1. Soils and their Preservation
Section 14

2. What is Soil?
the relatively thin surface layer of the Earth’s crust consisting of mineral and organic matter that has been modified by the natural actions of agents such as weather, wind, water, and organisms
necessary for shelter, food, and water for all organisms
no soil = no plants = no humans

3. How are Soils Formed? start with rock/parent material/bedrock
broken down into smaller and smaller particles by biological, chemical, and physical weathering processes
may take up to thousands of years to form a few centimeters of soil
organic material must also accumulate
the weathering of parent material beneath already-formed soil continues to add new soil

4. How Does Weathering Occur?
organisms produce CO2  diffuses into the soil  reacts with H2O to form H2CO3 (carbonic acid)
acids etch cracks in the rock
water seeps into the cracks
freezing/thawing of water causes the cracks to enlarge, breaking off pieces of rock
small plants establish roots in larger cracks  fractures rock further

5. Topography Matters!
a region’s surface features are involved in soil formation
steep slopes have little to no soil – why?
valleys tend to encourage formation of deep soils

6. Soil Composition 4 distinct components of soil:
mineral particles (45%)
organic matter (5%)
soil water (25%)
soil air (25%)

7. Component #1 – Mineral Particles
where do they come from?
why are they important?
provide anchorage and essential nutrients for plants
provide pore space for water and air
soils vary in mineral composition and chemical properties (based on the type of parent material)

8. Mineral Particles continued
the age of a soil affects its mineral composition
older soils are more weathered and generally are lower in essential nutrient minerals
e.g. large portions of Australia, South America, and India
geologically young soils are found in much of the Northern Hemisphere

9. Component #2 – Organic Matter
made up of:
litter (dead leaves and branches on the soil’s surface)
animal dung
dead remains of plants, animals, and microorganisms in various stages of decomposition
during decomposition, essential nutrients (ions) are released  bound by soil particles OR absorbed by plant roots OR washed away by water

10. Organic Matter continued
increases a soil’s water-holding capacity
humus: the black or dark brown organic material that remains after much decomposition has occurred
a mix of many organic compounds
binds ions and holds water

11. A Note About Pore Spaces
soil has pore spaces around and among the soil particles
makes up ~50% of a soil’s volume
generally, water is held in the smaller pores and air in the larger

12. Component #3 – Soil Water
soil water originates as precipitation (downward percolation) or as groundwater (upward movement)
some water is bound to the soil or absorbed by roots
contains low concentrations of salts

13. Leaching and Illuviation
leaching: the removal of dissolved materials from the soil by water percolating downward
illuviation: the deposition of leached material in the lower layers of soil
materials that commonly deposit include: Fe Al
humus
clay

14. Component #4 – Soil Air
contains the same gases as in the atmosphere, but in different proportions
there is generally more CO2 and less O2 because of cellular respiration

15. Soil Horizons the distinctive horizontal layers of soil
in order to look at the horizons, we can get a soil profile (a vertical section from surface to parent material)

16. Typical Horizons not all soils are created equal!
soil profiles vary depending on the biome you are in
typical horizons include: O A B C

17. Soil Profile

18. O-Horizon the uppermost layer of soil, rich in Organic material
where plant litter accumulates and decays
typically absent in deserts – why?
may be the dominant layer in grasslands
is this really soil?

19. A-Horizon
the topsoil, dark and rich in accumulated organic matter and humus
granular texture
somewhat nutrient-poor  gradual loss of nutrients to deeper layers by leaching
may be followed by an E-horizon, a thin layer made up of leached materials

20. B-Horizon
the subsoil; light-colored zone of illuviation in which nutrient minerals that are leached out of the topsoil and litter accumulate
typically rich in Fe and Al compounds and clay

21. C-Horizon
weathered pieces of rock; borders the unweathered solid parent material (R)
below the extent of most roots
often saturated with groundwater

