Forest SoilsForest Soils

Kenneth WilliamsFisheries Extension Specialist Langston University Aquaculture Extension Program

Elements of Forestry

Soil functionSoil function

Medium for plant growth

mechanical support

moisture

nutrients

Soil functionSoil function

Water transmitting mantle

Soil functionSoil function

Component of forest ecosystem– Materials added, transformed and moved

through natural cycling mechanisms.

Soil production factorsSoil production factors

Geologic substratumClimateOrganismsTopographyTime

Soil propertiesSoil properties

Physical properties/morphologyOrganic matterMoisture contentOrganismsChemical properties

Soil profileSoil profile

A cross-section through the soil from organic surface layers to parent bedrock.

Horizons – distinct layers in the soil profile.

Soil horizonsSoil horizons

Designated by capital letters– O, A, E, B,C and R– A – humus mixed

with mineral soil, often from earthworm activity

Soil horizonsSoil horizons

– O – partially decomposed organic matter. A source of nutrients, erosion protection from runoff.

Soil horizonsSoil horizonsA – humus mixed with

mineral soil, often from earthworm activity

Soil horizonsSoil horizons

E – mineral horizons that have lost silicate clay, iron and aluminum.

Mostly composed of sand and silt.

Soil horizonsSoil horizons B horizons - contain

weathering products, such as silicate clay, iron, aluminum and humus that has been moved from A, E, or O horizons or produced in place.

Most original rock obliterated through soil transforming processes.

Soil horizonsSoil horizons

C horizons – layers excluding hard bedrock that are little affected by soil forming processes.

Soil horizonsSoil horizons

R horizons – Hard bedrock. Not soil.

Soil horizonsSoil horizons

Lower case letters next to capitals indicate a more specific characterization of the soil.

Ex. - Bs

Physical soil propertiesPhysical soil propertiesSoil colorSoil textureSoil structure

gravel

sand clay silt

Soil color

Soil colorSoil color

Depends on mineral composition, organic matter and drainage.

Red- IronBlack – organic matterBlue or green – poorly aerated

soils.

Soil colorSoil colorBlue clay

Soil colorSoil color

Green clay used to “draw impurities” from the skin?

Soil colorSoil colorSome colored soils are used to

prepare artist’s pigments. Ex. Sienna, made from limonite clay, originally found in Siena, Italy. Its color comes from iron oxides in the clay. It is heated to form burnt sienna, a variant of the pigment.

Raw sienna Burnt sienna

Soil textureSoil texture

Proportion of sand, silt and clay particles in soil.

Texture affects: drainage nutrient supply

root penetrability

seedling emergence

Soil textureSoil texture

You can feel soil to determine its texture.Sandy soils support trees with low moisture

requirements. Ex. Pines, scrub oak and hemlock.

Silt and clay soils support trees that require large amounts of moisture and nutrients. Ex. Douglas-fir, maple, hickory, ash and oaks.

Soil triangleSoil triangle

Soil structureSoil structure

Organic matterOrganic matter

Binds mineral grainsIncreases aerationSource of energy for soil

organismsModerates soil

temperatureIncreases moisture holding

capacity of soil.

Organic matterOrganic matter

Litter – freshly fallen leaves, twigs, bark and flowers.

1000 – 4000 kg / haLitter is decomposed into humus by soil

organisms.

Organic matterOrganic matterOrganic matter can be

regulated in forest soils by selective management and harvest systems.

Harvest whole tree or only the salable stem?

Leave slash on ground Burning slash to release

nutrients.

Soil waterSoil water Usually in short supply Solvent that takes nutrients to

tree roots. “Available water” – can

readily be absorbed by tree roots. Factors that affect this:

amt. and freq. of rainfall

run-offsoil storageleachingdemands of vegetation

Soil waterSoil waterWater moves

through old root channels. Along living roots.

In animal burrowsPores in soil.Unsaturated flow is

by capillary action from water table/

Soil water/ transpirationSoil water/ transpiration

As much water can transpire from forests as evaporates from open water.

Forests usually more water efficient than cropland.

Silvaculture methods – thinning and weed control reduce moisture loss.

