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Chapter 8 . Soil: Foundation for Land Ecosystems. 8.1. Soil and plants. Soil vs. Dirt. Soil is much more than just “dirt”. Top Soil Formation. Productive topsoil involves dynamic interactions among the organisms, detritus, and mineral particles of the soil. Soil Characteristics - Texture.

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chapter 8

Chapter 8

Soil: Foundation for Land Ecosystems

soil vs dirt
Soil vs. Dirt
  • Soil is much more than just “dirt”
top soil formation
Top Soil Formation

Productive topsoil involves dynamic interactions among the organisms, detritus, and mineral particles of the soil

soil characteristics texture
Soil Characteristics - Texture
  • Parent material
    • Mineral material of the soil, has its origin in the geological history of the area
    • Parent material could be rock, or sediments deposited by wind, water, or ice
    • Eventually parent material is broken down by natural weathering
  • Gradual physical and chemical breakdown
    • Physical
      • Ex – ice wedging, erosion
    • Chemical
      • Formation of H2CO3 of organisms, or other chemical erosion
  • As rock weathers, it breaks down into small stones (soil separates)
soil separates
Soil Separates
  • Sand
    • Particles from 2.0 – 0.02 mm in size
  • Silt
    • Particles from 0.02 down to 0.002 mm
  • Clay
    • Particles finer than 0.002 mm
  • Sand, silt, and clay particles constitute the mineral portion of soil
  • Soil texture refers to the relative proportions of each type of the particle in a given soil
  • Loam
    • 40% sand, 40% silt, 20% clay
    • Best type of agricultural soil
Major classes of soil are indicated on the triangle. For clay, read across horizontally; for silt, read diagonally downwards; for sand, read diagonally upwards to the left. The texture content of any soil should total 100% if the triangle is read properly.
  • Larger Particles
    • Like sand
    • Have larger spaces separating them than smaller particles
      • Visualize difference between packing softballs and golf balls in the same size container
  • Smaller Particles
    • Such as silt and clay
    • Have more surface area relative to their volume than larger particles
      • (Visualize cutting a block in half again and again. Each time you cut it, you create two new surfaces, but the total volume of the block remains the same)
  • Nutrient ions and water molecules tend to cling to surfaces
    • Smaller particles will have a larger surface area, therefore tend to hold on to these ions and molecules
  • Soil texture affects workability
    • Ease with which a soil can be cultivated (typically for agriculture)
soil profiles
Soil Profiles
  • Soil formation creates a vertical gradient of layers that are often quite distinct
    • Layers are known as horizons
    • Vertical slice through a horizon is called soil profile
      • 5 horizons
o horizon
  • Consists of dead organic matter (detritus) deposited by plants, leaves, stems, fruits, seeds, etc.
  • High in organic content
  • Primary source of energy for the soil community
  • Towards the bottom of the O-horizon, the decomposition process is well advanced and the original materials may be unrecognizable
  • At this point the material is dark and is called HUMUS
    • Not to be confused with the chick-pea dip, hummus.
a horizon
  • Below O-Horizon
  • Mixture of mineral soil from below and humus above
  • AKA Topsoil.
  • Fine roots from O-horizon can permeate this layer
  • A-Horizon is usually dark because of the humus, and may be shallow or thick depending on the ecosystem
e horizon
  • E stands for eluviation
    • The process of leaching (dissolving away) many minerals due to the downward movement of water
  • This layer is often paler in color than the two layers above it
b horizon
B Horizon
  • Characterized by the deposition of minerals that have leached from the A and E horizons,
    • often high in FE, AL, CA, and other minerals.
  • Referred to as the SUBSOIL
  • Often high in clay and is reddish or yellow in color
c horizon
  • Parent material originally occupying the site
  • Weather rock, glacial deposits, volcanic ash
  • Affected little by the biological and chemical processes that go on in the overlying layers
soil classes
Soil Classes
  • Soil scientists have crated a taxonomy of soils
    • Order, suborder, group, subgroup, families
  • Literally hundreds of soil classes
  • There are 12 major orders
    • We will investigate 4 orders
  • Fertile, dark soils found in temperate grassland biomes
  • World’s best agricultural soil
  • Deep A-horizon and are rich in humus and minerals
  • Precipitation is insufficient to leach the minerals downward
  • Found in
    • Midwestern US, temperate Ukraine, Russia, Mongolia, Argentina
  • Tropical and subtropical rainforests
  • Layer of iron and aluminum oxides in the B-horizon
  • Little or no O-horizon
    • Due to rapid decomposition of the plant matter
    • Most minerals are in living plant matter
      • Most oxisols are of limited fertility for agriculture
  • Widespread
  • Moderately weathered forest soil
  • Not deep, but have well developed O, A, E, and B horizons
  • Typically of the moist, temperate forest biome
  • Suitable for agriculture if they are supplemented with organic matter or mineral fertilizers
  • Widespread soils of dry-lands and deserts
  • Relatively unstructured in soil horizons
  • Thin, lightly colored
  • Irrigation used on these soils usually leads to salinization
    • High evaporation rates draw salt to the surface
soil and plant growth
Soil and Plant Growth
  • Plants need a root environment that supplies optimal amounts of mineral nutrients, water, and air
  • The pH and salinity of the soil are also critically important
  • Soil Fertility
    • Soils ability to support plant growth
    • Refers to the presence of proper amounts of nutrients
mineral nutrient and nutrient holding capacity
Mineral Nutrient and Nutrient-Holding Capacity
  • Phosphate, Potassium, Calcium and other ions are present in rocks and become available to roots through weathering
    • Processes is usually too slow for plant growth
    • Nutrients that support plant growth are supplies mostly though breakdown of detritus
  • Nutrients may be washed from the soil as water moves though it
  • Lessens soil fertility
  • Contributes to pollution
  • Soils capacity to hold nutrient ions until they are absorbed by roots becomes very important
  • Unavoidable removal of nutrients from soil with each crop
    • Nutrients absorbed by plant are contained in harvested material
    • Therefore agricultural systems require input of fertilizers



