A. Physics & Geology

Soil formation

Soil formation, or pedogenesis, is the combined effect of physical, chemical, biological, and anthropogenic processes on soil parent material. Soil genesis involves processes that develop layers or horizons in the soil profile. 

These processes involve additions, losses, transformations and translocations of material that compose the soil. 

Minerals derived from weathered rocks undergo changes that cause the formation of secondary minerals and other compounds that are variably soluble in water, these constituents are moved (translocated) from one area of the soil to other areas by water and animal activity. The alteration and movement of materials within soil causes the formation of distinctive soil horizons.

The weathering of bedrock produces the parent material from which soils form. An example of soil development from bare rock occurs on recent lava flows in warm regions under heavy and very frequent rainfall. In such climates, plants become established very quickly on basaltic lava, even though there is very little organic material. 

The plants are supported by the porous rock as it is filled with nutrient-bearing water which carries, for example, dissolved minerals and guano. The developing plant roots, themselves are associated with mycorrhizal fungi that gradually break up the porous lava, and by these means organic matter soon accumulates.

But even before it does, the predominantly porous broken lava in which the plant roots grow can be considered a soil. How the soil "life" cycle proceeds is influenced by at least five classic soil forming factors that are dynamically intertwined in shaping the way soil is developed, they include: parent material, regional climatetopography, biotic potential and the passage of time.

Soil Profiles (or Horizons)

From Wikipedia article

soil horizon is a specific layer in the land area that is parallel to the soil surface and possesses physical characteristics which differ from the layers above and beneath. 

Horizon formation (horizonation) is a function of a range of geological, chemical, and biological processes and occurs over long time periods. 

Soils vary in the degree to which horizons are expressed. Relatively new deposits of soil parent material, such as alluviumsand dunes, or volcanic ash, may have no horizon formation, or only the distinct layers of deposition

As age increases, horizons generally are more easily observed. The exception occurs in some older soils, with few horizons expressed in deeply weathered soils, such as the oxisols in tropical areas with high annual precipitation.

Continue reading in Wikipedia

Types of Soils

There are roughly four different types of  soil, based on differences in structure and texture: clay, sand and gravel, silt, and loam. A fifth type, organic soil, is less common.    The make-up of the soil is important as we are trying to get a good balance of micro-pores and macro-pores (the gaps & spaces between the soil particles) in order to give the edaphic bacterial life a good home, to retain water & nutrients but also let gases & organisms circulate.  The rock components also provide the basic minerals which the bacteria & fungi then make available to the plants.

The descriptions that follow will help you identify your soil type.

1.Clay soils

Clay soil, composed of many small, flat particles, feels sticky or plastic in your hands. Clay comes in many colors: red, yellow, bluish gray, or almost black. 
Clay soils warm up and dry out slowly, take in water slowly, and can store reserves of nutrients better than most other soils.
Clay can become as hard as a rock duting dry, warm weather if not watered regularly. Once dried out, it is almost impossible to water clay soil adequately with sprinklers. 
The surface seals over and stops water from penetrating easily. If this happens, you should make irrigation furrows to hold the water until it can soak in. 
Improve clay soils by digging in generous amounts of organic matter, such as peat moss, compost, or well- rotted manure, to improve drainage and aeration. 
Gypsum also helps improve the texture of clay, but does not add nutrients to the soil. Adding only sand to clay will not improve it; the soil will continue to form crusts

2.Sand and gravel soils

If your soil looks and feels like a sandbox or gravel pit, you have sandy or gravelly soil. 
Sandy soil is easy to work and warms up rapidly. However, it dries out quickly and then may blow around. In direct sun it can reflect enough heat to damage a vegetable crop. 
Fortunately, most sandy soils contain enough clay particles to make them reasonably responsive to fertilizers. 
Pure sand contains almost no nutrients and has little capacity to store moisture. However, most sandy soils have enough clay particles to hold some nutrients.  Gravelly soils are usually a mixture of gravel and sand, silt, or clay. 
Generally low in organic matter, they are also low in natural fertility. The best way to improve sandy or gravelly soil is to remove the larger pebbles and stones; then add coarse organic matter, such as peat moss, compost, or well-rotted manure. 
Clay added to sandy or gravelly soils will tend to collect in impervious layers instead of improving the soil.

3.Silt soils 

Silt has an intermediate size between clay and sand. 
It consists of small, gritty particles that can pack down very hard. Silt ranges in color from gray to tan, yellow, and red. 
It’s usually not very fertile. Silt topsoils are often found over dense layers of clay that slow or stop drainage.
Both the topsoil and these lower layers should be broken up and kept loose by adding copious amounts of peat moss, compost, well-rotted sawdust, or wood shavings. 
Adding organic matter will improve the structure and fertility of silt soils. Adding clay or sand will not improve silt.

4.Loam Soils

Loam contains various proportions of clay, silt, sand, and organic matter. 
 The proportions of each determine how easy the soil is to cultivate and how productive it is. 
Sandy loam with a fairly high content of organic matter is the easiest to cultivate, water, and weed. 
A loam that contains more than one-third clay acts almost like solid clay and needs lots of added organic matter to make it easy to manage.

5.Organic soils

Dark in color, organic soils are composed largely of peat moss or leaf mold. Your soil is not likely to be organic unless your house is built on an old lakebed, bog, or forest site.      
Organic soils are easy to work, weed, and water, but may warm up slowly because they retain moisture.
Since organic soils are usually high in nitrogen, they can benefit from fertilizers high in phosphate and potassium. 
Micronutrient deficiencies (of iron, copper, cobalt, zinc, and manganese) are common in this kind of soil, but can be remedied by using special fertilizers containing the missing nutrients.

Soil Classification

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See Wikipedia link

Soil classification deals with the systematic categorization of soils based on distinguishing characteristics as well as criteria that dictate choices in use.

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Subpages (2): Introducing Soil SoilTests