Chemical

Soil Health for Organic Production

Charles Mitchell, Auburn UniversityAlisha Rupple, University of ArkansasHeather Friedrich, University of Arkansas

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

What is soil?

Agriculture / growing plants

50% Pore Space25% Water-filled25% Air-filled

45% Mineral Material5% Organic Matter

Three Main Soil Components

Soil Health

Physical Chemical

Biological

• Overlapping of the physical, chemical, and biological properties

• General picture of soil’s capacity to support plant growth without degradation (sustainability)

Physical Chemical

Biological

Proportion of sand, silt, and clay particlesThe 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 leachingClay: good water-holding capacity, high CEC, holds nutrients, easily compacted, poor drainage

Texture CLAY<0.002mm

0

1mm 2mm 3mm 4mm 5mm

SAND2.0 - 0.5 mm

0.5 - .002mm

SILT

Soil Particle Sizes

Soil Texture Triangle

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

• 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

Physical Chemical

Biological

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

Cation Exchange Capacity (CEC)

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

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 leachingCation exchange sites adsorb many metals (Cd2+, Zn2+, , Ni2+, , Pb2+, )that might be present in waste water.

CEC and Soil Management

-log [H+]; measure of acidity/alkalinity of soilSoils under field conditions vary from 3.5-10.05.5-8.5: range for most cropsStrongly acidic soils- Al3+ and Mn2+ at toxic level; microbial activity reduced; Ca2+, Mg2+ , and K+ limited; fungi favoredStrongly alkaline soils- Fe2+ , Zn2+ , Cu2+ , Mn2+, and P limited; salinity toxicity

pH

pH Effects on Nutrient Availability

Physical Chemical

Biological

Ranges from 1-5% in most soilsLiving fraction: roots, microorganisms, soil fauna

Alkaline soil favors bacteriaAcidic soil favors fungi, mites, collembolaNeutral soil favors earthworms, termites

Non-living fraction: surface litter, dead roots, microbial metabolites, humusGreatest concentration in the top 6 inches

Soil Organic Matter

Decomposing OM33-50%

Humus33-50%

Fresh Residue <10%

Living Organisms <5%

Components of Soil OM

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

Earthworms

Generally live in top 2m of soilUnfavorable conditions include: sandy, salty, arid, or acid soils; temperature extremes; presence of mice, mites, moles, and millipedes; tillage.

Decompose OMMineralize and recycle nutrientsFix nitrogenDetoxify pollutantsMaintain soil structureAble to suppress plant pestsParasitize and damage plants

Soil Microbes

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

Decomposition of plant residue to stable soil humus

Plants and Animals

Decomposable Organic Residues

Heterotrophic Biomass

Soil Humus(50-80% of OM)

SoilSurface

Biologically resistant organics

Microbial products

Nutrients

Stabilizes particles together as aggregates, esp. in sandy and clay soilsDecreases bulk density, providing resistance to compaction and improved porosityImproves water infiltration and retention

Able to retain 20x its weight in waterImproves friability, allowing for better root penetration

Effect of OM on Physical Properties

Increases CECIncreases nutrient retentionForms stable, chelated complexes with Fe3+,

Mn2+, Zn2+, Cu2+, and other cations

Effect of OM on Biological PropertiesProvides C source and energy for soil microbesImproves microbial population and diversityDiverse, active microbial population less likely to support spread of plant pathogens

Effect of OM on Chemical Properties

Proper use of tillageConventionally thought necessary for weed control, to incorporate OM, and allow root growthDamages structure, lowers OM content and overall soil productivityDecreasing tillage improves soil quality and fertilityNo-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/decompositionEncourage biodiversity of plant species

Management of Soil OM

Bob Kremer, USDA ARS

Use cover cropsIncorporate crop residues

Avoid pests/diseases by crop rotation, proper timing of incorporation, or compost residue away from field

Management of Soil OM

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

Integrate livestockDistribution of OM over landscapeGrazing stimulates root growth and subsequent release of C into rhizosphere soil

Add animal manuresSimultaneously add OM and nutrientsProblems with containing/storing /transporting/applying large quantities

Management of Soil OM

• Better for small, integrated farms• Nitrogen losses through

ammonification

CompostSize allows for uniform distributionOptimal C:N ratioFree 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

TemperatureMost effective bacteria thrive at 70°-100°F90°-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 decompositionRegular 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

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 20-30:1Net N immobilization- C:N ratio >30:1Blending different materials may be necessary to obtain optimum C:N ratio

C:N Ratios- important issue in composting

5000 lbs of wheat straw, 37%C and 0.5% NMicrobes assimilate 35% of CMicrobes C:N ratio is 8:1

5000lbs wheat straw X 0.37 (37% C)1850 lbs C in strawX 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?

Good soil tilthSufficient depthSufficient, but not excess, supply of nutrientsSmall population of plant pathogens and pestsGood soil drainageLarge population of beneficial organismsLow weed pressureFree of chemicals and toxins that may harm the cropResistant to degradationResilience when unfavorable conditions occur

Characteristics of a Healthy Soil

Indicator Best time to test Healthy ConditionEarthworm 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 subsoilPresence 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 easilySigns of erosion After heavy rainfall No gullies, runoff from field clearWater holding capacity After rainfall during

growing seasonSoil 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 cropNutrient holding capacity

Same time each year N, P, and K increasing or stable, but not into “high” zone

Indicators of Soil Health

Organic Soil Fertilitywww.extension.org/article/18565

NCAT-ATTRASustainable 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 SAREwww.sare.org/publications/soils.htm

Resources

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 ArkansasObadiah Njue, University of Arkansas at Pine BluffJeanine Davis, North Carolina State UniversityGeoff Zehnder, Clemson UniversityCharles Mitchell, Auburn UniversityRufina Ward, Alabama A&M UniversityKen Ward, Alabama A&M UniversityKaren Wynne, Alabama Sustainable Agriculture Network