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Soil health for organic production
1. Chemical
Soil Health for Organic
Production
Charles Mitchell, Auburn University
Alisha Rupple, University of Arkansas
Heather Friedrich, University of Arkansas
2. 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
What is soil?
8. 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
Texture CLAY
<0.002mm
0
1mm 2mm 3mm 4mm 5mm
SAND
2.0 - 0.5
mm
0.5 -
.002mm
SILT
Soil Particle Sizes
10. Arrangement of soil
particles into stabilized
aggregates
Affected by texture and
organic matter content
Soil Structure
Soil aggregates
Soil organisms break down organic residues,
producing glomalin that stabilizes aggregates
Ideal=granular or crumb
11.
12. •Resist wind and water
erosion
•Maintain low bulk
density
•Increased pore space
Benefits of Good Structure
•Ease of cultivation
•Allows root penetration
•Increased water storage
•Better water percolation
•Increased aeration
16. 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
Cation Exchange Capacity (CEC)
17. 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
CEC in Various Soil Types
18. Exchangeable Ca2+ , Mg2+ , and K+ major
source of plant Ca2+ , Mg2+ , and K+
Amount of lime needed to raise pH
dependent on CEC (>CEC = > 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.
CEC and Soil Management
19. -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
pH
24. Improve soil structure by ingesting organic
matter and soil and excreting stable
aggregates
Aerate and stir soil, which improves water
infiltration and root penetration
Earthworms
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.
25. Decompose OM
Mineralize and recycle
nutrients
Fix nitrogen
Detoxify pollutants
Maintain soil structure
Able to suppress plant pests
Parasitize and damage plants
Soil Microbes
USDA-NRCS Soil Biology Primer
26. 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. 85:221-267.
Important to maintain actively decomposing organic
material in soils
27. Decomposition of plant residue to
stable soil humus
Plants and
Animals
Decomposable
Organic Residues
Heterotrophic
Biomass
Soil Humus
(50-80% of OM)
Soil
Surface
Biologically
resistant
organics
Microbial
products
Nutrients
28. 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
Effect of OM on Physical Properties
29. 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
Effect of OM on Chemical Properties
30. 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
Management of Soil OM
31. 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
Management of Soil OM
Bob Kremer, USDAARS
32. Use cover crops
Incorporate crop
residues
Avoid
pests/diseases by
crop rotation,
proper timing of
incorporation, or
compost residue
away from field
Management of Soil OM
33. 33
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
Osage County, MO
‘Micro-insect’ larva attacking
Amaranthus (i.e., pigweed) seed
34. 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
Management of Soil OM
•Better for small, integrated farms
•Nitrogen losses through
ammonification
35. Compost
Size allows for uniform distribution
Optimal C:N 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
Management of Soil OM
36. Temperature
Most effective bacteria thrive at 70°-100°F
90°-140°F- rapid decomposition
>140°F- most weed seeds and pathogens killed;
bacterial activity significantly decreased
Aerobic conditions
Require O2 levels >5%
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
Compost
37.
38. Material C:N Ratio
Vegetable wastes 10-12:1
Coffee grounds 20:1
Grass clippings 12-25:1
Cow manure 20:1
Horse manure 25:1
Poultry litter 13-18:1
Leaves 30-80:1
Corn stalks 60:1
Bark 40-100:1
Paper 150-200:1
Wood chips &
sawdust
100-500:1
Microorganisms require
C for energy and N for
protein
Require N in a C:N ratio of
8:1
Net N mineralization-
C:N ratio <20:1
Stable- C:N ratio 20-30:1
Net N immobilization-
C:N ratio >30:1
Blending different
materials may be
necessary to obtain
optimum C:N ratio
C:N Ratios-
important issue in composting
39. 5000 lbs of wheat
straw, 37%C and 0.5%
N
Microbes assimilate
35% of C
Microbes C:N ratio is
8:1
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
Will N be mineralized or
immobilized?
40. 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
Characteristics of a Healthy Soil
41. Indicator Best time to test Healthy Condition
Earthworm presence With moist soil
(spring/fall)
>10 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
Indicators of Soil Health
42. Organic Soil Fertility
www.extension.org/article/18565
NCAT-ATTRA
Sustainable Soil Management, www.attra.ncat.org/attra-
pub/soilmgmt.html
Soil Management: National Organic Program Regulations,
www.attra.ncat.org/attra-pub/PDF/organic_soil.pdf
Cornell Soil Health
www.hort.cornell.edu/soilhealth/
Building Soils for Better Crops, 3rd Edition SARE
www.sare.org/publications/soils.htm
Resources
43. 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 CES
Heather Friedrich, University of Arkansas
Obadiah Njue, University of Arkansas at Pine Bluff
Jeanine Davis, North Carolina State University
Geoff Zehnder, Clemson University
Charles Mitchell, Auburn University
Rufina Ward, Alabama A&M University
Ken Ward, Alabama A&M University
Karen Wynne, Alabama Sustainable Agriculture Network
