© 2011 Pearson Education, Inc. CHAPTER 11 Soil: The Foundation for Land Ecosystems

© 2011 Pearson Education, Inc.

CHAPTER 11

Soil: The Foundation for Land Ecosystems


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


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


Desertification in China


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.


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?


Topsoil formation


Classification of soil• Good soil has equal parts of all of these particles:

• Sand: particles from 2.0 to 0.063 mm• Silt: particles range from 0.063 to 0.004 mm• Clay: anything finer than 0.004 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


Soil profile


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.


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.


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


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.


Salinization


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 wastesand provides a source of humus


Soil as a detritus-based ecosystem


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%


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)


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


Erosion


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


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.


Apparatus for no-till planting


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


Contour farming and shelterbelts


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


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


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


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


Deforestation

• When forests are cut and soils are left exposed• Topsoil becomes saturated with water and slides off

the slope• Subsoil continues to erode


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


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


Flood irrigation


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

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© 2011 Pearson Education, Inc. CHAPTER 11 Soil: The Foundation for Land Ecosystems