1. Soil: The Foundation for Land Ecosystems
CHAPTER 11
Soil: The Foundation for Land Ecosystems

2. Asking more of the land
Increasing human population forces us to use more land for crops and grazing
20% of cultivated land, 30% of forests, 10% of grasslands were degraded between 1981 and 2003
Soils are degraded by erosion, salt buildup, and other problems

3. Deserts are growing
Southeastern Russia has undergone severe desertification
Stable rolling grasslands have become drifting sands
Deserts in Russia are growing by thousands of acres annually, due to
Failed communist agricultural policies
Plowing the thin soil for crops
Grazing sheep with sharp hooves that broke the soil
The sand has buried over 25 towns
When marginal lands become deserts, this is called desertification

4. Desertification in China

5. Past neglect
Intensively plowing of the prairies during the 1930s (with tractors!) loosened the topsoil.
They had several years of drought, with winds that lifted that loosened soil, creating the Dust Bowl.

6. Rich soil is much more than dirt
The topsoil is the nutrient-rich layer on top. This is what supports the growth of crops and other plants.
So…What’s in good topsoil? Can you name ten things?

7. Topsoil formation

8. Classification of soil
Good soil has equal parts of all of these particles:
Sand: particles from 2.0 to mm
Silt: particles range from to mm
Clay: anything finer than mm
The sand allows for drainage; the clay holds water.
Clay soils are hard to work with: too sticky or too hard
Sandy soils are easy to work, but dry out too fast
Sand, silt, and clay constitute the mineral part of soil

9. Soil profile

10. Fertilizer
The harvesting of crops removes nutrients from the soil. These must be replaced.
Fertilizer provides nutrients to replace those that are lost. Fertilizer can be of two types:
Organic fertilizer: plant or animal wastes or both
Manure, compost (rotted organic material)
Inorganic fertilizer: synthetic formulations. Much of this fertilizer ends up being washed away This “runoff” can overfertilize waterways, leading to water pollution problems such as algae blooms.

11. Water, of course, is crucial for plants
Therefore, water-holding capacity is an important component of soil.
Organic “stuff” (manure, compost) in the soil holds a LOT of water and helps it stay moist.

12. Aeration Novice gardeners kill plants by overwatering (drowning)
Roots must breathe to obtain oxygen for energy
Land plants depend on loose, porous soil
Soil aeration: allows diffusion of oxygen into, and carbon dioxide out of, the soil
Overwatering fills air spaces
Compaction: packing of the soil
Due to excessive foot or vehicular traffic
Reduces infiltration and runoff
Strongly influenced by soil texture

13. Salt
Buildup of salt in the soil makes it impossible for roots to take in water
Long-term irrigation can lead to salt buildup in soil (salinization); that stunts crops.

14. Salinization

15. Good soil is “alive”
Dead leaves, roots, other detritus on and in the soil
Support a complex food web, including bacteria, fungi, mites, insects, millipedes, spiders, earthworms, snails, slugs, moles, etc.
Millions of bacteria are in a gram of soil
Humus: residue of partly decomposed organic matter
This is good stuff in soil!
Extraordinary capacity for holding water and nutrients
Composting fosters decay of organic wastes
and provides a source of humus

16. Soil as a detritus-based ecosystem

17. Soil structure and topsoil
Animals feeding on detritus also ingest mineral soil particles
earthworm excrement (castings) are clumps of “glued” inorganic particles plus humus. Earthworm castings make good fertilizer because the animal has already partially broken down the organic matter.
Burrowing of earthworms and animals helps to keep soil loose and aerated.
Topsoil is loose, humus-rich soil
Loss of topsoil reduces crop yield by 85–90%

18. Mineralization If detritus is lost, soil organisms starve
Soil will no longer be kept loose and nutrient-rich
If all the humus decomposes, water- and nutrient-holding capacities, infiltration, and aeration decline
Mineralization: loss of humus, loss of quality
All that remains are the minerals (sand, silt, clay)

