Download Soil Particles - Georgia Organics

Soil is described in terms of physical and
biological components. Both of these
components are important in
establishing a foundation of soil
knowledge, essential for good,
sustainable management.
Soil Components
Courtesy University of Georgia Agriculture Education Dept.
Organic Matter
 Living – about 15% - living leaf
 Recently dead - produce nutrients,
- dead leaf
 Long dead – stable, holds nutrients, humus-
decomposed leaf
Air and Water
 Occupy the same pore space
 Essential for soil life
Soil Particles
 Sand – largest
O
20-40x
 Silt – medium
o
25x
 Clay – smallest
.
1
 Pore space varies with size of particles
Soil Texture
Courtesy USDA
Characteristics of the Various Soil
Classes
Quality
Sand
Silt
Clay
Looseness
Good
Fair
Poor
Air Space
Good
Fair to Good Poor
Drainage
Good
Fair to Good Poor
Tendency to
form clods
Poor
Fair
Good
Ease of working Good
Fair to Good Poor
Moisture holding Poor
ability
Fertility
Poor
Fair to Good Good
Good
Fair to Good
Soil Horizon
Courtesy of USDA
Soil Structure Categories
Courtesy University of Georgia Agriculture Education Department
Factors which affect soil
structure
 Organic Matter Content
 Soil Organisms
 Soil Colloids
 Tillage
 Freezing and Thawing
 Water Movement
Organic Matter Content
May be increased by
 Adding manure
 Growing cover crops
May be decreased by
 Tillage
 Hot,
Humid
environment
Soil Organisms
 Bacteria in soil have sticky
substance on cell wall.
 This “glue” holds soil particles
to bacteria and to each other.
 This binds soil particles into larger particles.
 Fungi produce thread-like structures called
mycelia which bind soil particles and peds
making larger particles.
Soil Colloids
 Very small particles with a weak electric
charge.
+o
o The charge causes water to bind to particles.
 As water evaporates, colloids and larger
particles draw together, creating even larger
particles.
Tillage
Positive effect
 Creates air space in soil, allowing organisms
needed oxygen
Negative effect
 Disrupts the soil structure and kills the larger
organisms such as earthworms.
 If done when too wet or dry, may destroy the
soil structure of the tillage zone.
Freezing and Thawing
 As water in soil freezes it expands, pushing
soil particles apart.
 This opens pore spaces for air.
 As water thaws, pore spaces tend to remain
temporarily.
 This can be enough to encourage soil
organisms, by allowing more oxygen.
Water Movement
Positive +
 As water moves vertically through the soil some of the
water is stored, and can be used by crops.
Negative  Vertical movement of water can cause leaching of
minerals.
 Vertical movement in soil freshly tilled may cause
collapse and less pore space.
 Vertical movement in clay can result in a hard pan.
 Horizontal movement may cause erosion.
•
pH
 pH is the measure of the concentration of hydrogen
ions.
 It is measure on a logarithmic scale going from 1-14
with 1 being the most acidic (most hydrogen ions)
and 14 the most basic (least hydrogen ions).
 pH of soil is important because certain chemical
reactions only occur in certain range.
 Not as important for organic systems, because they
depend on biological reactions more than chemical
reactions.
Soil Ecosystem
 Biological model of soil management as
opposed to the chemical model is the major
key to good soil management.
 More individual life forms in soil than live on
the surface of the earth.
 4 billion bacteria, 1 million fungi per teaspoon
of soil
 Perform many functions to support plant life
Biodiversity
 Soil fertility is related to having a large
number of different types of life forms.
 Different crops depend on different
ecosystems.
 Annual field crops – bacteria dominated
 Woody perennials – fungi dominated
Copyright Soil Foodweb Inc., Dr. Elaine Ingham, used with
permission
Producers-herbivores-predators-higher predatorsdecomposers
How the soil food web benefits
plant life
 Builds soil structure
 Disease suppression
 Improves nitrogen retention
 Mineralizes nutrients
 Produces plant growth hormones
 Improves crop quality
Builds soil structure
 Pore space from burrowing.
 Sticky substance from bacteria holds particles
together.
 Fungal hyphae holds soil conglomerates
together
Disease Suppression
 Population density – “good” organisms take
up all the niches in the soil so there is no
room for disease organisms.
 Antibiotics – some fungi produce antibiotics
which destroy certain bacteria.
Improves nitrogen and other
nutrient retention
 Waste given off by organisms contain
nutrients.
 Soil organisms retain nutrients in their bodies.
 When organisms die the nutrients are
released.
Mineralizes nutrients
 Organisms can take nutrients that plants can’t
use and change them into a form that plants
can use, such as nitrogen.
 Chelation – Some nutrients are in a form that
is not available to plants. Soil organisms can
add an organic molecule, changing their form
and making them available to the plant.
Decomposition of plant toxins
 Natural plant toxins, such as phenols and
tannin, can be detoxified when there is a
diversity of soil organisms.
 Man-made toxins can also be detoxified by a
diversity of soil organisms.
Produces plant growth hormones
 Some plant hormones, such as auxin, can be
produced by some soil organisms.
 Auxin promotes root growth.
Improves crop quality
 Not well studied
 Relation between soil biodiversity and
nutrient quality of food grown in soil
 More proteins, vitamins, anti-oxidants present
when soil had a diversity of organisms
present
Importance of Carbon
 Basis for all life
 Cells 70-95% water – the rest carbon
compounds
 All organic compounds are carbon
compounds
Carbon Cycle
Courtesy of Windows to the Universe,
http://www.windows.ucar.edu
Importance of nitrogen
 Nitrogen is part of proteins and nucleic acids
so is needed by every living organism
 Free nitrogen (nitrogen gas) makes up about
78% of the atmosphere but is not available to
plants
 Plants must absorb nitrogen compounds from
the soil
Nitrogen Cycle
Courtesy of Hach Company
Summary
 Soil properties



Composition of soil
Texture – sand, silt, clay, loam
Structure – pore space
 Soil biology, ecosystems


Soil life produces nutrients, builds structure
Trophic levels
 Soil nutrient cycles


Carbon cycles through organic matter and carbon dioxide
Nitrogen cycles through nitrogen gas and nitrogen compounds to
proteins and nucleic acids