Alisha Rupple, University of Arkansas
Heather Friedrich, University of Arkansas

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

Agriculture / growing plants

Three Main Soil Components
50% Pore Space
25% Water-filled
25% Air-filled
45% Mineral Material
5% Organic Matter

Physical
•
Overlapping of the physical , chemical , and biological properties
Chemical
Biological
•
General picture of soil’s capacity to support plant growth without degradation
(sustainability)

Chemical
Biological

Texture
0
1mm
Soil Particle Sizes
2mm 3mm 4mm 5mm
SAND
2.0 - 0.5 mm
SILT
0.5 -
.002mm
CLAY
<0.002mm
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

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
 Ideal=granular or crumb

• 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

Physical
Biological

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

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

Exchangeable Ca 2+ , Mg 2+ , and K + major source of plant Ca 2+ , Mg 2+ , and K +
Amount of lime needed to raise pH dependent on CEC (>CEC = > lime)
Cation exchange sites hold Ca 2+ , Mg 2+ ,
K + , NH
4
+ , and Na leaching
+ ions and reduce
Cation exchange sites adsorb many metals (Cd 2+, Zn 2+, , Ni 2+, , Pb 2+, )that might be present in waste water.

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- Al 3+ and Mn 2+ at toxic level; microbial activity reduced; Ca 2+ ,
Mg 2+ , and K + limited; fungi favored
Strongly alkaline soils- Fe 2+ , Zn 2+ , Cu 2+ ,
Mn 2+ , and P limited; salinity toxicity

pH Effects on Nutrient Availability

Physical
Chemical

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

Components of Soil OM

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.

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

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

Nutrients
Decomposition of plant residue to stable soil humus
Plants and
Animals
Soil
Surface
Decomposable
Organic Residues
Heterotrophic
Biomass
Microbial products
Soil Humus
(50-80% of OM)
Biologically resistant organics

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

Effect of OM on Chemical Properties
Increases CEC
Increases nutrient retention
Forms stable, chelated complexes with
Fe 3+, Mn 2+ , Zn 2+ , Cu 2+ , and other cations
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

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
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

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

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
33

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

Management of Soil OM
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

Compost
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 O
2 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

C:N Ratiosimportant issue in composting
Material C:N Ratio
Vegetable wastes 10-12:1
Coffee grounds 20:1
Grass clippings 12-25:1
Cow manure
Horse manure
20:1
25:1
Poultry litter
Leaves
Corn stalks
Bark
Paper
Wood chips & sawdust
13-18:1
30-80:1
60:1
40-100:1
150-200:1
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

5000 lbs of wheat straw, 37%C and 0.5%
N
Microbes assimilate
35% of C
Microbes C:N ratio is
8:1
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
5000lbs wheat straw
X 0.37 (37% C)
1850 lbs C in straw
X 0.35 (35% assimilated)
647.5 lbs C assimilated
56 lbs N immobilized from soil

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

Indicators of Soil Health
Indicator
Earthworm presence
Best time to test
With moist soil
(spring/fall)
Color of OM
Presence of plant residues
When soil is moist
Anytime
Conditions of plant roots Late spring or during rapid growth
Degree of subsurface compaction
Before tillage or after harvest
Soil tilth or friability
Signs of erosion
When soil is moist
After heavy rainfall
Water holding capacity After rainfall during growing season
Water infiltration After rainfall pH Same time each year
Nutrient holding capacity Same time each year
Healthy Condition
>10 worms/ft3; many castings in tilled clods
Topsoil distinctly darker than subsoil
Residue on most of soil surface
Roots extensively branched, white, extended into subsoil
A stiff wire goes in easily to 2x plow depth
Soil crumbles easily
No gullies, runoff from field clear
Soil holds moisture at least a week w/o signs of drought stress
No ponding or runoff; soil surface does not remain excessively wet
Near neutral and appropriate for crop
N, P, and K increasing or stable, but not into “high” zone

Resources
Organic Soil Fertility www.extension.org/article/18565
NCAT-ATTRA
Sustainable Soil Management, www.
attra.ncat.org/attrapub/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

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