3.4 The Soil System

The Soil SystemSoil is a complex mixture of eroded rock,

mineral nutrients, decaying organic matter, water, air and billions of living

organisms (microscopic decomposers).

Soil formation is a slow process:

1. Weathering of rock (mechanical).

2. Deposition of sediments by erosion (mechanical).

3. Decomposition of organic matter in dead organisms (chemical).

soil system integrates the biotic and abiotic

Mature soils are arranged in a series of zones called SOIL HORIZONS:

“O” HORIZON = freshly fallen and partially decomposed leaves, twigs, animal waste. You can find fungi and other organic materials.

“A” HORIZON = porous mixture of partially decomposed organic matter (humus) and some inorganic mineral particles.

These top two layers are most fertile, have the highest concentration of organic

matter, and contain large amounts of living organisms.

Flatworm

Rove beetle

AntCentipede

Mite

Pseudoscorpion

Groundbeetle

Adultfly

Millipede

Flylarvae

Sowbug

Mite

Earthworm

Slug

Snail

Roundworms

Protozoa

Bacteria

Organic debris

Beetle Mite

Fungi

Springtail

Actinomycetes

“B” (subsoil) and “C” (parent material) HORIZON contain most of the soil’s

inorganic matter, broken-down rock.

Soil Content

• Clay (very fine particles)• Silt (fine particles)• Sand (medium-size particles)• Gravel (coarse to very coarse particles) SOIL TEXTURE is determined by the relative amounts

of the different types and sizes of mineral particles.

100%clay

Increasingpercentage silt

Increasingpercentage clay

0

20

40

60

80

80

60

40

20

0

100%sand 80 60 40 20 100%silt

Increasing percentage sand

sandyclay

clay

siltyclay

silty clayloam

clayloam

loam siltyloam

silt

sandy clayloam

sandyloam

loamy sandsand

• Soil texture helps determine SOIL POROSITY, a measure of the volume of spores or spaces per volume of soil and the average space between those spaces.

• INFILTRATION is the downward movement of water through soils.

• As the water seeps down, it dissolves various soil components in upper layers and carries them down to lower layers in a process called LEACHING.

SOIL PERMEABILITY is the rate at which water and air move from upper to lower soil layers.

Water Water

High permeability Low permeability

Properties of Soils with Different Textures

Texture Nutrient Capacity

Infiltration Water-Holding Capacity

Aeration Workability

Clay Good Poor Good Poor Poor

Silt Medium Medium Medium Medium Medium

Sand Poor Good Poor Good Good

Loam Medium Medium Medium Medium medium

Nitrogen fixingby lightning

Commercialinorganicfertilizer

10-6-4N-P-K

Organic fertilizers,animal manure,

green manure, compost

Cropplant

Deadorganic matter

Applicationto land

Nitrogen fixingby bacteria

Nitrogen fixing

Weatheringof rock

Nutrient removalwith harvest

Decomposition

Supply ofavailable plant

nutrients in soilNutrient lossby bacterialprocesses

such asconversion

of nitrates tonitrogen gas

Nutrient lossfrom soil erosion

Absorption of nutrientsby roots

Pathway of plant

nutrients in soil.

Soil erosion is the movement of soil components, especially surface litter and topsoil.

• The two main agents of erosion are wind and flowing water.

• Loss of plant cover by farming, logging, construction, overgrazing by livestock, off-road vehicles, deliberate burning of vegetation and other activities leave soil vulnerable to erosion.

Two major harmful effects of soil erosion:

1. Loss of soil fertility and its ability to hold water

2. Runoff of sediment that pollutes water, kills fish and shellfish, and clog irrigation ditches, boat channels, reservoirs, and lakes.

serious concern

some concern

Stable areas

Desertification is the enlargement of deserts through human

activities.

ConsequencesCauses

Worsening drought

Famine

Economic losses

Lower living standards

Environmentalrefugees

Overgrazing

Deforestation

Surface mining

Erosion

Salinization

Soil compaction

SALINATION

1. Irrigation water contains small amounts of dissolved salts.

2. Evaporation and transpiration leave salts behind.

3. Salt builds up in soil.

WATERLOGGING

• Precipitation and irrigation water percolate downward.

• Water table rises.

Both result in stunted plant growth,

lower crop yields,

dead plants and ruined land.

Evaporation

EvaporationTranspiration

Evaporation

Waterlogging

Less permeableclay layer

Soil Conservation involves reducing soil erosion and restoring soil

fertility.

Advantages Disadvantages

Reduces erosion

Saves fuel

Cuts costs

Holds more soil water

Reduces soil compaction

Allows several crops per season

Does not reduce crop yields

Can increase herbicide use for some crops

Leaves stalks that canharbor crop pests and fungal diseases and increase pesticide use

Requires investment in expensive equipment

Advantages and disadvantages of using Conservation Tillage.

Contour planting and strip cropping: each row acts as a small dam to help hold soil and slow water

runoff.

Alley cropping or agroforestry: several crops are planted together in strips or alleys between trees and shrubs that can

provide fruit or fuel-wood, shade, help retain and slowly release soil moisture, and fodder for livestock.

Windbreaks or shelterbelts of trees reduce wind erosion, help retain soil, supply wood for fuel, and provide habitats for birds,

pest-eating and pollinating insects, and other animals.

Terracing retains water for crops at each level

and reduces soil erosion

by controlling runoff.

Soil Restoration

• Organic fertilizer

• Manure

• Compost crop rotation

• No till farming

• Contour farming

• Terracing

• Nitrogen fixation-legumes