Soils
for Serious Gardeners
Patricia Steinhilber, Ph.D.
psteinhi@umd.edu
Ag Nutrient Management Program
University of Maryland College Park
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
• phenomena of nature (natural bodies)
mysciencebox.org
Department of Environmental Science and Technology
Agenda
•
•
•
•
•
•
soil composition
hydrologic cycle and soil water
importance of soil testing
fertilizers
soil acidity and liming
soil as biochemical reactors
Department of Environmental Science and Technology
What are Soils?
• reactive, dynamic, three-phase ecosystems
composed of solids, liquids and gases
topsoil
several
days after
rainfall or
irrigation
25%
25%
48%
Minerals
Air
Water
Organic Matter
2%
Department of Environmental Science and Technology
First Things First:
Soil Solids
• mineral or inorganic solids
−often classified based on particle size
• organic solids a.k.a. “organic matter”
Department of Environmental Science and Technology
Major Mineral Classes in a
Soil
Mineral
Class
Size of Mineral Particles
Feel of
Particles
sands
0.05 – 2 millimeters (mm)
gritty
silts
0.002 – 0.05 millimeters (mm)
smooth (like
clays
less than 0.002 millimeters (mm)
sticky when
wet
Department of Environmental Science and Technology
cornstarch or
talcum
powder)
Departmentof
of Environmental
Environmental Science
Department
Scienceand
andTechnology
Technology
60% silt
20% clay
20% sand
Departmentof
of Environmental
Environmental Science
Department
Scienceand
andTechnology
Technology
Department of Environmental Science and Technology
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
The Other Soil Solid
Material: Organic Matter
Humus
75%
10%
Biomass
15%
Residues &
By-Products
Department of Environmental Science and Technology
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
Department of Environmental Science and Technology
Microorganisms, Mesoorganisms and Macroorganisms
nematode
CAD of Ag
springtail (detritivore)
centipede (carnivore)
Department of Environmental Science and Technology
Source: USDA
Department of Environmental Science and Technology
Life in the Soil, Nardi, 2007
Department of Environmental Science and Technology
Biomass: What It Does
• participates in nutrient cycling
− comminute (shred or fragment) plant and animal
residues, using what they can utilize and leaving
behind what they cannot
• mineralization –
− conversion of organic form of an element to an
inorganic form
− amino acid to ammonium
Department of Environmental Science and Technology
Biomass: What It Does
(cont.)
• creation of biopores
−larger organisms move through soil
creating channels or pores
• channels promote water infiltration and create
a healthy balance between large and medium
pores
−disseminate spores and microbes
Department of Environmental Science and Technology
Resources about Soil Organisms
TED Talk – How Bacteria Communicate
http://www.ted.com/talks/bonnie_
bassler_on_how_bacteria_commu
nicate.html
Soil Biology Primer
http://soils.usda.gov/sqi/concepts
/soil_biology/biology.html
Department of Environmental Science and Technology
Residues and By-products:
What They Are
• dead stuff
− 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
Amazing By-product!
• Streptomycin
−bacterial exudate
−soil organism, Streptomyces
• Dr. Selman Waksman
−soil microbiologist at Rutgers University
−recognized the antibacterial function and
isolated the compound
−Nobel Prize in Medicine in 1952
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
Department of Environmental Science and Technology
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
Department of Environmental Science and Technology
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
Department of Environmental Science and Technology
The Other Half of Soil:
Soil Pores (Void Space)
• soil water
− adequate (but not too much) quantity
− adequate supply of nutrients
− minimize runoff and leaching
• soil air
− source of oxygen for roots and most soil
organisms
− constantly enriched with carbon dioxide from roots
and soil organisms
Department of Environmental Science and Technology
The Interplay of Air and Water:
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
Well-Structured Topsoil
(several days after rainfall)
In Soil Air
nitrogen
In the
Atmosphere
79%
oxygen
20.9%
20.6%
carbon dioxide
0.035%
0.300%
Gas
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 a form most plants can use
• micropores (very small pores)
− water is held too tightly to be use to most plants
Department of Environmental Science and Technology
Now, a Video Interlude • Dr. Ian Pepper, soil microbiologist,
Arizona State University
Department of Environmental Science and Technology
Let’s talk about the hydrologic
cycle and soil water…
Department of Environmental Science and Technology
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)
Department of Environmental Science and Technology
Water and Textural Groups
(% water)
coarse- mediumfinetextured textured textured
40
50
60
total
excess
25
15
15
available
12
27 *
20
unavailable
3
8
25
Department of Environmental Science and Technology
Department
Environmental Science
Technology
Department
ofofEnvironmental
Scienceand
and
Technology
Soil Solution
• keep well supplied with nutrients
• minimize adverse conditions for plant
growth
−acidity
−salinity
Department of Environmental Science and Technology
Soil tests can help
identify
limiting nutrient
factors.
Fertilizers can enrich
the soil solution when
soil tests indicate an
insufficiency.
Department of Environmental Science and Technology
Value of Soil Tests
• best pre-plant indicator we have of
potential nutritional products
• mine them for information!
• excellent investment
Department of Environmental Science and Technology
Soil Tests
• best if maintained in the high end of
the “optimum” range or the low end of
the “excessive” range
0
25
low
50
medium
100
optimum
Department of Environmental Science and Technology
excessive
Soil Test Value and Response
to Nutrients
Soil Test
Category
very low and
low
Fertility Index
value (FIV)
Likelihood of
Response
0-25
almost always
medium
26-50
sometimes
optimum
51-100
not often
excessive
>100
almost never
Department of Environmental Science and Technology
What is a fertilizer?
