1. Soils: the substrate upon which
many plants depend for water and nutrients
--primary medium to support plants
--also acts as shelter for billions of micro
and macro organisms
--bacteria, insects, worms, rodents: all help
mix the soil and keep it healthy
--nutrients are cycled from living to nonliving
components, soil always in a state of flux

2. Figure 4.8 (EFB)

3. Soil Processes
Parent material
Climate
Vegetation
Topography
Age

4. Weathering
Parent material
Vegetation
Climate

5. Parent Material Granite (igneous rock): mineral content includes
Al+, Fe+, Ca+, K+, Mo+ (molybdenum)
--provides diverse nutrients to vegetation
Serpentine (also igneous): mineral content is
Cr (chromium), Mg+, Ni+ in high concentrations
--limited nutrients = limited plant growth

6. Weathering --can be physical or chemical
--physical includes freeze/thaw, scouring
by wind and sand to break down rock

7. Weathering --chemical occurs when water dissolves
substances in rock from chemical reactions
--can release salts, calcium sulfides
--rocks react and erode differently based on
mineral content, e.g., limestone vs. granite

8. Water and pH
Pure water spontaneously associates and disassociates constantly
H2O H+ + OH-
--1 per 550 million molecules does this every instant
Water is rarely pure. Other substances added to it
will break bonds and form new ones with either H+ or OH-
Acids: substances that leave excess H+
Bases: substances that leave excess OH-
e.g., NH3 (ammonia) is a base: H2O + NH NH4 + OH-
CO2 dissolves in water: CO2 + H2O H2CO3 (carbonic acid)
H2CO3 disassociates in soil to H+ + HCO3- making
the soil more acidic, influences weathering rate of
parent material

9. pH scale is from 1-14 1-6 is acidic, higher amounts of H+
7 is neutral, or pure water
8-14 is basic, higher amounts of OH-
Most rain is at pH 5, or slightly acidic
--will weather limestone quickly, especially
if acidity increases
--higher acidity will also affect soil processes,
bacterial decay will slow, etc.

10. Vegetation Topography Age
--plants absorb and store nutrients until released
with decay
--storage time can be short or long, depending on
species of plant
--some vegetational debris can make soils more
acidic with decay, e.g., pine needles, and influence
weathering rates of parent material
Topography
--important for drainage, sedimentation, e.g., valley bottoms
vs. slopes where soils accumulate
--also rainshadow effect
Age
--time it takes to weather bedrock, release nutrients
and develop soils
--as soil develops, it forms horizons based on water and
nutrient flow

11. Figure 4.1 (EFB) surface litter mineral soil
zone of leaching (eluviation)
Figure 4.1 (EFB)
zone of accumulation
(illuviation)
weathered parent material
unweathered parent material

12. Soil Patterns Grasslands: grass species short-lived, rapid
decay every year, warm areas but low rainfall
Forests: trees are longer-lived species, slower
decay, more acidity in soil from pine needles,
more leaching of nutrients may occur

13. Soil Texture Based on particle sizes of soil and can influence
soil horizon development
Three size classes:
sand: – 2.0 mm diameter
silt: – 0.05 mm
clay: < mm
The relative proportion of each in a soil affects drainage
Chart shows how soils can be classified by texture

14. Figure 4.2 (EFB)

15. Why is clay loam best? Water moves through soil as either:
1. Gravitational: drains out
2. Capillary: held between grains
3. Hygroscopic: strong adhesion to surface
of smallest soil particles
Field capacity: maximum amount of capillary and
hygroscopic water a soil can hold
--determined 1 to 3 days after a soaking rain,
after gravitational water drains through
--most plants can absorb capillary water down to
-15 bar water potential (permanent wilting point)

16. Charged clay particles
Micelles
Charged clay particles
--formed by crystallization of minerals that weather
from bedrock, pH dependent to have charge
--negative charges capture and hold cations (+ ions)
in soil, don’t leach away
e.g., Ca+, Na+, K+, Mg+, Fe+, Si+, NH4+ (ammonium)
--plants break bonds and can absorb these nutrients
--problem is that each cation varies in strength of bond
it forms, with H+ as strongest
e.g., H+ Ca+ Mg+ K+ Na+
strong weak
--thus, H+ ions can displace all others off micelles and
cause more leaching. If acidity increases, this can be a
problem

17. Negative ions also can be held by micelles:
may attach to a cation held on micelle, which acts
as a ‘bridge’
e.g., NO3- (nitrate), PO4- (phosphate), SO4- (sulfate)
Figure 4.7 (EFB)
For these reasons, then, clay loam is best—it maximizes field capacity and has lots of
micelles to hold nutrients

18. Soils classifications by processes
that form them
Podzolization: highly acidic soils, e.g., pine forests,
resulting in highly leached A and E horizons, dark B
--typical of cold, northern coniferous forests, where
rainfall exceeds evaporation
Laterization: deeply weathered soils where decay is
rapid and high Fe content, e.g., tropical forests
--rainfall exceeds evaporation, no humic buildup
and little leaching to A horizon
--high Fe content gives reddish color, hardens like
bricks when exposed to sun, drying, when forests
are cleared

19. Soils classifications by processes
that form them
Salinization: salt buildup in top soils from reverse
leaching, e.g., dry arid regions and deserts
--evaporation exceeds rainfall, causes reverse
leaching from ground water
--can be salts, but also carbonates (alkali) that form
hard crust on surface, limiting plant growth
Also peat can form under certain
conditions and act as a carbon sink.

20. Sphagnum Moss Releases H+ ions into soil, making them more
acidic so less decay as a result
Hyde County, NC, also has lots of peat so not
just in cold, wet areas
These soils are ‘carbon sinks’ in that all the carbon
can remain locked in the soils for hundreds to
thousands of years (also tundra and permafrost soils)
Sphagnum Moss