Chemical formulations of required nutrients, with out any organic matter included

Much more prone to leaching than organic fertilizers

  • Includes plant or animals wastes or both
  • Manure and compost
    • Rotten organic material
  • Includes nitrogen fixing plants
    • Alfalfa, soy (legumes), peas, lentils
water water holding capacity
Water/Water Holding Capacity
  • Transpiration
    • Water is absorbed by the roots of plants, passed up through plant, and exit as water vapor through microscopic pores in leaves (stomata)
  • Water is resupplied to soil by rainfall or irrigation
  • Soils ability to allow water to soak in is important
water holding capacity
Water-Holding Capacity
  • Poor water holding capacity implies that most of the infiltrating water percolates on down below the reach of the roots
  • Evaporative water loss from the soil surface
  • Depletes soil’s water reservoir without serving the needs of plants
plant soil water relationships
Plant-Soil-Water relationships

Water lost from the plant by transpiration must be replaced from a reservoir of water held in the soil. In addition to the amount and frequency of precipitation, the size of this reservoir depends on the soil’s ability to allow water to infiltrate, to hold water, and to minimize direct evaporation

  • Roots need to “breathe”
  • Living organs need a constant supply of oxygen for energy via metabolism (respiration)
  • Overwater fills air spaces preventing aeration
  • Compaction
    • Or packing of the soil due to large amounts of traffic (foot or vehicular) reduces aeration
relative acidity ph
Relative Acidity (pH)
  • Refers to the acidity or alkalinity of any solution
  • Most plants do best with a pH near neutral
  • To support good crop soil must
    • Have good supply of nutrients and a good nutrient holding capacity
    • Allow infiltration and have a good water holding capacity/resist evaporative water loss
    • Have a porous structure that permits good aeration
    • Have a pH near neutral
    • Have a low salt content
  • Detritus accumulated on and in the soil supports a complex food web
    • Including bacteria, fungi, protozoans, mites, insects, millipedes, spiders, centipedes, earthworms, snails, slugs, moles, and other burrowing animals
    • Most numerous and important – BACTERIA
  • As organisms feed, bulk of detritus is consumed through cellular respiration
  • 6CO2 + C6H12O66CO2 + 6H20 + ENERGY
  • However, each organism leaves a part of the detritus undigested
    • This is humus
humus and the development of soil structure
Humus and the development of soil structure

On the left is a humus-poor sample of loam. It is relatively uniform, dense “clod”. On right is a sample of the same loam, but rich in humus. Note that is has a very loose structure, composed of numerous aggregates of various sizes

soil structure
Soil Structure
  • Refers to soil particle arrangement (whereas texture refers to size)
    • Ex a loose soil structure is ideal for infiltration, aeration, and workability
soil interactions
Soil Interactions
  • Between soil and biota
  • Roots of some plants and a fungi called mycorrhizae
    • Mycorrhizae go deep into the detritus, absorbs nutrients, and transfers them directly to the plant
      • No loss of nutrients due to leaching
      • Example of a mutualistic relationship!
soil interactions1
Soil Interactions
  • Nematodes
    • Small words that feed on living roots
      • Can be very destructive to some agricultural crops
    • In flourishing soil ecosystems nematodes are controlled by other soil organisms
      • Like fungus
soil enrichment
Soil Enrichment
  • Green plants protect soil
    • Protects from erosion
    • Reduces evaporative water loss
      • Relationship between plants and soil can be broken easily
        • Topsoil depends on additions of detritus and water
  • Loss of humus and the consequent collapse of topsoil
  • When key soil attributes required for plant growth or for other ecosystem services deterorate over time, the soil is considered degraded
importance of humus to top soil
Importance of humus to top soil