• a compound that contains at least 1
plant nutrient
−ammonium nitrate (NH4NO3)
• 33-0-0
−potassium nitrate (KNO3)
• 13-0-44
−muriate of potash (KCl)
• 0-0-60
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
How do we describe the
nutrient content of
fertilizers?
Ag-Gro-Pro
5-10-15
50 lbs.
This bag contains:
5% nitrogen--10% phosphate--15% potash
or
2.5 lbs. nitrogen (N)
5 lbs. phosphate (P2O5 )
7.5 lbs. potash (K2O)
Department of Environmental Science and Technology
The fertilizer guarantee
• % nitrogen (total)
• % phosphate, P2O5 (citrate-soluble)
• % potash, K20 (soluble)
P2O5 x 0.44 = P
K20 x 0.83 = K
P x 2.27 = P2O5
K x 1.20 = K20
Department of Environmental Science and Technology
Common Commercial N
Fertilizers
Material
Analysis
ammonia
82-0-0
ammonium nitrate
34-0-0
urea
46-0-0
ammonium sulfate
21-0-0
UAN (urea+ammonium 28-0-0 to 32-0-0
nitrate)
Department of Environmental Science and Technology
Less Common Commercial N
Fertilizers
sulfur-coated urea
(SCU)
40-0-0
ureaform
38-0-0
methylene urea
28-0-0
isobutylidene diurea
(IBDU)
30-0-0
Department of Environmental Science and Technology
Common Commercial P
Fertilizers
Material
Analysis
superphosphate
0-18-0
triple superphosphate
0-46-0
monoammonium
phosphate (MAP)
11-48-0
diammonium
phosphate (DAP)
18-46-0
Department of Environmental Science and Technology
Common Commercial K
Fertilizers
Material
muriate of potash
sulfate of potash
potassium-magnesium
sulfate
potassium nitrate
Analysis
0-0-60
0-0-50
0-0-22-22S-11Mg
13-0-44
Department of Environmental Science and Technology
Common Organic Fertilizers
• animal manures
• composts
• by-products of animal and crop
harvesting
−fish meal, bone meal, blood meal
−corn gluten, soybean meal
Department of Environmental Science and Technology
soil acidity:
the adverse
condition on
humid
regions
Department of Environmental Science and Technology
Adverse Conditions Due to
Soil Acidity (Low pH)
Adverse Condition
Soil Component Affected
inadequate supply of
some nutrients
calcium and magnesium may be present in
inadequate amounts in acidic soils
excessive or toxic supply
of some nutrients or
non-essential elements
aluminum, iron, zinc, manganese and copper
can be present in excessive and even toxic
amounts in acidic soils
some nutrients bound in
unavailable forms
phosphorus and molybdenum are so tightly
bound to soil clays that they are not available
to plants in acidic soils
inhospitable environment bacteria, such as those needed for nitrogen
fixation, can not thrive in acidic soils
Department of Environmental Science and Technology
Department of Environmental Science and Technology
Soils are Biochemical
Reactors
• the various components (soil air, soil
water, minerals and organic matter)
interact
• a wide array of chemical and
biochemical processes occur
Department of Environmental Science and Technology
Clays and Humus
• center of chemical reactivity in soil
• engine driving chemical transformations in soil
geoscienceworld.org
fullwiki.org
Department of Environmental Science and Technology
Soil Model
micro meso macro
biomass
organic
solids
residues &
by-products
soil
solution
soil
air
sand
silt
clay layer
minerals
humus
Pore space
inorganic
solids
oxides
clay
Department of Environmental Science and Technology
SSSAJ v. 75
Department of Environmental Science and Technology
SSSAJ v. 75
Solid-Liquid Interface
• adsorption of water and nutrients occurs at
surfaces
• small particles have more surface area than
the same weight of large particles
• surfaces are often charged; most charge is
negative
• negatively charged surfaces attract positively
charged ions (cations)
Department of Environmental Science and Technology
Cation exchange is a big deal!
• cation exchange capacity (CEC)
− a soil’s ability to hold cations on charged surfaces
− a measure of the negative charge on the particles
− correlated to clay and humus content of soils
• exchange phase
− those cations held on surfaces of charged particles
Department of Environmental Science and Technology
Importance of CEC
• nutrient cations exist on exchange
phase
−are protected from leaching
−can replenish the soil solution when plant
uptake or leaching removes nutrients
−the “storehouse” of cationic nutrients
Department of Environmental Science and Technology
Relationship of Textural Class
to Clay Content and CEC
Textural Group
Range of Clay
Content (%)
Cation Exchange Capacity
(CEC)*
sand and loamy
sands
0 - 15
1-5
sandy loams
15 - 20
5 - 10
loams and silt
loams
0 - 25
5 - 15
clay loams, sandy
clay loams, silty
clay loams
20 - 40
15 - 30
clays, sandy clays,
silty clays
>49
>30
*cmol/kg or centimoles per kilogram, or an older mode of expression,
milliequivalent per 100 grams.
Department of Environmental Science and Technology
Soil:
The Most Complex
Biomaterial on Earth
Questions or comments?
Department of Environmental Science and Technology