Why does topsoil differ in thickness

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In his famous lyceum address Abraham Lincoln compared dedication to one's country to. It's important to understand the difference between temperate clays and tropical clays and why both types are found in the tropics. The figure refers to the ratio of silicate to aluminum plates in a clay particle's laminated structure. Soils with a good amount of these temperate clays are very sticky and plastic when wet; some kinds such as montmorillinite shrink and swell readily, forming large cracks upon drying out.

These clays have lost lots of silicate due to centuries of weathering and leaching. Unlike the clays, these "tropical" clays are much less sticky and plastic and are easier to work with, even when clay content is high. However, they usually have much less negative charge and lower natural fertility than temperate clays. Soils whose clay portion is largely "tropical" can usually be identified by their red or yellow colors.

Distinguishing ''tropical'' soils from ''temperate'' soils Note that "tropical" clays don't necessarily make up the major portion of the clay in all soils of the tropics. In fact, temperate clays are surprisingly common, especially in younger soils or those formed under drier conditions or where drainage isn't good.

A true tropical soil one whose clays are mainly I:1 or hydrous oxides requires good drainage, centuries of weathering, and lots of rainfall and leaching to form.

Iikewise, not all clays in the temperate zone are clays, especially in areas that may have once been tropical thousands of years ago. Some soils are mixes of both types. Spotting "tropical" soils: A distinct red or yellow color, especially in the subsoil may be one indication. Such soils are unlikely to form in depressions but are found on gentle to steep slopes where drainage is good.

The extent of tropical soils in the tropics: Overall, true tropical soils account for about half the soils in the tropics and often exist side by side with "temperate" ones. They're fairly diverse themselves and are grouped into 2 broad categories based on the current USDA U.

They are moderately to very acidic and may have a high capacity to "tie up" added phosphorus, preventing its full use by plants.

They're acidic and have high clay contents mainly of hydrous oxides , but don't tend to be very sticky when wet. Like ultisols, they may tie up added phosphorus readily.

One well known member of this order group are laterite soils whose subsoils are rich in a clayey material called plinthite that contains red mottles blotches and highly weathered oxides of iron and aluminum. Plinthite can harden irreversibly into ironstone formerly called laterite when exposed by erosion, as has occured following deforestation.

Organic matter - a soil's best friend Most cultivated soils contain about percent organic matter by weight in the topsoil. Despite its small proportion, organic matter has a remarkably beneficial effect on soil behavior and crop yields, especially in the form of humus partially decomposed organic matter that has become dark and crumbly; humus continues decomposing, but at a slower rate. Key Points The O horizon, or topsoil, is made of decaying organisms and plant life; it is responsible for plant production.

The A horizon is of a mixture of organic material and inorganic products of weathering; it is the beginning of true mineral soil. The materials engage students in understanding the earth system as it intertwines with key societal issues. The collection is freely available and ready to be adapted by undergraduate educators across a range of courses including: general education or majors courses in Earth-focused disciplines such as geoscience or environmental science, social science, engineering, and other sciences, as well as courses for interdisciplinary programs.

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Or you can download these pages in several formats that you can include in your course website or local Learning Managment System. Learn more about using, modifying, and sharing InTeGrate teaching materials. Soil Formation and Geography: How do soils form in different places? Below are some of the most important factors that contribute to creating a soil: Climate : climate has a big influence on soils over the long term because water from rain and warm temperatures will promote weathering, which is the dissolution of rock particles and liberating of nutrients that proceed in soils with the help of plant roots and microbes.

Weathering requires rainfall and is initially a positive process that replenishes these solubilized nutrients in soils year after year and helps plants to access nutrients. However over the long run thousands to millions of years and in rainy climates, rain water passing through a soil leaching leaves acid-producing elements in the soil like aluminum and hydrogen ions, and carries away more of the nutrients that foster a neutral pH e.

Old soils in rainy areas, therefore, tend to be more acidic, while dry-region soils tend to be neutral or alkaline in pH.

Acid soils can make it difficult for many crops to grow. Meanwhile, dry climate soils retain nutrients gained in weathering of rock -- a good thing -- but may lack plant cover because of dry conditions. A lack of plant cover leaves the soil unprotected from damage by soil erosion and means that dry climate soils often lack dead plant material residues to enrich the soil with organic matter.

Both dry and wet climate soils have advantages as well as challenges that must be addressed by human knowledge in managing them well so that they are protected as valuable resources. Parent Material : soils form through gradual modification of an original raw material like rock, ash, or river sediments.