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Soil and the Soil Ecosystem

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Why Study Dirt? 90% of the world’s food comes from land based

agriculture 22% of the land used for crops, grazing, and

forestry has become degraded during the past fifty years (See Table 8-1 p.187).

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http://www.globalchange.umich.edu/globalchange2/current/lectures/land_deg/concern.gif

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Areas of the planet threatened by soil erosion

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Loss of Farmland to Development is Something We Experience Locally

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No healthy topsoil = No us.

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What is Healthy Soil?

Add to this air, water, soil nutrients, salts, and pH.

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Soil Formation—Mineral Soil Soil building begins with the physical and

chemical fragmentation of the Parent Material.

Mechanical Weathering– Temperature change– Abrasion

Chemical Weathering– Acid deposition– Oxidation

Type of parent material and the climate determine the type of mineral soil.

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Soil Formation—Detritus Detritus is Decaying plants and animals.

Supplies nutrients for plantsLowers soil pH– Makes inorganic nutrients available to plants by

increasing their solubility– Typical range pH 6-8. 6-6.5 optimum for biological activity

in most soil ecosystems.

Creates a loose texture that increases aeration and water infiltration / water holding capacity.

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Soil Formation—DetritusOptimum organic matter content for most non-wetland soil ecosystems is 3-5%Detritus that has undergone most of its decomp. Is referred to as humus

humus-detritus that has been almost completely digested by the detritus feeder food web.

humus will decomp @ 2%-5% of vol./yr.

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Soil Formation—Detritus Food Web Microflora (the decomposers) help to unlock the

nutrients stored in detritus and make them available to the rest of the DFW.

Burrowing animals aerate the soil and mix the organic and inorganic components of the soil.

1 ha of healthy soil may contain 500k earthworms that can process 9 metric tons (20 k lbs) of soil/year.

See handout for other denizens of the soil

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Bacteria are the most important organisms in the detritus food web. They are the first heterotrophs in any food chain.

15 tons/ acre

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Rate of Topsoil Formation Varies depending on the conditions of the area,

but primary sources list a range from 1” every 50 to 500 years.

That’s a long time to wait for a meal.

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Soil Characteristics Soils differ greatly from one area to another. Even on relatively small plots of land, soil types

may differ. Soils are generally studied according to their

ProfileTextureStructureColor

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page 188

Soil Profile

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Page 189

Soil Texture

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Soil TextureSand are the largest particles and they feel

"gritty."Silt are medium sized, and they feel soft, silky

or "floury."Clay are the smallest sized particles, and they

feel "sticky" and they are hard to squeeze.

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Loam is common and generally considered ideal for plant growth.

Properties of Soil Particles Worth Noting:large particles have large spaces separating themsmall particles have a larger surface area to volume ratioNutrient ions and water molecules cling to surfaces

Workability (Tilth)-the ease with which a soil can be cultivated

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Soil Consistence Loose Friable Firm Extremely Firm

Source

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Soil Structure

Granular: Resembles cookie crumbs and is usually less than 0.5 cm in diameter. Commonly found in surface horizons where roots have been growing.

Blocky: Irregular blocks that are usually 1.5 - 5.0 cm in diameter.

Prismatic: Vertical columns of soil that might be a number of cm long. Usually found in lower horizons.

Platy: Thin, flat plates of soil that lie horizontally. Usually found in compacted soil.

Single Grained: Soil is broken into individual particles that do not stick together. Always accompanies a loose consistence. Commonly found in sandy soils.

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Soil Structure

Columnar: Vertical columns of soil that have a salt "cap" at the top. Found in soils of arid climates. Massive: Soil has no visible

structure, is hard to break apart and appears in very large clods.

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Soil Structure

Soil Structure-the “feel” of the soil (clumpy, soft, loose, etc.)

Soil Structure vs. Soil Texture

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Soil Color Quantified by horizon

and inclusions using the Munsell Soil Color Charts.

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Soil Color

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Soil and Plants Plant considerations pertaining to soil:

nutrientswaterairpHsalinity

These factors together affect the fertility “tilth” of the soil.

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Nutrients Nitrogen (N2, NO3

-, NO2-, NH3)

Phosphate (PO43- )

Potassium (K+) Calcium (Ca2+) These are added to the soil by weathering of

parent material, fixation by bacteria from the atmosphere, and through decomposition of detritus.

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Nitrogen Needed for chlorophyll synthesis. Leafy growth above ground. Hasten maturity of crops. Enhance fruit development. Optimum amounts present in soil depend

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Phosphorus Encourages cell division Encourages root development 50 – 75 lbs / acre typically available to plants for

growth.

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Potassium Enhances disease resistance Activates enzymes needed for development Cuticle development Regulates turgor pressure Necessary for protein metabolism

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Other Soil Properties leaching nutrient-holding capacity ion-exchange capacity

Organic vs. Inorganic Fertilizer

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Water Infiltration

What it isWhy it mattersHow to measure it

Water ContentWhat it isWhy it mattersHow to measure it

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Water Water Holding Capacity

What it isWhy it mattersHow to measure it

Evaporative Water LossWhat it isWhy it mattersHow to measure it

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Air the ability of the underground organs of plants to

exchange gases with the outside environment is essential for their survival.

Roots need air to grow. Many things affect the aeration of soil.

mineral particlesOMCActions of Detritus Feeders and other animalsActions of plantsWeather

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pH from water from detritus from mineral soil

evaporative qualities

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Salinity Plants must maintain an osmotic balance with

their external environment. addition of salt to the soil prevents plants from

taking up water into their roots

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Soil As an Ecosystem Nebel p.192

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There is an 85% to 95% decline in GPP when plants are grown on subsoil compared to when they are grown on topsoil.

Loss of humus in topsoil is called mineralization

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