Soil

This section is in addition to Chapter 3

Soil

Produced slowly (200-1000 years typically) by weathering of rock, deposition of sediments, and decomposition of organic matter

Soil horizons – separate zones within soilSoil profile – cross-section view of soil

Horizons

O horizon – surface litterA horizon – top soil, made up of inorganic

particles (clay, silt, sand) and humus (organic particles from decomposed organisms) Dark topsoil is richer in nutrients Releases water and nutrients slowly Provides aeration to roots Healthy soil contains many nematodes and

bacteria, fungi, etc.

Fig. 10.12, p. 220

Oak tree

Fern

O horizonLeaf litter

A horizonTopsoil

B horizonSubsoil

C horizonParent

material

Root system

Mature soil

Red earthmite Springtail

Bacteria

Fungus

Actinomycetes

Nematode

Mite

Pseudoscorpion

Young soil

Regolith

Bedrock

Immature soil

RockfragmentsMoss and

lichen

Organic debrisBuilds upGrasses and

small shrubsMole

Dog violet

Honeyfungus

Millipede

Earthworm

Lords andladiesWord

sorrel

Poor topsoil

Grey, yellow and red are not the colors of healthy topsoil

Generally means that soil is lacking nutrients

Best soil is called loam with equal parts sand, silt, clay and humus

Leaching – dissolving and carrying nutrients (or pollutants) through soil into lower layers

B – horizon and C - horizon

B – Subsoil mostly broken down rock with little organic matter

C- parent material broken down rock on top of the bedrock

Soils

Texture – relative amount of different sized particles present (sand, silt, clay)

Porosity – volume of pore space in the soil

Permeability – the ability of water to flow through the soil

Water Water

High permeability Low permeability

Sandy soil Clay soil

Soils

Clay – high porosity, low permeabilitySand – high permeability, low porosity

Acidity is another factorWhere rain is low, calcium and other

alkaline compounds may build up (sulfur can be added – turns to sulfuric acid by bacteria)

Fig. 10.15b, p. 223

Acidiclight-coloredhumus

Iron andaluminumcompoundsmixed withclay

Forest litterleaf mold

Humus-mineralmixture

Light, grayish-brown, silt loam

Dark brownFirm clay

Acid litterand humus

Humus andiron andaluminumcompounds

Light-coloredand acidic

Tropical Rain Forest Soil(humid, tropical climate)

Deciduous Forest Soil(humid, mild climate)

Coniferous Forest Soil(humid, cold climate)

Fig. 10.15a, p. 223

Weak humus-mineral mixture

Mosaicof closelypackedpebbles,boulders

Dry, brown toreddish-brownwith variableaccumulationsof clay, calciumcarbonate, andsoluble salts

Desert Soil(hot, dry climate)

Grassland Soil(semiarid climate)

Alkaline,dark,and richin humus

Clay,calciumcompounds

Soil erosion

Causes – mainly water and windHuman induced causes – farming,

logging, mining, construction, overgrazing by livestock, off-road vehicles, burning, and more (go us!)

Soil erosion

TypesSheet

Uniform loss of soil, usually when water crosses a flat field

Rill Fast flowing water cuts small rivulets in soil

Gully Rivulets join to become larger, channel

becomes wider and deeper, usually on steeper slopes or where water moves fast

Global soil loss

This is a major problem world wide Have lost about 15% of land for agriculture to soil

erosion Overgrazing Deforestation Unsustainable farming

Also 40% of ag land is seriously degraded due to soil erosion, salinization, water logging and compaction

Fig. 10.21, p. 228

Moderate Severe Very Severe

Desertification of arid and semiarid lands

Fig. 10.19, p. 226

Areas of serious concern

Areas of some concern

Stable or nonvegetative areas

Global soil erosion

Desertification

Turning productive (fertile) soil into less productive soil (10% loss or more) Overgrazing Deforestation Surface mining Poor irrigation techniques Poor farming techniques Soil compaction

Salinization

As water flows over the land, salts are leached out When water irrigates a field it is left to evaporate

typically This repeated process causes the dissolved salts to

accumulate and possibly severely reduce plant productivity

Fields must be repeatedly flushed with fresh water to remove salt build up

Waterlogging

When fields are irrigated they allow water to sink into the soil.

Winds can dry the surfaceAs more water is applied the root area of

plants is over saturated reducing yield

As clay is brought to subsoil levels it can act as a boundary for water infiltration

Fig. 10.22, p. 229

Evaporation

Evaporation TranspirationEvaporation

Waterlogging

Less permeableclay layer

Soil conservation

Conservation tillage – (no till farming) disturb the soil as little as possible

Reducing erosion also helps – save fuel, cut costs, hold water, avoid compaction, allow more crops to be grown, increase yields, reduce release of carbon dioxide

Soil conservation

Terracing – making flat growing areas on hillsides

Contour farming – planting crops perpendicular to the hill slope, not parallel

Strip cropping – planting alternating rows of crops to replace lost soil nutrients (legumes)

Alley cropping – planting crops between rows of trees

Fig. 10.24b, p. 230

Control planting and strip cropping

Fig. 10.24c, p. 230

Alley cropping

Fig. 10.24a, p. 230

Terracing

Soil conservation

Gully reclamation – seeding with fast growing native grasses, slows erosion or “reverses” it Also building small dams traps sediments Building channels to divert water or slow water

Windbreaks – trees planted around open land to prevent erosion Retains soil moisture (shade, less wind) Habitats for birds, bees, etc.

Land classification – identify marginal land that should not be farmed

Fig. 10.24d, p. 230

Windbreaks

Soil fertility

Inorganic fertilizers – easily transported, stored, and applied Do not add humus – less water and air

holding ability, leads to compaction Only supply about 3 of 20 needed nutrients Requires large amount of energy for

production Releases nitrous oxide (N2O) during

production, a green house gas

Soil fertility

Organic fertilizers – the odor is a problemAnimal manure – difficult to collect and

transfer easily, hard to storeGreen manure – compost, aerates soil,

improves water retention, recycles nutrientsCrop rotation – allows nutrients to return to

soil, otherwise same crop continually strips same nutrient, keeps yields high, reduces erosion