Soils for Great Gardeners:
Intermediate Version
Patricia Steinhilber, Ph.D.
Ag Nutrient Management Program
University of Maryland College Park
psteinhi@umd.edu
Department of Environmental Science and Technology
What is Soil?
• the living skin of the Earth (Ian
Pepper, soil microbiologist)
• crucible of terrestrial life (Daniel Hillel,
soil physicist)
• the pedosphere
• the interface between the lithosphere,
hydrosphere and atmosphere
(ecologists)
• a medium for plant growth
− edaphology
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mysciencebox.org
Agenda
• review of soil components
• optimizing physical properties
• nutrients storage in soil
• supplying additional nutrients
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What are Soils?
• reactive, dynamic, three-phase ecosystems
composed of solids, liquids and gases
Minerals
Air
Water
Organic Matter
25%
25%
48%
2%
topsoil of a cultivated field
several days after
rainfall or irrigation
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Effect of Soil Texture on Soil
Properties
Soil Property
Textural Groups
coarse-textured
medium-textured
fine-textured
rapid
slow to moderate
very slow
water- and
nutrientholding
capacity
low
moderate
high
susceptibility
to erosion
low
high
moderate
leaching
potential
high
moderate
low
aeration
good
moderate
poor
drainage
Department of Environmental Science and Technology
Organic Matter
Humus
75%
10%
Biomass
15%
Residues &
By-Products
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Biomass: What It Is
• the living component of the soil
• consists of a range of creatures
− as small as microscopic viruses & bacteria
− as large as roots, worms and other
creatures that are visible to the unaided
eye
− and everything between
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Source: USDA
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Biomass:
Microorganisms
• mineralization
−conversion of organic form of an element
to an inorganic form
−proteins to amino acids to ammonium
−nucleic acids to phosphate
−bacteria are responsible for 90% of
mineralization
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Biomass:
Meson-organisms and Macroorganisms
• litter transformers
− comminute (shred or fragment) plant and
animal residues, using what they can utilize
and leaving behind what they cannot
−inoculate materials with gut microbes
−increases surface area and access for
microbes
Department of Environmental Science and Technology
Biomass:
Meso-organisms and Macro-organisms
• creation of biopores (ecosystem
engineers)
−move through soil creating channels or
pores
• channels promote water infiltration and create
a healthy balance between large and medium
pores
−disseminate spores and propagules
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Helping One Another:
Symbiosis in Soil
mycorrhiza – a symbiotic relationship
between certain fungi and higher plants
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Below-ground
Mutualism
10% - 20% of C fixed
by photosynthesis is
transported to microbes
In the rhizosphere
(4” per hour)
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Residues and By-products:
What They Are
• dead stuff (detritus)- crop residues, dead
roots and bodies of soil creatures
• by-products - materials that plant roots
and soil creatures release or exude into
the soil
Department of Environmental Science and Technology
Residues and By-products:
What They Do
• fuel and nutrients for soil organisms
− energy and nutrient source for most of the soil
creatures
• formation and maintenance of soil aggregates
(structure or architecture)
− sticky and gummy by-products of residue
decomposition hold soil particles together in
clumps or aggregates
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Last But Certainly Not Least:
Humus
• relatively stable end product of residue
decomposition
• composes the majority of organic matter
• resists further decomposition (1% per
year)
• it is not a good nutrient or energy source
for soil creatures
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Humus: What It Does
• very small in particle size & high surface
area
• charged sites at many locations on the
surface
• effective at holding water and nutrients
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Keep the Creatures Content
(well fed)
• better water infiltration
• improved aeration
• improved soil tilth, soil health or soil
quality
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The Interplay of Air and Water in
Soil Pores: Soil Aeration
• The exchange of O2 and CO2 between the soil
pores and the ambient atmosphere
Hillel
Department of Environmental Science and Technology
Comparison of Gasses in the
Atmosphere and in Soil Air of a
Garden Soil
(several days after rainfall)
Gas
In Soil Air
nitrogen
In the
Atmosphere
79%
oxygen
20.9%
20.6%
carbon dioxide
0.035%
0.300%
79%
Department of Environmental Science and Technology
Balance Between Water
and Air
• macropores (large pores)
- drain quickly after rain or irrigation
- allow rapid infiltration of rainfall and
replenishment of oxygen in the root zone
• mesopores (medium-sized pores)
- “storage pores”
- hold water in form most plants can use
• micropores (very small pores)
- water is held too tightly to be use to most plant