Soil organismsSoil organisms

Decompose organic matter

Release nutrientsMix organic matter

with mineral soil.

Soil organismsSoil organisms

Roots may be most important

RootsRoots

Add organic matter to soilStimulate microbial action by root chemical

exudatesProduce organic acids that dissolve

nutrientsHold and exchange nutrients in the soil.Give off toxic compounds that prevent

growth of other plants. (allelopathy)

Soil organisms Soil organisms - Fungi- Fungi

Increase ability of trees to uptake nitrogen, phosphorous, potassium, calcium and magnesium.

Trees in soil with fungi live longer and grow better.

Soil organisms – earthworms and Soil organisms – earthworms and nematodesnematodes

Mix soil

Soil chemical propertiesSoil chemical properties

Soil reactionCation exchange capacityEssential chemical elements

Soil reaction - pHSoil reaction - pH

pH important because it affects microbial populations

Availability of micro- and macronutrientsRate of nitrificationTree litter is usually acidicAcidity also can come from acid rainForest soils more acidic than grassland or

ag soils.

Cation exchangeCation exchangeAbility of soil to hold and exchange

positively charged nutrient ions.Dominant cations(+):

– Hydrogen– Aluminum– Calcium– Magnesium– Potassium– ammonium– Sodium

Cation exchangeCation exchangeCation exchange capacity depends on

amount of organic matter, amount and types of clays and pH.

Low in sandy soil higher in fine textured soils.

Essential chemical elementsEssential chemical elements

Macronutrients– Nitrogen– Phosphorous– Potassium– Calcium– Magnesium– Sulfur

Essential chemical elementsEssential chemical elements

Micronutrients– Iron– Manganese– Boron– Copper– Molybdenum– Zinc– Chlorine

Soil-site factors and tree growthSoil-site factors and tree growth

Tree growth can be predicted from soil features. Valuable knowledge for foresters for planning Important features include:

depth

texture

drainage

slope steepness

orientation

Nutrient deficienciesNutrient deficiencies

Difficult to tell from insect or disease damage/

Need to know soil nutrient content and requirements for tree species.

Tissue analysis (foliage) often used.Deficiencies corrected with fertilizers.Usually in nurseries or other small scale

applications.

Soil surveySoil survey

Contains the following information:– Soil description– Use– Management– Formation– Classification– Lab data– Area information

Soil surveySoil survey

Soil survey used to:– Match harvesting with soil conditions– Locate roads and landings– Match tree species with soil type– Planning recreational facilities– Evaluate impacts of grazing, mining, waste

disposal, etc.

Multiple factor classification Multiple factor classification systemssystems

Classify ecologically significant segments of forests and predict site quality using:– land form– Soil initial material– Forest cover type– Soil taxonomic unit

Habitat systemHabitat system

Classifies according to climax plant associations

Used to predict site quality and succession relationships

Soil classificationSoil classification

Seven levels of classification: Order Suborder Great group Subgroup Family Series Type

Soil orders important to North Soil orders important to North American forestryAmerican forestry

Ultisols – less than 35% of exchange sites containing calcium, magnesium potasium and sodium.

Occur in areas moist and warm to tropical.Loblolly and shortleaf pine in S.E. U.S. and

oak- hickory and oak-pine in south central U.S.

Soil orders important to North Soil orders important to North American forestryAmerican forestry

Alfisols – soils with greater than 35% of exchange sites containing Ca, Mg, K, and Na.

Found in oak-hickory forests of central U.S., northern hardwoods area and ponderosa - lodge pole pine areas of W. North America

Soil orders important to North Soil orders important to North American forestryAmerican forestry

Spodosols – enriched organic soils with iron or aluminum oxides. Develop from coarse textured acidic initial material.

Found in cold, humid climates.Major forest covers are spruce- fir, eastern

white pine and northern hardwoods.

Soils and environmental qualitySoils and environmental quality

Forest management practices effect soil and water quality.– shortened rotations– Close utilization– Fast growing hybrids– Mechanical and chemical site preparation– Improper road building – Wet weather logging

Top 4 affect forest nutrient balance

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