8-II Soils, Mass Movements, and Soil Conservation

Soils are the result of weathering Without soil there could be no life on land. Soil is made of loose, weathered rock and organic material in which

plant with roots can grow. The rock material in soil contains three noticeable parts: sand, clay,

and silt.

The material from which a soil is formed is called its parent material Often this is the bedrock beneath the soil. Soil that has bedrock as its parent material is residual soil Soils formed from materials left by wind, rivers, and glaciers are

called transported soils

A Mature Soil Profile The cross section of earth exposed by

digging through layers of soil is called the soil profile.

In most mature soils, three distinct zones(horizons) can be seen:

The A-horizon is topsoil, its colour is gray to black. It has organic material, or humus formed from decayed plant and animal materials

The B-horizon begins with the subsoil. Much of the clay in the topsoil is washed to the subsoil. Its colour is usually red or brown from iron oxides.

The C-horizon is made of slightly weathered parent material, such as rock fragments. Near the bottom they sit on unweathered bedrock.

Soil Types and Climates

The most important factor affecting soil is the climate.

A tropical soil forms in the areas that have constant high temperature and heavy rainfall.

Warm, wet conditions speed up the chemical weathering, and soil forms quickly.

The soil profile may be more than three metres thick.

Frequent heavy rains wash nutrients out of the soil, making tropical soils relatively infertile.

Grassland soils form in areas that receive enough rainfall for heavy grass, but not enough for trees.

The soil profile is usually less than a metre thick.

The A-horizon are black or deep-brown.

Grassland soils are very rich in organic matter, and are very fertile soils.

Forest soils form in humid regions that have cool seasons and forests of hardwood and evergreen trees.

They have well developed A, B, and C-horizons. The soil profile is usually less than a metre in total thickness. Not very fertile.

• Desert soils form in very dry climates.

• Profiles are seldom more than a few centimetres from top to bottom.

• Nutrients are not washed away from the soil, and can be very fertile when watered

• Arctic soils form at high elevations and high latitudes.

• The surface is poorly drained and boggy.

• The bottom layers are constantly frozen (permafrost).

• Soil profile is often only a few centimetres thick.

This is a soil profile, from the top surface down through horizons, including some parent material (pedogenically unaltered and similar to that in which the soil formed). This soil is Kuslinad loamy-mixed, nonacid Histic Pergelic Cryaquept found in the

Gulkana River Corridor. (photograph courtesy of Mark Clark, Soil Scientist, NRCS)

Mass Movements• Wherever the ground slopes, gravity causes soil and rock

fragments to fall, slide, or move at very slow speeds to lower levels.

• These movements are called mass movements; there are several important types:

• Creep is a slow, imperceptible downslope movement of the soil.

• Talus is a pile of rock fragments at the base of a cliff, and is the result of mass movements near steep slopes

• A landslide is the sudden movement of a mass of bedrock or loose rock down the slope of a hill, mountain, or cliff.

• An avalanche is a landslide made from masses of snow, ice, soil, or rock, or mixtures of these materials.

• Landslides are likely to occur on steep slopes, especially those caused by erosion or by mining.

• Slumps occur when small blocks of land tile and move downhill

• A mudflow is the rapid movement of a water-saturated mass of clay and silt.

• A mud avalanche is an especially fast and large mudflow, often caused by volcanic eruptions quickly melting glaciers.

The top image of the lahar channel shows the area right after the collapse of New Zealand's Crater Lake's walls. Below, the lahar channel is pictured just before the geologic event in New Zealand. (Credit: University of Hawaii) Link

Soil Conservation• Soil erosion is the removal of topsoil by the action of running

water or wind, it is a serious environmental and economic problem.

• Soil erosion can be reduced by a number of methods.

• Planting windbreaks, which are belts of trees along the edges of fields. The trees slow the wind and reduce wind erosion.

• A second method of reducing erosion is contour farming.

• Crops are planted in rows parallel to land contours.

• This prevents water from flowing rapidly downhill and carrying soil with it.

• Flattening hillslopes into terraces also slows stream flow and reduces soil erosion.

Contour farming improves water qualityby reducing sedimentation and runoff

• Another method of reducing soil erosion is strip cropping.

• A crop that leaves bare ground between rows is alternated with a crop that completely covers the ground.

• The no-till method uses the technique of plowing, planting, fertilizing and weed control being done at the same time.

• The soil is not disturbed again until harvest.

Strip cropping is a highly effective way to reduce or prevent erosion. The alternating strips of forage or cereal crops cover the soil and slow water movement across the field.