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 soil  Soil profile – cross-section view of soil

Horizons

 O horizon – surface litter  A 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.

Oak tree Fern Word sorrel Lords and ladies Dog violet Earthworm Millipede Mole Honey fungus Grasses and small shrubs Organic debris Builds up Moss and lichen Rock fragments

O horizon

Leaf litter

A horizon

Topsoil

B horizon

Subsoil

C horizon

Parent material Root system

Mature soil

Red earth mite Springtail Bacteria Bedrock

Immature soil

Regolith

Young soil

Pseudoscorpion Mite Nematode Actinomycetes Fungus

Fig. 10.12, p. 220

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

Sandy soil

Low permeability

Clay soil

Soils

 Clay – high porosity, low permeability  Sand – high permeability, low porosity  Acidity is another factor  Where rain is low, calcium and other alkaline compounds may build up (sulfur can be added – turns to sulfuric acid by bacteria)

Tropical Rain Forest Soil (humid, tropical climate) Acidic light colored humus Iron and aluminum compounds mixed with clay Deciduous Forest Soil (humid, mild climate) Forest litter leaf mold Humus-mineral mixture Light, grayish brown, silt loam Dark brown Firm clay Acid litter and humus Light-colored and acidic Humus and iron and aluminum compounds Coniferous Forest Soil (humid, cold climate) Fig. 10.15b, p. 223

Desert Soil (hot, dry climate) Mosaic of closely packed pebbles, boulders Weak humus mineral mixture Dry, brown to reddish-brown with variable accumulations of clay, calcium carbonate, and soluble salts Grassland Soil (semiarid climate) Alkaline, dark, and rich in humus Clay, calcium compounds Fig. 10.15a, p. 223

Soil erosion

 Causes – mainly water and wind  Human induced causes – farming, logging, mining, construction, overgrazing by livestock, off-road vehicles, burning, and more (go us!)

Soil erosion

 Types  Sheet  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

Moderate Severe

Desertification of arid and semiarid lands

Very Severe Fig. 10.21, p. 228

Global soil erosion

Areas of serious concern Areas of some concern Stable or nonvegetative areas Fig. 10.19, p. 226

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 surface  As 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

Evaporation Evaporation Transpiration Evaporation Waterlogging Less permeable clay layer Fig. 10.22, p. 229

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

Control planting and strip cropping Fig. 10.24b, p. 230

Alley cropping Fig. 10.24c, p. 230

Terracing Fig. 10.24a, p. 230

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

Windbreaks Fig. 10.24d, p. 230

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 (N 2 O) during production, a green house gas

Soil fertility

 Organic fertilizers – the odor is a problem  Animal manure – difficult to collect and transfer easily, hard to store  Green manure – compost, aerates soil, improves water retention, recycles nutrients  Crop rotation – allows nutrients to return to soil, otherwise same crop continually strips same nutrient, keeps yields high, reduces erosion