19. Erosion
Erosion: the process of soil and humus particles being picked up and carried away by water and wind
Occurs any time soil is bared and exposed
Soil removal may be slow and gradual (e.g., by wind) or dramatic (e.g., gullies formed by a single storm)
Vegetative cover prevents erosion from water
Reducing the energy of raindrops
Allowing slow infiltration
Grass is excellent for erosion control
Vegetation also slows wind velocity

20. Erosion

21. A main cause of erosion: overcultivation
Plowing to grow crops exposes soil to wind and water erosion
Soil remains bare before planting and after harvest; it is then vulnerable to wind as well as to splash-erosion
Rotating cash crops with hay and clover as cover crops helps to reduce erosion and enrich soil.
Tractors compact soil, reducing aeration and infiltration

22. No-till planting
No-till agriculture: a technique allowing continuous cropping while minimizing erosion
After spraying a field with herbicide to kill weeds
A planting apparatus cuts a furrow through the mulch
Drops seeds and fertilizer and then closes the furrow
So the soil is never exposed
Low-till farming uses one pass (not 6–12) over a field
The waste from the previous crop becomes detritus, which is eventually decomposed, adding humus to the soil and thus increasing its fertility.

23. Apparatus for no-till planting

24. Reducing soil erosion
Contour strip cropping: plowing and cultivating at right angles to contour slopes
Shelterbelts: protective belts of trees and shrubs planted along plowed fields
The U.S. Natural Resource Conservation Service (NRCS) was established in response to the Dust Bowl
Regional offices provide information to farmers regarding soil and water conservation practices
U.S. soil erosion has decreased in recent years

25. Contour farming and shelterbelts

26. Degraded rangelands
The National Public Lands Grazing Campaign documents harmful effects of livestock grazing
Competition of livestock with native animals for food
One-third of endangered species are in danger due to cattle-raising practices (predator control, fire suppression)
Wooded zones along streams are trampled and polluted
Desertification impacts 85% of North America’s drylands
The most widespread cause is livestock grazing

27. Public lands Overgrazing occurs because rangelands are public lands
The U.S. Bureau of Land Management (BLM) and Forest Service leases grazing rights on 2 million km2 of land
Federal land is owned by taxpayers
Animal unit = one cow-calf pair or five sheep
The grazing fee ($1.35/animal unit/month) is 10% what would be paid on private land

28. Why are rangelands overgrazed?
The 1934 Taylor Grazing Act prohibits reducing grazing levels or keeps grazing fees below market level
The U.S. government lost $115 million in 2004
$500 million more was lost in ecological costs (to watersheds, streams, wildlife, endangered species)
When Congress and the BLM try to raise fees, western congressmen threaten to cut the BLM budget

29. Solutions to overgrazing
Better management could restore rangelands
Benefiting wildlife and cattle production
Conservation Stewardship Program (NRCS)
Provides information and support to land-owning ranchers to burn woody plants, reseed land, rotate cattle
The government could buy up some of the 26,000 permits
Retire rangelands
Generously pay ranchers for their permits
Use the land for wildlife, recreation, watershed protection

30. Deforestation When forests are cut and soils are left exposed
Topsoil becomes saturated with water and slides off the slope
Subsoil continues to erode

31. The other end of the erosion problem
Water that does not infiltrate enters streams and rivers
Causing flooding and pollution
Sediment: eroded soil carried into streams and rivers
Clogs channels, intensifies floods, fills reservoirs
Kills fish and coral reefs
Damages streams, rivers, bays, estuaries
Excess sediments and nutrients from erosion are the greatest pollution problem in many areas

32. Irrigation Irrigation: supplying water to croplands artificially
Dramatically increases production
Is a major contributor to land degradation
Flood irrigation: river water flows into canals to flood fields
Center-pivot irrigation: water is pumped from a well into a giant pivoting sprinkler
The U.S. Bureau of Reclamation is involved with supplying irrigation water to the western states
Irrigating 4 million hectares (10 million acres)
Worldwide irrigation is huge and is still rising

33. Flood irrigation

34. Goals of sustainable agriculture:
Maintain productive topsoil
Prevent erosion
Keep food safe
Reduce chemical fertilizers and pesticides
Keep farms economically viable
Use sustainable options like contouring, crop rotation, terracing, little or no chemicals