Soil
There are 23,000 soil types in various combinations with different slopes and surface textures in the U.S.
Soil Forming
Factors:
Parent Material
Climate
Living Organisms
Topography
Time

8. Soil Formation
The A horizon is the topsoil, which is rich in organic matter. The dark color of the topsoil is due to the carbon in the decomposing organic material.
The B horizon is a subsoil containing clay and soluble minerals.
The C horizon comprises weathered parent material and rock fragments. A B C

9. Disintegrating parent rock
Soil Formation
Disintegrating parent rock
Bedrock
Stage 1
Weathered parent rock
(C horizon)
Bedrock
Stage 2
C horizon
A horizon
Bedrock
Stage 3
C horizon
A horizon
B horizon
Bedrock
Stage 4

10. Climatic Influence on Soil
Photo: USDA
Photo: USDA
ARID
Desert soils
These are alkaline mineral soils with variable amounts of clay, low levels of organic matter, and poorly developed vertical profiles.
Horizon A
Horizon A
Horizon B
Horizon B
HUMID
Tropical soils
Leaching and chemical weathering make these soils acidic.
Aluminum and iron oxides accumulate in the deep B horizon.

11. Climatic Influence on Soil
Photo: USDA
Photo: USDA
MID-LATITUDE
Grassland soils
Mature, alkaline, deep, well drained soils. They are typically nutrient-rich and productive with a high organic content.
Horizon A
Horizon A
Horizon B
Horizon B
POLAR
Very low temperatures slow the decomposition of organic matter and maintain the permafrost layer in these frozen soils.

12. Climatic Influence on Soil
Photo: USDA
Photo: USDA
Horizon A
Horizon B
TEMPERATE
Weathered forest soils Well developed soils with a deep organic layer and accumulated clay at lower levels.
Horizon A
Horizon B
SEASONALLY WET
Swelling soils
Marked seasonal rainfall results in deep cracks as the soil alternately swells and shrinks.

13. SOIL
Component definition: Mixture of mineral matter, organic matter, water, and air.
Example:
Air 25%
Mineral Matter 45%
Water 25%
Organic Matter 5%

14. Soil Texture
Soil texture depends upon the amount of sand, silt and clay present.
Percentage clay
Percentage silt
Percentage sand 90 80 70 60 50 40 30 20 10
Read silt in this direction
The optimal soil texture is called a loam and is around 40% sand, 40% silt and 20% clay.
Clay
Sandy
clay
Silty
Sandy clay
loam
Clay loam
Loam
Sandy loam
Silty clay
Loamy sand
Sand
Silt
Other loam exist but are named after their various components.
Read clay in this direction
Read sand in this direction

15. Examples of Soil Structure

16. Porosity
Porosity is the amount of pore space in a rock (the open spaces between the grains) and is independent of grain size.
Depends on:
Sorting of the grains (or uniformity of grain size)
Are the grains all the same size (well sorted), or are a variety of grain sizes present, with finer grains filling the spaces between the larger grains (poorly sorted)?
Shape of the grains
Packing and arrangement of grains. Cubic packing. Porosity = 47.64% Rhombohedral packing. Porosity = 25.95%
Examples of rock/soil with high porosity: sands Examples of rock/soil with low porosity: well-cemented sandstone, shale, mudstone, very poorly sorted material

17. Microscopic structure of shale, sandstone, and limestone with water in pore spaces. Note differences in scale among views of each rock type.
%Porosity = V1(volume of pores or average volume water) x 100
Total sediment volume

19. Permeability
Permeability is the ease with which fluids flow through a rock or sediment. A rock/soil is permeable if fluids pass through it, and impermeable if fluid flow through the rock/soil is negligible
Depends on:
Grain size
Coarser-grained sediments are more permeable than fine-grained sediments because the pores between the grains are larger
Sorting
Grain shape
Packing (controls pore size)