Soil formation

Teaching Guide for DiscussionScience Grade 7

MA. RACHEL B. ESPINOBuhatan National High School

Sorsogon City

SOIL FORMATION

Soil covers the entire Earth. Temperature, rainfall, chemical changes and biological action act together to continuously form soil. Climate, expressed as both temperature and rainfall effects, is often considered the most powerful soil-forming factor.

What factors affect soil formation?

Temperature controls how fast chemical reactions occur. Many reactions proceed more quickly as temperature increases. Warm-region soils are normally more developed or more mature than cold-region soils. Mature soils have more silt and clay on or near the surface. Thus, soils in the tropical areas are observed to sustain various farming activities and account for why the primary source of livelihood in the Philippines and other countries in the tropical region is their fertile land.

Effect of Temperature to Soil Formation

Climate (temperature and rainfall) is a significant factor not only in soil formation but also in sustaining diversity of plants and animals in the country. On the other hand, water also directly affects the movement of soluble soil nutrients from the top soil to deep under the ground (leaching). These nutrients may no longer be available to shallow rooted plants. Acidic rainwater may also contribute to the loss of minerals in soil resulting in low yield. So rainfall determines the kind of vegetation in an area. In turn, the degree of vegetation cover, especially in sloping areas, determines how much soil is removed.

Effect of Climate to Soil Formation

Over million of years, the forces of weathering and erosion, slowly but constantly have produced a layer of broken pieces of rock all over the earth’s surface. The size of these rock pieces varies from boulders to the finest bits. The layer composed of pieces of rock is called ROCK MANTLE. As the forces of wind and water go on breaking and decomposing the rock mantle, a layer of fine loose materials is formed. This is referred to as SOIL. It is made up of tiny grains of rocks and materials.

How is soil formed?

The natural processes of weathering and erosion bring about important changes on the surface rock. These processes consistently break, carry off and deposit loose surface materials from where they are formed to other places. The continued action of water and wind changes the rock pieces into finer particles, thus forming boulders, gravel, sand and clay.

To these surface materials are added the decayed remains of plants and animals called HUMUS. Organic substances are returned into the soul after they decompose. Because of their finer texture, these enable the soil to hold much water.

A

B

C

The natural processes that slowly breaks rocks into little pieces is called WEATHERING. It is a very slow but continuous process. It wears down the surface of the earth.

Soil is formed by weathering

The Hundred Islands in the Philippines is made through continuous weathering of

rocks around the islands.

Mechanical weathering includes processes that break rocks to pieces without changing their chemical composition. It is brought about by severe environmental conditions.a) Rapid changes in temperature. During the day

the rock are heated and then expand. At night, they are cooled and then contract. Since rocks are composed of minerals, these minerals expand and contract too. This alternate expansion and contraction causes the rocks to crack, especially if the minerals expand and contract in unequal amounts.

Mechanical Weathering

Sometimes this alternate expansion and contraction may cause the rocks to peel off in thin sheets. This peeling off process is called exfoliation.

b) Force of freezing water. Water may enter the cracks or crevices in rocks. When it freezes it expands by about 9% of its volume, causing the rock to split.

c) Roots of trees. Some seeds are accidentally thrown into the cracks in rocks. When they grow, the roots creep in and the force of their growing can split rocks.

d) Burrowing animals. Animals such as earthworms, ants and the larvae of other insects dig into the rocks, thus exposing fresh surfaces to further weathering.

e) The wind. The wind with its load of fine rock particles may rub or abrade exposed rock surfaces.

f) Rocks. Rocks also wear away other rocks. Rocks on a hillside may roll down and rub against other rock surfaces. Rocks dropping from a higher place often break the rocks they hit farther down. Small pieces of rocks like sand and gravel may scratch other rocks as they are carried off by water.

g) The beating force of water can break rocks. The exposed rock in a river bank can be observed to have smooth rounded surfaces. The sharp edges have been worn away by the constant flowing of the water. The stones you can see in river beds differ from those in other places. Their surfaces are smoother and they are more rounded in shape. Peebles on the seashore are smooth and rounded for the same reason.

In chemical weathering, the composition of the rock is altered. A new product which easily crumbles to pieces is formed. Chemical weathering is brought about by a chemical reaction between the minerals in the rocks and carbon dioxide, oxygen or water.

Chemical Weathering

Biological weathering involves the disintegration of rock and minerals due to the chemical and/ or physical agents of an organisms.

Biological Weathering

Limestone Rock Formation in Caramoan, Camarines Sur

Rock Formations in the Philippines caused by weathering

Biri Rock FormationBiri, Northern Samar

Rock Formation in Magpupingko PoolPilar, Siargao

Rock Formation in Paguriran IslandSawanga, Bacon District, Sorsogon City

Rock Formation in Tikling IslandMatnog, Sorsogon

To sum it up, the processes of soil formation is as follows:

1. When a piece of rock is exposed to the sun, its outer part expands (becomes bigger) because it heats up faster than the inner part .

2. On cooling at night time, the outer part of the rock contracts or shrinks because the outer part of the rock cools faster than the inner portion. The process of expansion and contraction are repeated over the years and produce cracks in the rock causing the outer surface to break off.

3. Once broken, water enters the cracks, causing some minerals to dissolve. The rock breaks apart further.

3. Once broken, water enters the cracks, causing some minerals to dissolve. The rock breaks apart further.

4. Air also enters the cracks and oxygen in the air combines with some elements such as iron to produce iron oxide (rust or kalawang) which is brittle and will easily peel off. In a similar way, CO2 from the air reacts with water to form an acid causing the rock to soften further. Once soft and broken, bacteria and small plants start to grow in the cracks of the rock.

After some time, the dead plants and animals die and decay causing the formation of more acidic substances which further breaks the rocks. The dead bodies of plants and animals are acted upon by microorganism and breakdown into smaller compounds while the minerals from the rock return to the soil.

1. Regulates the flow of water on land.2. Sustains plant and animal life.3. Filters potential pollutants. The minerals

and microbes in soil are responsible for filtering, buffering, degrading, immobilizing and detoxifying organic and inorganic materials including industrial and municipal by-products and atmospheric deposits.

4. Aids in cycling nutrients.5. Supports structures of Earth.

Five Essential Functions of Soil

Science Grade 7 Learners Material (Part Two). DepEd Philippines, 2012

Integrated Science I. DepEd Philippines, 2009

Salandanan, Gloria G. Earth Science. Quezon City: Phoenix Publishing House, 1988

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