SOIL porosity

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This section is in addition to Chapter 3
Produced slowly (200-1000 years typically)
 by weathering of rock, deposition of
 sediments, and decomposition of organic

Soil horizons – separate zones within soil
Soil profile – cross-section view of soil
O horizon – surface litter
A horizon – top soil, made up of inorganic
 particles (clay, silt, sand) and humus (organic
 particles from decomposed organisms)
   Dark topsoil is richer in nutrients
  Releases water and nutrients slowly
  Provides aeration to roots
  Healthy soil contains many nematodes and
   bacteria, fungi, etc.
                    Oak tree             Lords and
                               Word      ladies
                                                     Dog violet
                                        Earthworm                                  Organic debris
                                                                    Grasses and    Builds up                  Rock
                                             Millipede              small shrubs
                                                            Mole                               Moss and       fragments
                    Fern                   Honey                                               lichen

O horizon
Leaf litter
A horizon

B horizon                                                                                              Immature soil

                                                                                             Young soil
C horizon
 Parent                                                                            Mite

              Root system                    Red earth                              Fungus
                                             mite      Springtail

                               Mature soil                           Bacteria
                                                                                                    Fig. 10.12, p. 220
               Poor topsoil
Grey, yellow and red are not the colors of
 healthy topsoil
Generally means that soil is lacking nutrients

Best soil is called loam with equal parts sand,
 silt, clay and humus

Leaching – dissolving and carrying nutrients
 (or pollutants) through soil into lower layers
  B – horizon and C - horizon
B – Subsoil mostly broken down rock
 with little organic matter

C- parent material broken down rock on
 top of the bedrock
Texture – relative amount of different
 sized particles present (sand, silt, clay)
Porosity – volume of pore space in the
Permeability – the ability of water to
 flow through the soil
         Water              Water

High permeability   Low permeability

 Sandy soil           Clay soil
Clay – high porosity, low permeability
Sand – high permeability, low porosity

Acidity is another factor
Where rain is low, calcium and other
 alkaline compounds may build up (sulfur
 can be added – turns to sulfuric acid by
                                                                 Forest litter
                                                                 leaf mold                                   Acid litter
                            Acidic                                                                           and humus
                            colored                              Humus-mineral                               Light-colored
                            humus                                mixture                                     and acidic
                                                                 Light, grayish-
                                                                 brown, silt loam
                            Iron and
                            aluminum                                                                         Humus and
                                                                 Dark brown
                            compounds                                                                        iron and
                            mixed with                           Firm clay
                            clay                                                                             compounds

Tropical Rain Forest Soil                Deciduous Forest Soil                      Coniferous Forest Soil
(humid, tropical climate)                (humid, mild climate)                       (humid, cold climate)

                                                                                                     Fig. 10.15b, p. 223
                     of closely
                     Weak humus-                             dark,
                     mineral mixture                         and rich
                                                             in humus
                     Dry, brown to
                     with variable
                     accumulations                           Clay,
                     of clay, calcium                        calcium
                     carbonate, and                          compounds
                     soluble salts

   Desert Soil                            Grassland Soil        Fig. 10.15a, p. 223
(hot, dry climate)                      (semiarid climate)
            Soil erosion
Causes – mainly water and wind
Human induced causes – farming,
 logging, mining, construction,
 overgrazing by livestock, off-road
 vehicles, burning, and more (go us!)
              Soil erosion
  Uniform loss of soil, usually when water
   crosses a flat field
  Fast flowing water cuts small rivulets in soil
  Rivulets join to become larger, channel
   becomes wider and deeper, usually on
   steeper slopes or where water moves fast
              Global soil loss
 This is a major problem world wide
 Have lost about 15% of land for agriculture to soil
    Overgrazing
    Deforestation
    Unsustainable farming
 Also 40% of ag land is seriously degraded due to soil
  erosion, salinization, water logging and compaction
Moderate                 Severe              Very Severe

                                                Fig. 10.21, p. 228
Desertification of arid and semiarid lands
                      Areas of serious concern

                      Areas of some concern

                      Stable or nonvegetative areas

                                                 Fig. 10.19, p. 226
Global soil erosion
Turning productive (fertile) soil into less
 productive soil (10% loss or more)
  Surface mining
  Poor irrigation techniques
  Poor farming techniques
  Soil compaction
 As water flows over the land, salts are leached out
 When water irrigates a field it is left to evaporate
 This repeated process causes the dissolved salts to
  accumulate and possibly severely reduce plant

 Fields must be repeatedly flushed with fresh water to
  remove salt build up
When fields are irrigated they allow water to
 sink into the soil.
Winds can dry the surface
As more water is applied the root area of
 plants is over saturated reducing yield

As clay is brought to subsoil levels it can act
 as a boundary for water infiltration
              Evaporation   Transpiration


                                     Less permeable
                                     clay layer

                                                          Fig. 10.22, p. 229
         Soil conservation
Conservation tillage – (no till farming)
 disturb the soil as little as possible

Reducing erosion also helps – save
 fuel, cut costs, hold water, avoid
 compaction, allow more crops to be
 grown, increase yields, reduce release
 of carbon dioxide
          Soil conservation
Terracing – making flat growing areas on
Contour farming – planting crops
 perpendicular to the hill slope, not parallel
Strip cropping – planting alternating rows of
 crops to replace lost soil nutrients (legumes)
Alley cropping – planting crops between rows
 of trees
Control planting and strip cropping
                                      Fig. 10.24b, p. 230
Alley cropping
                 Fig. 10.24c, p. 230
            Fig. 10.24a, p. 230
             Soil conservation
 Gully reclamation – seeding with fast growing native
  grasses, slows erosion or “reverses” it
    Also building small dams traps sediments
    Building channels to divert water or slow water
 Windbreaks – trees planted around open land to
  prevent erosion
    Retains soil moisture (shade, less wind)
    Habitats for birds, bees, etc.
 Land classification – identify marginal land that
  should not be farmed

             Fig. 10.24d, p. 230
              Soil fertility
Inorganic fertilizers – easily transported,
 stored, and applied
  Do not add humus – less water and air
   holding ability, leads to compaction
  Only supply about 3 of 20 needed nutrients
  Requires large amount of energy for
  Releases nitrous oxide (N 2O) during
   production, a green house gas
               Soil fertility
Organic fertilizers – the odor is a problem
Animal manure – difficult to collect and
 transfer easily, hard to store
Green manure – compost, aerates soil,
 improves water retention, recycles nutrients
Crop rotation – allows nutrients to return to
 soil, otherwise same crop continually strips
 same nutrient, keeps yields high, reduces