Chemical

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Soil Health for Organic Production
Chemical
Charles Mitchell, Auburn University Alisha
Rupple, University of Arkansas Heather Friedrich,
University of Arkansas
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What is soil?

• Surface mineral layer of the earth that is mixed
  with organic matter (living and non-living) that
  serves as a growing media for land plants
• Combination of biological, physical, and chemical
  processes, particular to regions and climates

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Agriculture / growing plants
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Three Main Soil Components

• 50 Pore Space
• 25 Water-filled
• 25 Air-filled
• 45 Mineral Material
• 5 Organic Matter

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Soil Health

• Overlapping of the physical, chemical, and
  biological properties
• General picture of soils capacity to support
  plant growth without degradation (sustainability)

Physical
Chemical
Biological
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Physical
Chemical
Biological
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Texture

• Proportion of sand, silt, and clay particles
• The ideal texture depends on which crop will be
  grown.
• Potatoes grow best in a sandy soil while rice
  grows best in clay soil.
• Sand good drainage, ease of cultivation, dries
  easily, nutrients lost to leaching
• Clay good water-holding capacity, high CEC,
  holds nutrients, easily compacted, poor drainage

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Soil Texture Triangle
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Soil Structure

• Arrangement of soil particles into stabilized
  aggregates
• Affected by texture and organic matter content

Soil aggregates

• Soil organisms break down organic residues,
  producing glomalin that stabilizes aggregates
• Idealgranular or crumb

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Benefits of Good Structure

• Resist wind and water erosion
• Maintain low bulk density
• Increased pore space

• Increased water storage
• Better water percolation
• Increased aeration

• Ease of cultivation
• Allows root penetration

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Chemical
Physical
Biological
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Cation Exchange Capacity (CEC)

• Cation Exchange the replacement of one adsorbed
  cation by another cation free in solution
• CEC quantity of exchangeable cation sites per
  unit weight dry soil
• Dependent on structure, texture, and organic
  matter content
• Greatly influences nutrient availability and
  retention

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CEC in Various Soil Types
Soil Type Typical CEC meq/100 g
Light colored sand 3-5
Dark colored sand 10-20
Loams 10-15
Silt loams 15-25
Clay and clay loams 20-50
Organic soils 50-100
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CEC and Soil Management

• Exchangeable Ca2 , Mg2 , and K major source of
  plant Ca2 , Mg2 , and K
• Amount of lime needed to raise pH dependent on
  CEC (gtCEC gt lime)
• Cation exchange sites hold Ca2, Mg2 , K, NH4,
  and Na ions and reduce leaching
• Cation exchange sites adsorb many metals (Cd2,
  Zn2, , Ni2, , Pb2, )that might be present in
  waste water.

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pH

• -log H measure of acidity/alkalinity of soil
• Soils under field conditions vary from 3.5-10.0
• 5.5-8.5 range for most crops
• Strongly acidic soils- Al3 and Mn2 at toxic
  level microbial activity reduced Ca2, Mg2 ,
  and K limited fungi favored
• Strongly alkaline soils- Fe2 , Zn2 , Cu2 ,
  Mn2, and P limited salinity toxicity

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pH Effects on Nutrient Availability
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Physical
Chemical
Biological
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Soil Organic Matter

• Ranges from 1-5 in most soils
• Living fraction roots, microorganisms, soil
  fauna
• Alkaline soil favors bacteria
• Acidic soil favors fungi, mites, collembola
• Neutral soil favors earthworms, termites
• Non-living fraction surface litter, dead roots,
  microbial metabolites, humus
• Greatest concentration in the top 6 inches

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Components of Soil OM
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Earthworms

• Improve soil structure by ingesting organic
  matter and soil and excreting stable aggregates
• Aerate and stir soil, which improves water
  infiltration and root penetration

Generally live in top 2m of soil Unfavorable
conditions include sandy, salty, arid, or acid
soils temperature extremes presence of mice,
mites, moles, and millipedes tillage.
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Soil Microbes

• Decompose OM
• Mineralize and recycle nutrients
• Fix nitrogen
• Detoxify pollutants
• Maintain soil structure
• Able to suppress plant pests
• Parasitize and damage plants

USDA-NRCS Soil Biology Primer
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Soil bacterial colonization of POM (Active C
fraction of SOM)
Microbes are concentrated on/near POM rather
than distributed homogenously in soil Haynes,
2005. Adv. Agron. 85221-267. Important to
maintain actively decomposing organic material in
soils
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Decomposition of plant residue to stable soil
humus
Plants and Animals
Soil Surface
Decomposable Organic Residues
Nutrients
Heterotrophic Biomass
Biologically resistant organics
Microbial products
Soil Humus (50-80 of OM)
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Effect of OM on Physical Properties

• Stabilizes particles together as aggregates, esp.
  in sandy and clay soils
• Decreases bulk density, providing resistance to
  compaction and improved porosity
• Improves water infiltration and retention
• Able to retain 20x its weight in water
• Improves friability, allowing for better root
  penetration

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Effect of OM on Chemical Properties

• Increases CEC
• Increases nutrient retention
• Forms stable, chelated complexes with Fe3, Mn2,
  Zn2, Cu2, and other cations
• Effect of OM on Biological Properties
• Provides C source and energy for soil microbes
• Improves microbial population and diversity
• Diverse, active microbial population less likely
  to support spread of plant pathogens

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Management of Soil OM

• Proper use of tillage
• Conventionally thought necessary for weed
  control, to incorporate OM, and allow root growth
• Damages structure, lowers OM content and overall
  soil productivity
• Decreasing tillage improves soil quality and
  fertility
• No-till practices may initially decrease yields
  and increase fertility needs

