Joel Gruver

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A metaphorical approach to understanding soil
science
Joel Gruver Soil Science NCSU
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What is a soil pedon ?
a soil individual
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INPUTS
OUTPUTS
Parent material Climate Topography Organisms T
ime
SOIL FORMATION
TRANS- LOCATIONS
TRANS- FORMATIONS
Adapted from Stewart (1990)
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Soils in the southeast are diverse !
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Young Soils
Old Soils
Very acid Thin top soil Low phosphorus
http//www.mo15.nrcs.usda.gov/features/gallery/
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Sand
(0.05 2 mm)
Silt
(0.002 0.005 mm)
Primary mineral particles
Clay
(lt 0.002 mm)
http//ianrpubs.unl.edu/fieldcrops/graphics/soilh2
o2.gif
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12 textural classes
Textural triangle
http//www.oneplan.org/Images/soilMst/SoilTriangle
.gif
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Loamy soils
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Texture strongly influences soil hydraulic
properties
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Clay and humus are the soil skin
http//www.ccma.csic.es/dpts/suelos/
Brady and Weil (2002)
clay minerals
humus
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• What is humus ????
• While it is unlikely that any 2 humus molecules
  are identical the diverse products of
  humification have many common characteristics
• Extreme chemical complexity
• Resistance to further decomposition
• High specific surface and negative charge
• Dark color

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Why is the soil skin important ?

• Adsorption of water films
• Adsorption of organic and inorganic chemicals
• Ion exchange
• Catalysis of chemical reactions
• Habitat for bacteria

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Crumb structure
Compacted soil
The soil fabric
http//www.grdc.com.au/growers/gc/gc48/conference1
.htm
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Granular crumb structure
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20 yrs of bluegrass, then 5 yrs CT corn
25 yrs of CT corn
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After adding water
25 yrs of conventional corn
20 yrs of bluegrass, then 5 yrs conventional corn
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Plasticity vs. Friability
Friable soils crumble easily when subjected to
mechanical stress. Tillage requires less draft !
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Angular blocky structure enhances drainage and
root growth below the plow layer
Ray Weil
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Biofilms cover lt 5 of soil surfaces
Soil is habitat
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Soil presents its inhabitants with many
constraints
Tortuous, loosely connected and highly
constricted porosity
Low quality nutritional resources
Structural rigidity
Environmental fluctuations
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Average microbial growth rates are hundreds of
times slower in field soil than in a petri dish
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The metabolic potential of soil microbial
communities greatly exceeds available organic
substrates
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Why do microorganisms behave so differently in
soil than they do in a petri dish ?
http//picturethis.pnl.gov/im2/8208417-5cn0/820841
7-5cn.jpg
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La Paradoja de la Bella Durmiente
The Sleeping Beauty Paradox
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Who are the prince charmings ?
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Soil organisms are concentrated
in HOT SPOTS !
Porosphere
Drilosphere
Aggregatusphere
Detritusphere
Rhizosphere
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Most of the bacteria in soil are unculturable
volume gt 0.1 ?m3, gt 40 culturable volume lt 0.1
?m3, lt 1 culturable
t2
t3
t1
Starvation time
(Morita, 1989)
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Aestivating earthworm in East Texas forest soil
SSAJ 69(3) cover
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Most plants are in a state of dormancy
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Soil fauna have a disproportionate impact on soil
nutrient and energy fluxes
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Soil fauna are relatively mobile
but have limited digestive ability
Soil microflora are relatively immobile but have
almost unlimited digestive ability
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Microfoodwebs
I want some bacteria for lunch !
Microfauna (e.g. protozoa and nematodes) harness
the microfloras digestive abilities by grazing
on them.
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Exhabitational digestion
When microarthropods feed on fresh litter, their
fecal pellets contain shredded, moistened and
mixed but largely undigested residues.
Reingestion of fecal pellets after a few days of
microbial activity greatly increases
assimilation.
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Rumen digestion enables earthworms and termites
to utilize complex substrates
Ecosystem Engineers
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The Soil Stomach

• Bacteria
• Fungi
• Algae
• Protozoa
• Nematodes
• Microarthropods
• Enchytraeids
• Earthworms
• Ants, termites, spiders
• Mollusks
• Others rodents, snakes,
• voles, amphibians, etc.

Body size increasing
Microflora
Microfauna
Mesofauna
Macrofauna
Megafauna
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The fence post principle
Schriefer (2000)
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The soil pore network serves as a respiratory
and circulatory system
http//www.mtm.kuleuven.ac.be/Research/NDT/IDO_SHe
rman_final.ppt
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Tillage affects the pore network
white zones are pores
No-till soil
Tilled soil
(Young and Ritz, 2000)
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Intensive tillage
Long term no-till
Ontario Ministry of Ag and Food
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Pore diameter
Drainage pores
10-30 µm
Field Capacity
Most available
Plant available water
Permanent wilting point
0.2 µm
Hygroscopic water
Adapted from Buol (2000)
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Least Limiting Water Range
? LLWR for loose well-aggregated soil ?
?LLWR for compacted soil?
Root growth rate
20 25 30 35 40 45 50 55
dry Soil water content, cm3/cm3 100 wet
From Weil, 2003
Ray Weil
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Crusts impede seedling emergence, infiltration of
water and gas exchange
http//www.oznet.ksu.edu/soybeanscene/may24.htm
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high residue
high infiltration
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Ontario Ministry of Ag and Food
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Whats in the soil soup ??
Ca2
DOM
NO3-
Ca2
Mg2
NO3-
H2PO4-
Ca2
DOM
K
NO3-
K
Mg2
Ca2
DOM
NO3-
Mg2
DOM
Ca2
SO4-2
Adapted from Brady and Weil (2002)
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Which forms of nutrients are available to plants ?
solution
exchangeable
active OM
passive OM
weatherable minerals
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Re-seasoning the soup
Highly buffered
Poorly buffered
Modified from Havlin et al. (1999)
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Understanding nutrient uptake
H20
Root exudates activate soil microbes
Transpirational stream
H20
Root extension
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?
Feed the soil vs. Feed the crop ?
Both are important
Healthy roots need available nutrients !
Unhealthy roots use nutrients inefficiently
Acute root disease
Chronic root malfunction
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The acid infertility complex
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Whats wrong with soil acidity ?
Aluminum becomes insoluble at pH 5.5
Aluminum is soluble at low pHs
Aluminum is toxic to plant roots
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http//www.ppi-ppic.org/
Avoid overliming !!
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• Routine soil testing
• Goals
• Rapid
• Cheap
• Predictive
• Widely applicable

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Meaningful interpretation of soil test results
requires field calibration
100 yield
50 yield
http//www.ipm.iastate.edu/ipm/icm/2003/11-17-2003
/mehlich3.gif
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Crop yield
Deficiency Symptoms
Nutrient availability
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Why doesnt the NCDA lab test for N ???
Preseason soil mineral N is a poor predictor of
seasonal availability of N
Total soil N is also a poor predictor of seasonal
availability of N
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