Soil Horizons

Organic Layer (O-horizon)

The uppermost layer; it is rich in organic material.

Plant litter accumulates in the O-horizon and gradually decays.

In desert soils the O-horizon is completely absent, but in certain organically rich soils it may be the dominant layer.

Topsoil (A-horizon)

It is dark and rich in accumulated organic matter and humus.It has a granular texture and is somewhat nutrient-poor due to the loss of many nutrient minerals to deeper layers and by leaching.

Subsoil (B-horizon)The light-colored subsoil beneath the A-horizon; it is often a zone of illuviation where nutrient minerals have leached out of the topsoil and litter accumulate. It is typically rich in iron and aluminum compounds and clay.

Parent Material (C-horizon)

This contains weathered pieces of rock and borders the unweathered solid parent material. Most roots do not go down this deep and it is often saturated with groundwater.

Fig. 3-23, p. 68

Fern

Mature soil

Honey fungus

Root system

Oak tree

Bacteria

Lords and ladies

Fungus

Actinomycetes

Nematode

Pseudoscorpion

Mite

RegolithYoung soil

Immature soil

Bedrock

Rockfragments

Moss and lichen

Organic debrisbuilds upGrasses and

small shrubs

Mole

Dog violet

Woodsorrel

EarthwormMillipede

O horizonLeaf litter

A horizon

Topsoil

B horizonSubsoil

C horizon

Parent material

Springtail

Red Earth Mite

Layers in Mature Soils

Infiltration: the downward movement of water through soil.

Leaching: dissolving of minerals and organic matter in upper layers carrying them to lower layers.

The soil type determines the degree of infiltration and leaching.

Fig. 3-24a, p. 69

Mosaic of closely packed pebbles, boulders

Weak humus-mineral mixture

Dry, brown to reddish-brown with variable accumulations of clay, calcium and carbonate, and soluble salts

Alkaline, dark, and rich in humus

Clay, calcium compounds

Desert Soil(hot, dry climate)

Grassland Soilsemiarid climate)

Fig. 3-24b, p. 69

Tropical Rain Forest Soil(humid, tropical climate)

Acidic light-colored humus

Iron and aluminum compounds mixed with clay

Fig. 3-24b, p. 69

Deciduous Forest Soil(humid, mild climate)

Forest litter leaf moldHumus-mineral mixtureLight, grayish-brown, silt loamDark brown firm clay

Fig. 3-24b, p. 69

Coniferous Forest Soil(humid, cold climate)

Light-colored and acidic

Acid litter and humus

Humus and iron and aluminum compounds

Case Study: Industrialized Food Production in the United States

Industrialized agriculture uses about 17% of all commercial energy in the U.S. and food travels an average 2,400 kilometers from farm to plate.

Figure 13-7Figure 13-7

Fig. 13-7, p. 277

4%

Food production

Food distribution and preparation

Food processing

LivestockCrops5%6%2% 17%

of total U.S. commercial energy use

Traditional Agriculture: Low Input PolycultureMany farmers in developing countries use low-input agriculture to grow a variety of crops on each plot of land (interplanting) through:

Polyvarietal cultivation: planting several genetic varieties.

Intercropping: two or more different crops grown at the same time in a plot.

Agroforestry: crops and trees are grown together.

Polyculture: different plants are planted together.