Physical Landscape: Soils

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Soils are the interface between mineral geology and life. They are formed through the physical and chemical weathering of parent material (bedrock or surficial sediments.) The addition of decomposed organic material, air and water will further speed the soil building process. In some places like the tropics, soils have been forming for millions of years, but in a glaciated landscape such as Vermont, the most soils are a mere 13,500 years old. No matter how old they are, soils continue to develop as long as there are organic and mineral resources available. Some soils form more rapidly than others, and in some cases, such as soil

Samples from different soil horizons

formation in floodplains, it is one of the few geologic processes that we can observe on in our own lifetime.

Soil supports plant growth in several ways. It provides a substrate for plants to anchor their roots, and, depending on its composition, it supplies varying amounts of water, air and nutrients for the plant as well. Of the eight essential plant nutrients five are derived from soils. How much and which kinds of nutrients are available in the soil depends largely on the chemistry of the parent material. Soils that have a high clay or organic material content tend to have the largest amount of biologically available nutrients. In contrast, soils that are mostly sand, gravel or silt are more inert and provide fewer nutrients for plants and animals. In both cases weathering is the process that causes the break down of material and makes the nutrients available to living organisms.

What soils form where and how they affect the landscape is a broad and complex science. Most soil scientists use a soil classification system that organizes all of the world's soils into eleven orders. These orders are further broken down until they are finally split into families and series. The characteristics used to identify soils include grain size, moisture, existing soil horizons, mineralogy and sometimes temperature. Soil series are named after the location where the series was first described. As you become familiar with the soil series of your area, look for connections between soil type and landuse.

The Soils of Shelburne

The diversity expressed by the 30 different soil series found in the town of Shelburne is largely a reflection of the landscape's underlying geology. Many of the soils in town have a high silt and clay content, which is not surprising given the fact that the entire landscape was covered by a glacial lake at one point. Soils rich in clay have the potential to be nutrient-rich due to the high cation exchange capacity the negatively-charged clay particles. They also tend retain moisture well (sometimes too well for a farmer trying harvest crops from muddy field).

Many of the soils in Shelburne also tend to be rich in nutrients because of the calcareous nature of the underlying bedrock, which has a neutralizing effect on the pH of the soil. This is especially true on the ridges and knolls in town where calcium-enriched glacial till (and/or bedrock) is at or near the surface. This enrichment is clearly relflected in the natural vegetation of these area, which tend to be dominated by "rich-site indicators" such as American basswood, butternut, maidenhair fern, bloodroot, wild Ginger, and blue cohosh. In the areas immediately surrounding the mouth of the Laplatte River and Shelburne Pond, the wetland soils are categorized as peat and muck.