- habitat for microbes
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Soil Solution (Soil Water):
Ideal Management
• adequate supply at all time for growing
plants
• adequate supply of dissolved nutrient at
all times
• minimize losses of water (and nutrients)
• minimize adverse condition for plant
growth
−acidity
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Biological Classification
of Soil Water
• excess or gravitational water
- water that drains from soil 1-3 days after a rainfall
or irrigation (in macropores)
• available
- water that is in a form crop plants can use (in
mespores)
• unavailable
- water that is held to tightly by the soil to be
usable by most crop plants (in micropores)
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Nature and Properties
Of Soils, Brady and Weil
similar diagram on
p. 67 of the MANMH
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Why Liming to Reduce Soil
Acidity is Helpful…
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Soils Store Stuff
• some types of materials but no others
• adsorption
−“ad” not “ab”
−attraction of a material for a surface
−layer silicate minerals, metal oxides and
humus
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Soil Model
biomass
organic
solids
residues &
by-products
soil
solution
soil
air
sand
silt
clay layer
minerals
humus
Pore space
inorganic
solids
oxides
clay
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SSSAJ v. 75
Department of Environmental Science and Technology
SSSAJ v. 75
The Adsorption Story
• layer silicate minerals (clay minerals)
−net negative charge (temperate zone)
−adsorb cations (Ca+2, H+, …)
• metal oxides
−iron oxides in humid East
−adsorb large oxyanions (phosphate)
• humus
−several types of reactive surfaces
−adsorbs cations & organic molecules
Department of Environmental Science and Technology
Cation Exchange Capacity
(CEC)
• adsorption and desorption of cations at
negatively charged sites
−layer silicates and humus
−protected from leaching
−replenish the soil solution when plant
uptake or leaching removes nutrients
−the “storehouse” of cationic nutrients
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Soil Texture and Cation
Exchange Capacity
Soil Texture
Cation Exchange Capacity
sands
fine sandy loams
loams and silt loams
clay loams
clays
cmolc/kg
1-5
5-10
5-15
15-30
>30
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Soil Testing for Fertility Status
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Nutrient Applications
• commercial inorganic fertilizers
• commercial organic fertilizers
• compost and manure
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What is a fertilizer?
• a compound that contains at least 1 plant
nutrient
− ammonium nitrate (NH4NO3) – 34-0-0
− potassium nitrate (KNO3) – 13-0-44
− Epsoma’s Garden-Tone – 3-4-4
• hydrolyzed feather meal, poultry litter, cocoa meal, bone
meal, alfalfa meal, greensand, humates, potassium
sulfate, sulfate of potash-magnesia
• most materials are by-products of animal or crop
production
Department of Environmental Science and Technology
The fertilizer guarantee
• % nitrogen (total)
• % phosphate, P2O5 (citrate-soluble)
• % potash, K20 (soluble)
• Epsoma’s Garden-tone (3-4-4)
−0.8% water-soluble N
−2.2% water-insoluble N (organic N)
Department of Environmental Science and Technology
Where do N, P and K in
fertilizers originate?
• N originates from the atmosphere
−natural gas is an energy source and a
hydrogen source
• P originates from rock phosphates
• K originates K-bearing minerals, usually
chlorides or sulfates
Department of Environmental Science and Technology
By-Product and Organic
Sources
• animal manures and composts
• rock phosphate & green sand
• bone meal, crab meal, blood meal,
feather meal
• alfalfa meal, corn gluten, cottonseed
meal
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Let’s Compare
synthetic
flexibility
tailor nutrient
sources to soil
test and crop
requirements
long-term
finite supply of
sustainability Earth materials
by-product
get what you get
utilize materials that
already exist and
might otherwise be
considered a waste
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Let’s Compare
effect on
soil
microbes
effect on
water
quality
(due to
runoff and
leaching)
synthetic
minimal and
very localized
minimal when
used at
appropriate
rates
by-product
organic materials
will temporarily
cause a flush of
microbial growth
often leads to
high P levels in
soil and can
contribute to
poor water
quality
Department of Environmental Science and Technology
Want adequate plant-available
nitrogen for a vegetable garden?
(raised bed 4 x 12 feet)
100 pounds
30
pounds
4
ounces
urea
6 pounds
layer manure
dairy manure
horse manure
Department of Environmental Science and Technology
Are Organic Sources
Sustainable?
• rock phosphate and green sand – no
−mined earth materials with very low plant
availability
• by-products
−where are they coming from?
−composted cow manure from Oklahoma
Department of Environmental Science and Technology
Lessons from the Ag
Community
• test soil at least every 3 years
• be informed about manure content
• control erosion
• use cool-season cover crops
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Questions? Comments?
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