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Management of Soil OM

• Proper management of OM is a major factor in
  sustainable production
• Maintain constant inputs of organic materials to
  replace loses from harvest/decomposition
• Encourage biodiversity of plant species

Bob Kremer, USDA ARS
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Management of Soil OM

• Use cover crops
• Incorporate crop residues
• Avoid pests/diseases by crop rotation, proper
  timing of incorporation, or compost residue away
  from field

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Maintenance of vegetative residues through cover
cropping, refuge areas, buffer strips, etc not
only restores organic matter but also provides
habitats for natural insect predators of weed
seeds
Micro-insect larva attacking Amaranthus (i.e.,
pigweed) seed
Osage County, MO
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Management of Soil OM

• Integrate livestock
• Distribution of OM over landscape
• Grazing stimulates root growth and subsequent
  release of C into rhizosphere soil
• Add animal manures
• Simultaneously add OM and nutrients
• Problems with containing/storing
  /transporting/applying large quantities

• Better for small, integrated farms
• Nitrogen losses through ammonification

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Management of Soil OM

• Compost
• Size allows for uniform distribution
• Optimal CN ratio
• Free from weed seeds (if composted correctly)
• Can suppress soil diseases
• Vermicompost- compost produced through action of
  worms, esp. good for small farms, gardens
• Eisenia foetida (red worm)- known for composting
  ability

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Compost

• Temperature
• Most effective bacteria thrive at 70-100F
• 90-140F- rapid decomposition
• gt140F- most weed seeds and pathogens killed
  bacterial activity significantly decreased
• Aerobic conditions
• Require O2 levels gt5
• Allows for most rapid and effective decomposition
• Regular mixing/turning enhances aeration
• Moisture content of 40-60
• Excess moisture causes nutrient leaching, odor,
  slowed decomposition
• squeeze test- damp to the touch, with a few
  drops of liquid extracted with tightly squeezed

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CN Ratios- important issue in composting
Material CN Ratio
Vegetable wastes 10-121
Coffee grounds 201
Grass clippings 12-251
Cow manure 201
Horse manure 251
Poultry litter 13-181
Leaves 30-801
Corn stalks 601
Bark 40-1001
Paper 150-2001
Wood chips sawdust 100-5001

• Microorganisms require C for energy and N for
  protein
• Require N in a CN ratio of 81
• Net N mineralization- CN ratio lt201
• Stable- CN ratio 20-301
• Net N immobilization- CN ratio gt301
• Blending different materials may be necessary to
  obtain optimum CN ratio

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Will N be mineralized or immobilized?

• 5000 lbs of wheat straw, 37C and 0.5 N
• Microbes assimilate 35 of C
• Microbes CN ratio is 81
• 5000lbs wheat straw
• X 0.37 (37 C)
• 1850 lbs C in straw
• X 0.35 (35 assimilated)
• 647.5 lbs C assimilated

• 647.5 lbs C 8 81 lbs N
• (x) Lbs N 1 needed
• 0.005 x 5000lbs 25 lbs N in straw
• 81 lbs N needed- 25lbs N in straw 56 lb N
  deficit
• 56 lbs N immobilized from soil

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Characteristics of a Healthy Soil

• Good soil tilth
• Sufficient depth
• Sufficient, but not excess, supply of nutrients
• Small population of plant pathogens and pests
• Good soil drainage
• Large population of beneficial organisms
• Low weed pressure
• Free of chemicals and toxins that may harm the
  crop
• Resistant to degradation
• Resilience when unfavorable conditions occur

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Indicators of Soil Health
Indicator Best time to test Healthy Condition
Earthworm presence With moist soil (spring/fall) gt10 worms/ft3 many castings in tilled clods
Color of OM When soil is moist Topsoil distinctly darker than subsoil
Presence of plant residues Anytime Residue on most of soil surface
Conditions of plant roots Late spring or during rapid growth Roots extensively branched, white, extended into subsoil
Degree of subsurface compaction Before tillage or after harvest A stiff wire goes in easily to 2x plow depth
Soil tilth or friability When soil is moist Soil crumbles easily
Signs of erosion After heavy rainfall No gullies, runoff from field clear
Water holding capacity After rainfall during growing season Soil holds moisture at least a week w/o signs of drought stress
Water infiltration After rainfall No ponding or runoff soil surface does not remain excessively wet
pH Same time each year Near neutral and appropriate for crop
Nutrient holding capacity Same time each year N, P, and K increasing or stable, but not into high zone
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Resources

• Organic Soil Fertility
• www.extension.org/article/18565
• NCAT-ATTRA
• Sustainable Soil Management, www.attra.ncat.org/at
  tra-pub/soilmgmt.html
• Soil Management National Organic Program
  Regulations, www.attra.ncat.org/attra-pub/PDF/orga
  nic_soil.pdf
• Cornell Soil Health
• www.hort.cornell.edu/soilhealth/
• Building Soils for Better Crops, 3rd Edition SARE
• www.sare.org/publications/soils.htm

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Acknowledgements

• This presentation address general organic
  production practices. It is to be to use in
  planning and conducting organic horticulture
  trainings. The presentation is part of project
  funded by a Southern SARE PDP titled Building
  Organic Agriculture Extension Training Capacity
  in the Southeast
• Project Collaborators
• Elena Garcia, University of Arkansas CESHeather
  Friedrich, University of ArkansasObadiah Njue,
  University of Arkansas at Pine BluffJeanine
  Davis, North Carolina State UniversityGeoff
  Zehnder, Clemson UniversityCharles Mitchell,
  Auburn UniversityRufina Ward, Alabama AM
  UniversityKen Ward, Alabama AM UniversityKaren
  Wynne, Alabama Sustainable Agriculture Network