Topsoil is the result of a balance between detritus additions and humus-forming processes, and their breakdown and loss. If additions of detritus are insufficient, the soil will gradually deteriorate

  • Global Assessment of Soil Degradation map
    • Data came from questionnaires, very little information has been validated by collecting data on actual soil conditions or information on crop productivity (but it is still cited)
    • Starting to change
      • Ex – Burkina Faso
  • Soil and humus particles are picked up and carried away by water or wind.
  • Occurs anytime soil is bared and exposed to the elements
splash erosion
Splash Erosion
  • Water erosion starts here
    • Impact of falling raindrops breaks p the clumpy structure of the topsoil
    • Dislodge particles wash into paces between aggregates, clogging pores, and thereby decreasing infiltration and aeration
    • Results in more water running off
sheet erosion
Sheet Erosion
  • Once pores are closed in, sheet erosion occurs
  • Water converges into rivulets and streams
    • Great velocity and can pick up more soil
    • Result is the erosion into gullies (gully erosion)
desert pavement
Desert Pavement
  • Differential removal of particles
    • Lighter particles of hums and clay are first to be removed
    • Rocks, stones, coarse sand remain behind
      • Remaining soil becomes more and more coarse, and finally rocky
  • Cover 41% of Earth’s land area
  • Defined by precipitation, not temperature
    • 10 – 30 inches yearly
  • Home to 2 billion people
  • United Nations Convention to Combat Desertification
    • Issues on funding projects to reverse land degradation
    • Local communities gather and share traditional knowledge of effective dryland use
causing and correcting erosion
Causing and Correcting Erosion
  • 3 major practices expose soil to erosion and lead to degradation
    • Overcultivation
    • Overgrazing
    • deforestation
  • Fist step in growing crops – plowing for weed control
  • Exposes loose soil
  • Soil may remain bare for a while before vegetative cover is planted
negatives of plowing
Negatives of Plowing
  • Splash erosion
    • Seals surface so that aeration and infiltration are decreased
  • Weight of tractors add to compaction of soil
  • Accelerates oxidation of humus and evaporation
crop rotation
Crop Rotation
  • Cash every third year
    • Hay and clover in between
      • Hay – adds organic matter
      • Clover – adds nitrogen
  • Not very cost effective for farmers
no till agriculture
No Till Agriculture
  • Field is first sprayed with herbicide to kill weeds
  • Crop seeds are planted
  • At harvest, process is repeated
    • Waste from previous crop becomes detritus
    • Soil is never left exposed
  • Inorganic versus organic fertilizers
    • Chemical fertilizer lacks organic matter to support soil organisms to build soil structure
contour plowing
Contour Plowing

Cultivation up and down a slope encourages water to run down furrows and may lead to severe erosion. This problem is reduced by plowing along the contours at a right angle to the slope


Belts of trees around farm fields that break the wind and protect soil

  • Natural Resource Conservation Services
    • Established in response to the dust bowl
    • Provides info regarding soil or water conservation practices
    • Performs inventory of erosion losses in the U.S.
  • 65% of dryland areas are rangelands
  • Native plants cleared out for grass for cattle to graze on
  • Usually occurs because rangelands are public lands not owned by the people who own the animals
    • “tragedy of the commons”
  • Forest ecosystems are efficient at holding and recycling nutrients and for holding water
  • Sediments are carried into streams and rivers, clogging changes
    • Intensifies floods
    • Fills reservoirs
    • Kills fish
    • Eutrophication
    • Groundwater decreases
  • Supplying water to croplands by artificial means
    • Increased crop production in arid regions
    • Water is usually diverted from rivers though canals and flooded through furrows in fields
      • Flood irrigation
center pivot irrigation
Center-Pivot Irrigation
  • Water is pumped from a central well through a big sprinkler that slowly turns
  • Result of accumulation of salts in and on the soil where plant growth is suppressed
  • Occurs because irrigation water contains at least 200-500 ppm dissolved salts
soil conservation
Soil Conservation
  • Practiced at two levels
    • Individual land owner
    • Public policy