Mulching and Soil BiologyWhat’s Going On?

Rex A. BastianThe Davey Tree Expert Company

The Care of Trees2371 South Foster

Wheeling, IL 60090

Mulching

Overall, proper

mulching is considered

one of the best, long

term tree health

practices» Usually focus on the

physical and chemical

benefits over the

biological benefits

Soil properties and root development levels resulting from wood chip mulch treatments 1979-1983

Character Mulched Control

Bulk

Density

0.73* 1.14

Moisture 30.8** 18.9

Root

Density

6.16** 3.2

Mycorrhizal

Density

1.95* 0.46

Soil pH 5.8* 6.7

Himelick and Watson, 1990

Watson, Morton Arboretum

Natural vs. Urban Systems

soils.usda.gov

Components of Soil Organic Matter

Decomposing organic matter(active fraction) 33% - 50%

Stabilized organic matter

(humus) 33% - 50%

Fresh residue <10%

Living organisms

<5%

Soil Foodweb Components

Bacteria» Decomposers

– (Low C:N ratio “Green Stuff”)» Detoxifiers» Nutrient storage» Nitrogen fixers» “slime” soil» Disease suppression

Fungi» Decomposers

– (high C:N ratio “Brown Stuff”)» Nutrient storage» Nutrient “pumpers”» Mycorrhizae» Soil binders» Disease suppression

soilfoodweb.com

Protozoans» Nutrient cycling» Bacterial grazing» Disease suppression

Nematodes» Nutrient cycling» Many types

– Grazing– Some nematode

predators

Soil Foodweb Components

Olympus America, Inc.

soilfoodweb.com

www.freewebs.com

Arthropods» Shredders» Herbivores, predators» burrowing

Earthworms» Decomposers» Soil “mixers”» Stimulate microbial activity» Provide for improved water

holding capacity and root penetration

» Bury organic matter

Soil Foodweb Components

www.wildaboutbritain.co.uk

The Soil Food Web

Consider These Two Systems

Would the Same Types and Quantities of Soil Microorganisms Present be the Same in Both?

CA Integrated Waste Management Board

Ecological Succession

Affects Soil

Biology, Too

Soil organism mix

changes as the

above ground

system changes

More Fungi

More Arthropods

More Complex

More Acidic

More ammonium vs. nitrate uptake

Typical Numbers of Soil Organisms in Healthy Ecosystems

Grass Forest

Organisms per gram (teaspoon) of soil

Bacteria 100 mil. -1 bil. 100 mil. -1 bil.

Fungi

F:B Ratio

10s – 100’s of yds

1:3 to 2:1

1-40 miles (conifers)

10:1 to 100:1

Protozoa 1000’s 100,000’s

Nematodes 10’s – 100’s 100’s

Organisms per square foot of soil

Arthropods 500-2000 10,000-25,000

Earthworms 10-50 10-50

USDA, National Resources Conservation Service

Higher pH

Lower pH

Acidic Tree Root

Secretions

Tree

Root

Rhizosphere

Zone of intense biological and chemical activity

Biological Effects on Tree Rhizosphere Chemistry

Acidic Fungal

Secretions

Selective

NH4 vs. NO3

Uptake

Root

Biological Effects on Rhizosphere Chemistry

Biological Make-up» Host Plant Exudates» Dominant Vegetation

Type » Physical Soil

Characteristics

Consume Foods Occupy Space Tie-Up/Cycle/Release

Nutrients Release Exudates

Fe-Reduction

G. Neumann, 2005

Garlic MustardInfluence on Tree Mycorrhizae

Garlic mustard root

extracts suppressed

the growth of native

tree seedlings by

disrupting soil

mycorrhizae

Stinson, et al., 2006

Soil Biology/Soil Chemistry InteractionNitrate vs. Ammonium Nitrogen

Nitrate (NO3- ) is more common in turf systems

More alkaline environment, nitrification bacteria more

common, nitrate turf fertilizer use

NO3-

HCO3-

HCO3- + H2O = H2CO3 + OH-

pH Increases

Root Rhizosphere

• Ammonium (NH4+) is more common in woody

(mulched) systems

• More acidic environment, fewer nitrification bacteria

NH4+

H+Rhizosphere

pH Drops

Soil Biology/Soil Chemistry Interaction

Root Rhizosphere

Rhizosphere acidification

G. Neumann, 2005

Natural and Cultural Differences Impact Soil Biology

Mulching can Help Reestablish a Woodland Biology

Woodland Systems» More “brown” stuff

– Higher C:N ratios

» Lower N levels [NH4+ ]

» Fungal food sources

» Acidic Secretions (tree roots

and fungal dominance)

– Lower rhizosphere pH

Leads to higher

Fungal:Bacterial Ratio» 5:1 and higher

Turfgrass Systems» More “green” stuff

– Lower C:N ratios

» Higher N levels [NO3- ]

» Bacterial food sources» Higher chemical/fertilizer use» Alkaline Secretions (grass

roots and bacterial dominance)– Higher rhizosphere pH

Leads to lower Fungal:Bacterial Ratio» 1:1 or less

– Often 1:2 or 1:3