GEOG 3000 Resource Management SOIL AS A RENEWABLE RESOURCE

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GEOG 3000 Resource ManagementSOIL AS A
RENEWABLE RESOURCE

• M.D. Lee CSU Hayward Winter 2004

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Soil - a (conditionally) renewable resource
"A Nation that destroys its soil destroys
itself." President Franklin D. Roosevelt. Quoted
from a letter sent to US Governors on February
26, 1937.
(Photo source Silsoe College, UK)
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Soil as a renewable resource

• The worlds soils are fragile and valuable
  renewable resources that must be actively
  conserved while increasingly being exploited
• (photo from USDA http//www.statlab.iastate.edu/s
  oils/photogal/).

• The soil is the uppermost layer mantling the
  earth.
• It is the medium in which crops are grown and
  through which plants get water and nutrients.
• It is a mix of organic material decaying from the
  top down, geological (parent) material weathering
  from the bottom up, and organic and inorganic
  material deposited from above by wind, water or
  mass movement.

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Soil fertility

• Although abiotic, the soil is often thought of as
  a living entity - a healthy soil is crammed with
  millions of microbes and other organisms per
  cubic centimeter.
• For soils to remain useful, it is important that
  they remain fertile, able to provide the
  vegetation being grown on them with nutrients.
• Fertility is a function of many biotic and
  abiotic factors - presence of microrhizomes (N
  fixing bacteria), soil texture and structure, NPK
  nutrient availability, micronutrient
  availability, pH, etc.

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Soil sustainability

• For soils to remain useful, it is also necessary
  that they remain stable, i.e. attached to the
  land surface, neither removed by wind, water nor
  mass movements.
• Soil sustainability requires nutrients to remain
  available to plants in adequate amounts and for
  the rate of soil loss to be balanced by the rate
  of soil formation.
• Soils are being lost from the land surface all
  over the world at rates greater than the rate of
  soil formation.
• For example, Iowa has lost half the depth of its
  topsoil this century and erosion all over the
  midwest is so extreme that Louisiana is growing
  by 10 sq. mi./yr due to the silt transported by
  the Mississippi River to the Gulf of Texas.

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Erosion of soils

• Agricultural soils are over-compacted through
  over-grazing and over-use of machinery, leading
  to erosive runoff.
• Vegetation is completely removed each harvest and
  soils left exposed to erosional forces wind,
  raindrops, etc.
• Soils are used continuously with little fallowing
  (rest).
• Pesticides that are used to kill crop pests also
  sterilize soils, killing useful organisms.
• Many soils have been converted into nothing more
  than mediums for the delivery of powdered
  chemical nutrients which have replaced organic
  manure as fertilizer.
• Soil structure, microbiology and chemistry have
  deteriorated so that a soil often cannot hold on
  to chemical fertilizers which then leach away to
  groundwater.

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Principal erosion mechanisms

• Erosion can be accomplished by one or more
  processes acting in unison that detach and move
  particles toward drains, streams or rivers or up
  and away into the air.
• Rainfall intensity, slope gradient and soil
  erodibility (a function of many variables both
  micro e.g. particle arrangement and macro e.g.
  surface roughness) are major determinants of
  erosion potential.
• Rain splash - raindrops detach soil particles
  from each other and throw them into the air.
• Overland flow - running water entrains, pushes or
  jumps (saltates) particles downhill.
• Wind - turbulent winds pick up small particles or
  saltate larger soil particles over the surface.

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Erosion impacts

• Erosion causes a variety of important off-site
  (ex-situ) resource impacts in addition to
  affecting agricultural or forestry productivity
  on-site (in-situ).
• The silting up of navigable waterways and water
  storage reservoirs affecting transport, carryover
  storage, hydroelectric power production, etc.
• The destruction of clean water habitats such as
  trout streams and offshore coral reefs affecting
  fishery productivity and tourism/recreation.
• The pollution of drinking water supplies with
  suspended sediment and chemicals such as
  pesticides and metals carried down attached to
  soil particles.

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Soil Conservation Approaches

• Soil conservation can best be achieved through
  effective crop management and through the use of
  physical structures.
• A critical element in most soil erosion controls
  is the prevention of the uncontrolled movement of
  water across a sloping soil surface.
• Similarly, efforts against wind erosion are
  designed to reduce wind velocity and surface
  shear above a soil surface.
• Controls also seek to boost soil cohesiveness and
  increase surface friction coefficients, holding
  soil particles together to resist movement.

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Predicting Soil Loss

• Soil scientists have had great success with the
  RUSLE (see p 147 CRO).
• A RKLSCP
• Where A soil loss, R rainfall erosivity, K
  erodibility of soil, L length of slope, S
  steepness of slope, C cover-type and P
  conservation BMP.
• R, K, L and S are fixed factors for a farmers
  land but C and P can be greatly modified by
  changing land management practices.
• Look at box 7.2 for the impacts of typical
  changes.
• The main conservation BMPs that can can be used
  to bring down the soil loss for a given field are
  taught to farmers through Agricultural Extension
  agencies and the USDA Resource Conservation
  Service.

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Some key conservation BMPs

• Minimal or no-till farming - doesnt destroy soil
  structure or remove residues, drills in seeds
  w/out furrows.
• Contour farming - plants rows at right angles to
  water flow
• Strip cropping and crop rotation - pairs mutually
  beneficial crops avoids leaving bare soil
• Alley cropping/agroforestry - grows trees and
  crops together, provides shade, leaf litter,
  nitrogen fixing, etc.
• Terracing - breaks steep slopes into shallow,
  flat steps
• Gully control/reclamation - gullies can quickly
  swallow up topsoil and must be stopped by walls,
  plugs, etc.
• Windbreaks/shelterbelts - slow down winds and
  wind shear.

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Example BMPCrop Residue Management

• Leaving last year's crop residue on the surface
  before and during planting operations provides
  cover for the soil at a critical time of the
  year.
• The residue is left on the surface by reducing
  tillage operations and turning the soil less.
• Pieces of crop residue shield soil particles from
  rain and wind until plants can produce a
  protective canopy.

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Example BMPContour Farming

• Crop row ridges built by tilling and/or planting
  on the contour create hundreds of small check
  dams.
• These ridges or dams slow water flow and increase
  infiltration which reduces erosion.

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Using soil conservation

• The steeper the slope being farmed and the more
  erosive the rainfall, the more important it is
  that farmers use physical barriers to erosion
  (e.g. terraces, drains and spillways) rather than
  more simple surface treatments.
• In developing countries, many techniques have
  been developed, even for peasant farmers working
  on very steep hillsides e.g. Fanya Juu terraces
  in Kenya, stone check dams in Burkina Faso,
  contour terracing in Central America.
• Terracing of slopes for soil stability is an
  ancient art going back to the earliest
  agricultural civilizations.