Weathering and Soil Chapter 7

Weathering and its effects Sec 7-1 H.W. pg 187 ques. 1-4 Weathering is the mechanical breakdown of rock into sediment like sand, silt and clay. There are two types of weathering: mechanical weathering and chemical weathering Mechanical weathering occurs when rocks are broken apart by physical processes and the overall chemical makeup of the rock does not change. Every fragment of rock has similar characteristics to the original rock.

Mechanical Weathering cont’d Plants and animals cause mechanical weathering in nature. As plants grow, their roots enlarge and crack soil and animals that burrow, loosen sediment and push it to the surface where other weathering can occur. Ice weathers rock by getting into cracks and freezing, therefore pushing or cracking the rock. Mechanical weathering increases the surface area of rock sediments and speeds up chemical weathering.

Chemical Weathering This occurs when chemical reactions dissolve the minerals in rocks or change them into different minerals. Here the chemical composition of the rock is changed and can weaken the rock. Natural acids like carbonic acid, forms when water reacts with carbon dioxide in the air, and cause rock to dissolve (limestone). Feldspar is broken down into a clay mineral called kaolinite when it comes into contact with a natural acid. Clay is the end product of weathering.

Effects of Rock Type The type of rock depends on how easily it is weathered. In wet climates, marble weathers more rapidly than granite.

The Nature of Soil Sec 7-2 H.W. pg 194 ques 1-4 Soil is a mixture of weathered rock, decayed organic matter, mineral fragments, water, and air. Soil takes thousands of years to form and can be 60 meters thick or just a few centimeters. Climate, slope, types of rock, types of vegetation, and the length of time that rock has been weathered, all affect what type of soil is formed.

Composition of Soil In addition to the things we spoke of in soil already, most organic matter in soil comes from plants. The other organic matter comes from microorganisms. Decayed organic matter, such as plants, turns into a dark colored material called humus, which serves as a source of nutrients for plants. Humus gets mixed into the ground by worms, insects and rodents that burrow into the soil.

Soil Profiles Soils have different layers called horizons. These horizons altogether form a soil profile. There are 3 soil horizons, A,B, and C. A horizon, also called topsoil, is the top layer of soil. Its usually covered with litter, which is made up of leaves, twigs, and other organic material that can be changed into humus by decomposing organisms. Litter helps prevent erosion and water evaporation from the soil. Topsoil has more humus than mineral and rock particles than other soil profiles. Humus gives the topsoil its dark color and provides nutrients for plant growth.

Soil Profiles B Horizon lies underneath the A horizon and has less organic material. This makes it lighter in color than an A horizon, and there is less humus. A B horizon is less fertile, and contains material that has moved down from the A horizon by a process called leeching. Leeching is the removal of minerals that have been dissolved in water (coffeemaker).

Soil Profiles C horizon consists of partially weathered rock and it is the bottom most and thickest horizon in a soil profile. It has hardly any organic matter, its not affected by leeching, and is lightest in color of all horizons. You find a rock in a C horizon. This rock will eventually become horizon A and B through erosion and weathering. We call the rock in C horizon parent material.

Soil profile

Glacial Deposits Glaciers deposit a thick layer of sediment on land. This sediment contains clay, silt, sand, and boulders that cover much of the northern U.S. this type of sediment are extremely fertile and are important for agriculture in many regions like the midwest.

Soil Types Not all soils are the same. Some are thick and red, while others could be brown. Climate can reflect soil type. Places with different climates such as deserts and forests will have different soil types. A desert will contain little organic material, and are also thinner than soils in a wetter environment. Prairies will have a thick, dark A horizon because of contributions of the growing grass.

Other Factors of Soil Parent material has an effect on the type of soil that will develop. Clay soils often develop on rocks like basalt, because minerals in the rock weather to form clay. The type of rock also effects the type of vegetation that grows in the region, because different rocks provide different amounts of nutrients. Slope is another factor that effects soil. On steep slopes, soil is poorly developed, because the materials cannot be weathered much.

Soil Erosion Sec 7-3 H.W. pg 199 ques 1-4 Erosion occurs as water flows over earths surface, or when wind picks up and transports sediments. Erosion is more severe on steep slopes and in areas of little vegetation. Soil production, and soil erosion, are usually in balance because soil can form at the same rate as it erodes. Humans can cause this balance to be disrupted, by removing ground cover, which is vegetation that covers soil and protects it from erosion.

Soil Erosion Top soil contains many nutrients and water that are good fro plant growth. If this topsoil is eroded, you reduce the quality of the soil. This will cause less agriculture to grow in that area which can negatively effect farming.

Soil Erosion Removing a forest, will increase erosion because soil is being exposed. Soils in tropical rain forests, appear to be rich in nutrients, however it is almost infertile because most of the topsoil has been eroded. Overgrazing is also a problem. If too much or too many sheep and cattle graze in an area, they are removing the ground cover that protects the topsoil, which can then be eroded away.

Preventing Soil Erosion H.W. pg 204 ques. 1-14 & pg 206 ques 1-10 No-till farming provides cover for the soil year round, and reduces soil erosion. Contour farming is another way to reduce soil erosion. Contour farming is when you plant along the natural contours of the land. This slows the flow of water down the slope, and prevents erosion of soil. On steep slopes, terracing is a method in which steep sided, level topped areas are built on the sides of steep hills so crops can be grown. This reduces the slope which reduces erosion.

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