1. Topic 5 Part 1
soil

2. What is soil and why do we care about it?
complex mixture:
weathered mineral materials from rocks
partially decomposed organic molecules
Ecosystem services
Soil consists of
1/2 mineral
plant and animal residue,
air,
water,
living things

3. Soil formation Many factors contribute to this LONG process
ClORPT
Help determine soil type
Cl=climate
O= organisms
R= relief
P= parent rock
T= time
Young soil has less organic material and fewer nutrients than mature soils.
Very old soils can be nutrient-poor also b/c of leaching and plant uptake. 3

4. Climate. As temperature increases weathering increases
Climate. As temperature increases weathering increases. Rain or water added also increase weathering.
Temperature and Moisture influence the speed of chemical reactions, which in turn, control how fast rocks weather and dead organisms decompose.

5. Warm and wet – tropics – weather faster than cold and dry (Antarctica) or even hot and dry
Soils develop fastest in warm, moist climates, and slowest in cold and arid ones.

6. Relief – Shape and aspect (where the surface faces) affect the soil development. Steep slopes show less development as they are likely to erode and are unstable. South facing slopes in the northern hemisphere will show more development as they are warmer.
The shape of the land and the direction in faces makes a difference in how much sunlight the soil gets, and how much water it keeps.

7. Deeper soils form at the bottom of a hill than at the top because gravity and water move soil particles down the slope.
Stable and or flat landscapes show more development – deeper and more horizons. Also these areas may be an area where particles (sediment)from up slope accumulate.

8. Parent Material
Just like you inherited some characteristics from your parents, every soil inherits traits from the material from which it formed.

9. Parent Material
Soils that form in limestone bedrock are rich in calcium, Soils that formed from materials at the bottom of lakes are high in clay.

10. Time
More development means more horizons, redder color in the B, more clay in the B. However time does not always just mean true chronologic age to a soil scientist. For example a soil in Antarctica will show very little development and appear young since it is not highly weathered. On the other hand a tropical soil (warm and wet) will show more development as compared to the Antarctic soil even though it is chronologically or less age (younger)
Older soils differ from younger soils because they have had longer to develop

11. Soils in the northern US and Canada are younger as compared to those in the Southeast.
EXAMPLE: In the Northern U.S., soils tend to be younger, because glaciers covered the surface during the last ice age, which kept soils from forming. In the southern U.S., there were no glaciers. There, the soils have been exposed for a longer time, so they are more weathered.

12. Soil Texture
Ratio of soil particle size determines some soil properties
Head of pin, baseball, basketball

13. Determining soil type
% Increasing Clay 13

14. Soil Properties: Porosity
Affects water infiltration, water holding capacity, aeration, workability

15. Soil Properties: Chemical
Cation exchange capacity (aka nutrient holding capacity) 15

17. SOIL Profile
O: organic material (leaf litter, dead plants and animals, not technically part of the soil, but will eventually breakdown and become part of the A horizon
A: the topsoil, contains a mix of humus and mineral material, most plant roots are in this layer
B: the subsoil, mostly mineral material, where nutrients collect, plants with deep root systems will penetrate to this depth
C: the parent rock material.
E: eluviation zone, zone where nutrients have leached through due to acidic conditions. Very little organic matter to hold nutrients.
This is a generic soil profile. Profile vary greatly depending on age of the soil and climate.
Topsoil vs. humus

18. Soil Profiles in Different Biomes
mollisols
aridisols
Mosaic
of closely
packed
pebbles,
boulders
Alkaline,
dark,
and rich
in humus
Weak humus-
mineral mixture
Dry, brown to
reddish-brown, with
variable accumulations
of clay, calcium
carbonate, and
soluble salts
Aridisols are characteristic of arid regions of the world. There is generally no O layer because there is so little vegetation. The A horizon is very shallow with little humus and poor-mineral content. The B horizon tends to be light reddish or light brown in color due to the high clay content and build up of salts.
Mollisols are characteristic of grasslands. There is usually an O layer. The A horizon is deep due to the large inputs of organic material every year. There is a rich humus layer very dark in color, and neutral to slightly basic soil pH. Precipitation is not significant enough to leach out nutrients and minerals.
Clay,
calcium
compounds
Desert Soil
(hot, dry climate)
Grassland Soil
(semiarid climate)

19. Soil Profiles in Different Biomes
alfisols
oxisols
Forest litter
leaf mold
Acidic
light-
colored
humus
Humus-mineral
mixture
Light, grayish-
brown, silt loam
Oxisols are characteristic of tropical rain forests. The O horizon is present but very shallow due to the high decomposition rates. The A horizon has acidic humus that is mineral-poor due to constant leaching of ions and nutrient-poor due to the rapid assimilation of nutrients into living vegetation. The A horizon is acidic due to the organic acid by-products of decomposition. The constant rain leaches minerals to the B horizon, where heavy metals tend to accumulate.
Alfisols are characteristic of temperate forests, like those here in New England. Generally there is an O horizon present, the depth of which fluctuates seasonally. The A horizon is rich in humus and minerals. None of the horizons are very deep, the soils tend to be old and thus weathered.
Iron and
aluminum
compounds
mixed with
clay
Dark brown
firm clay
Deciduous Forest Soil
(humid, mild climate)
Tropical Rain Forest Soil
(humid, tropical climate)

20. Soil Properties: Biotic
surface = algae
top few cm of soil =
bacteria & fungi
roundworms, segmented worms, mites, insects
farther down: burrowing animals--gophers, moles, insect larvae, worms
even farther: some plant roots
Sweet aroma of freshly turned soil is caused by actinomycetes, bacteria that grow in fungus-like strands (and give us streptomycin & tetracyclines = antibiotics)

21. Soil Community

22. Soil Community -- Mycorrhizae
mycorrhizal symbiosis - mutualism between plant roots & fungi
Pine seedling showing how mycorrhizal roots from one tree spread to inoculate other tree roots.
Redwood seedlings with (right) and without (left) mycorrhizae.