22. Soil Profile Revisited

23. Soil Organisms
include: bacteria, fungi, algae, worms, protozoa, plant roots, insects, moles, snakes, groundhogs, etc.
provide many ecosystem services:
maintain soil fertility by decaying and cycling organic material
prevent soil erosion
break down toxic materials
cleanse water
affect the composition of the atmosphere

24. The Importance of Earthworms
provide castings (bits of soil that have passed through the gut of an earthworm that are deposited on the soil surface)
nutrients from deep layers are brought to upper layers
earthworm tunnels aerate the soil
corpses add organic material to the soil

25. Soil Texture
determined by the percentages (by weight) of different sized inorganic mineral particles of sand, silt, and clay that it contains
*gravel and stones are larger than 2 mm in diameter, and so are not considered soil particles because they do not have any direct value to plants*

26. Soil Particles
sand: largest soil particles ( mm in diameter); large enough to be seen easily with the eye
silt: medium-sized particles ( mm in diameter); ½ the size of flour particles, barely visible to the eye
clay: small particles (less than mm in diameter); can only be seen under an electron microscope

27. Why are Clay Particles so Important?
have the greatest surface area of all soil particles
each clay particle is predominantly negatively charged
bind to cations (e.g. K+, Mg2+) that are essential for plants

28. The Optimum Soil
soil contains a mixture of different-sized particles  proportions vary from soil to soil
loam: has an optimum combination of different soil particle sizes; ideal agricultural soil
40% sand, 40% silt, 20% clay
larger particles provide:
structural support
aeration
permeability
smaller particles hold nutrient minerals and water

29. Less than Optimum Soils
soils with large proportions of sand 
soils with large proportions of clay 

31. Soil Acidity usually ranges from 4-8 (optimum 6-7)
the solubility of minerals varies with differences in pH
low pH 
high pH 
affects leaching
acidic soil has a reduced ability to bind cations
organisms can affect soil pH
e.g. conifer needles
decomposition of humus + cellular respiration lower pH

32. Major Soil Groups
5 common soil types, based on variation in climate, local vegetation, parent material, underlying geology, topography, and soil age
spodosols
alfisols
mollisols
aridisols
oxisols

33. Major Soil Groups

34. Sustainable Soil Use
when humans use soil resources without a reduction in the amount or fertility of the soil, so that it is productive in the future
two main soil problems:
soil erosion
nutrient mineral depletion

35. Soil Erosion the wearing away or removal of soil from the land
water and wind are very effective at moving soil
rain loosens soil particles  moving water transports them
wind loosens soil  blows it away (especially if it is barren and dry)

36. Soil Erosion continued
reduces the amount of soil in an area and limits plant growth
loss of soil fertility  minerals and organic matter are removed
reduces productivity of agricultural soil
increases the need to replace nutrients with fertilizer
can have an impact on other natural resources
e.g. sediments can affect water quality and fish habitats, especially if they contain pesticides and fertilizers

37. Human Acceleration of Soil Erosion
humans can accelerate erosion through poor soil management
poor agricultural practices
removal of natural plant communities for construction
unsound logging practices such as clearcutting

38. Nutrient Mineral Depletion
harvesting crops disrupts nutrient cycling
plant material is removed from the cycle  no decay and release of minerals
severe in tropical rainforest soils, due to a combination of the climate, the typical soil type, and the removal of the natural forest community
nutrients are stored primarily in the vegetation
when a forest is cleared, nutrients leach out of the system

39. Worldwide Soil Problems
in the U.S., erosion is a serious threat to many regions (e.g. Iowa, Missouri, Texas, Tennessee)
water erosion is particularly severe along the Mississippi and Missouri Rivers
worldwide, erosion is greatest in parts of Asia, Africa, and Central/South America
only 16% of the world’s farmland does not have some kind of fertility problem

40. The African Sahel soil damage is compounded by a high r
desertification: the degradation of once-fertile rangeland or forest into nonproductive desert caused partly by soil erosion, forest removal, overcultivation, and overgrazing
people HAVE to use land to grow food  BUT with each year, the land can support fewer and fewer people

41. How Can We Lessen Deforestation & Degradation?
a possible technique is agroforestry (agricultural and forestry techniques that encompass the planting of trees and crops together, to improve soil fertility in degraded soils)
decay of leaf litter improves soil fertility
leaf layer improves the soil’s ability to hold moisture  less evaporation

42. Soil Conservation and Regeneration
conventional tillage: prepares the land for crops by removing all plant cover
what problems does this cause?
conservation tillage: residues from previous crops are left in the soil, partially covering it and helping to hold it in place

43. Conservation Tillage
different types have been developed to fit different areas of the country and different crops
one example of conservation tillage is called no-tillage (leaves the soil undisturbed over the winter)
during planting, special machines cut a narrow furrow in the soil for seeds

44. Benefits of No-Tillage
reduces soil erosion
increases the organic material in soil  improves water-holding capacity
farmers save on fuel costs, machinery wear and tear, and labor

45. Crop Rotation
the planting of a series of different crops in the same field over a period of years
lessens damage by insects and disease
helps maintain soil fertility
typical rotation: corn  soybeans  oats  alfalfa

46. Cultivation of Hilly Terrain
hilly terrain must be cultivated carefully because it is very prone to erosion
methods include:
contour plowing
strip cropping
terracing

47. Contour Plowing
fields are plowed and planted in curves that conform to the natural contours of the land, rather than in straight rows
furrows run AROUND hills

48. Strip Cropping
a special type of contour plowing that produces alternating strips of different crops along natural contours

49. Terracing
produces level areas on steep slopes, thereby reducing soil erosion

50. Preserving Fertility Through Use of Fertilizers
organic fertilizers: contain natural materials such as animal manure, crop residues, bone meal, and compost
minerals become available as organic material decomposes  long-lasting AND/BUT slow-acting
commercial inorganic fertilizers: manufactured from chemical compounds (exact compositions are known)
soluble  immediately available to plants
BUT quickly leach away

51. Why Should We Limit Use of Inorganic Fertilizers?
environmental reasons
very mobile 
do not improve the water-holding capacity of the soil like organic fertilizers do
they are sources of N-containing gases (e.g. NOx)
require a great deal of fossil fuel energy to produce
economic reasons
can cost A LOT of money!

52. Soil Reclamation involves 2 steps
stabilizing the land to prevent further erosion
bare ground is seeded with plants – why?
plant windbreaks (rows of trees that lessen the impact of wind)
restoring the soil to its former fertility
dead plants are converted to humus, which holds nutrients AND improves water-holding capacity
a SLOW process  land cannot be farmed/grazed during recovery

53. Soil Conservation Policies in the U.S.
3 main laws/programs
the Soil Conservation Act of 1935
the Food Security Act of 1985
the Grasslands Reserve Program

54. The Soil Conservation Act of 1935
authorized the formation of the Soil Conservation Service (now the Natural Resources Conservation Service) to work with U.S. citizens to conserve natural resources on private land
the NRCS assesses soil damage and develops policies

55. The Food Security Act of 1985
aka “The Farm Bill”
contained provisions for 2 main soil conservation programs
a conservation compliance program
the Conservation Reserve Program (CRP)

56. Provisions of the Farm Bill
conservation compliance program
requires farmers with highly erodible land to develop and adopt a 5-year conservation plan for their farms that includes erosion-control measures
noncompliance = loss of agricultural subsidies
the Conservation Reserve Program (CRP)
a voluntary program that pays farmers an average of $50/acre/year to stop producing crops on highly erodible farmland
requires planting native grasses or trees  “retire” the land for 10 years
benefits?

57. The Grasslands Reserve Program
pays farmers to protect up to 2 million acres of virgin and improved pastureland for a period of at least 10 years