Plymouth County Soil Survey Update
FACTORS OF SOIL FORMATION
Soil Profile Development
PowerPoint presentation on Soil Formation
Link to Soil Formation/Classification NRCS Web Page
Soils are formed through the interaction of five major factors: time, climate, parent material, topography and relief, and organisms. The relative influence of each factor varies from place to place, but the combination of all five factors normally determines the kind of soil developing in any given place. In Plymouth County parent material and relief as it relates to drainage, account for many of the differences among the named soils. The following are brief explanations of the factors contributing to soil formation in Plymouth County Massachusetts.
The formation of soils is a continuing process and generally takes several thousand years for significant changes to take place. The soils of Plymouth County began to develop immediately after the ice retreat of the last glaciation (Pleistocene), approximately 14,000 years ago. The soils are considered to be relatively young soils with slight alteration of parent material and weak soil horizon development. Most of the soil orders mapped in Plymouth County are Inceptisols, Entisols, and Spodosols.
Climate, particularly temperature, precipitation and frost action have a profound influence on the soil forming processes which occur within a region. The kind of climate largely determines the nature of the weathering processes that will occur and the rates of these chemical and physical processes. It directly affects the type of vegetation in an area which in turn will affect those soil forming processes related to vegetation. Plymouth County is in a transitional climate zone with conditions characteristic of both humid marine and humid continental climates. The winters are quite mild for New England due to the moderating effect of the surrounding Atlantic waters. Temperature and precipitation (approximately 40 in/yr.) govern the rate of chemical and physical weathering of the soils and allows for the accumulation of organic matter in the surface layer of the soils. Moisture is sufficient enough to promote leaching of water-soluble material down through the soil. Cold winter temperatures allow for frost action which physically break apart rock fragments.
Parent Material: Link to General Geology
of Plymouth County, Massachusetts
Parent material is the unconsolidated mineral and organic deposits in which soils are developing. It determines the mineralogical composition and contributes largely to the physical and chemical characteristics of the soil. The kind of parent material also influences the rate at which soil forming processes take place.
Due to the effects of the Wisconsinan glaciation, insufficient time has elapsed to significantly alter the glacial deposits. The influence of parent material is very apparent in the soils of Plymouth County. Parent material is roughly divided into two broad groups; glacial (Late Pleistocene) deposits and post glacial (Holocene) deposits. There are four general types of glacial deposits recognized in the survey area, they are; till, fluvial, lacustrine, and ice-contact deposits. Post glacial deposits are sediments which were deposited after the last glacial period which ended approximately 10,000 years ago. Post glacial deposits recognized in the survey area include, eolian silts and sands, alluvial (floodplain) deposits, freshwater and marine organic deposits, and coastal beaches and sand dunes desposits.
Glacial till is dominantly unsorted and unstratified sediments, deposited directly (beneath and within) by glacial ice. Till consists of a heterogeneous mixture of clay, silt, sand, gravel, stones and boulders. Two broad groupings of till have been recognized by soil scientists in Plymouth County. One is referred to as dense or basal till and is characterized by a loamy texture, with a moderate percentage of coarse fragments, and a dense, firm, slowly permeable substratum; locally referred to as hardpan. The Paxton and Montauk soils are representative soils that developed in these sediments. The other till referred to as ablation till, has a sandy texture, with a high percentage of coarse fragments, and a loose, permeable substratum. The Gloucester and Canton soils are representative soils that developed in this material.
Glacial fluvial (outwash) material is stratified sand and gravel deposited by glacial meltwater streams. Soils that have developed in glacial outwash have a sandy or gravelly, loose, very permeable substratum. Glacial fluvial deposits are associated with aquifer recharge areas. Soils commonly mapped in glacial outwash areas include Carver, Hinckley and Merrimac soils. Click here for a glacial fluvial landform image.
Glacial lacustrine, commonly referred to as lakebed deposits are varied ranging from sands and gravels to silts and clays. These materials were deposited within glacial lakes which have since drained or have filled with sediments. Soil patterns are complex within these areas and often vary dramatically, both chemically and physically, over short distances. Soils that have developed in glacial lakebed deposits include the Scio, Hinesburg, Raynham and Birdsall soils.
Ice contact deposits are varied ranging from till to bouldery sand and gravel. These materials were deposited in contact with an ice mass. Examples of ice contact features include; kames, heads of outwash, and kettles. Soils that have developed in ice contact deposits include the Plymouth, Barnstable, Canton, and Hinckley soils.
Post glacial eolian sediments are sand and silt deposited by wind during the period after the glacial ice melted from the area and before a permanent vegetative cover was established, stabilizing the newly exposed land surface. Eolian sands and silt occur as a discontinuous mantle or cap, variable in thickness, over glacial deposits. Most of the upland soils in Plymouth County are capped with an eolian mantle. Haven soils developed in areas where this mantle is thick (18 to 36 inches) and overlies glacial outwash. The loamy surface and upper solum of the Barnstable and Merrimac soils is the result of a thin capping of eolian material that has been mixed, through natural processes, with the underlying material. Carver soils have a coarse sand eolian mantle.
Organic deposits are accumulations of plant materials of varying degrees of decomposition that have formed in wet areas and are 16 inches or greater in thickness. The Freetown and Swansea soils developed in freshwater organic deposits. The Ipswich and Pawcatuck soils developed in organic tidal marsh deposits.
Alluvial deposits are recent material which has been deposited by flowing water. Alluvial deposits occur along the major rivers of the county. The Winoski and Limerick soils are examples of alluvial soils.
Beaches are unvegetated, wave washed accumulations of sand that do not meet the criteria for a soil and are mapped as a miscellaneous area. Sand dunes are windblown deposits of sand. Hooksan soils developed within areas of sand dunes.
Topography and Relief
The shape of the land surface, its slope and position on the landscape, greatly influence the kinds of soils formed. In Plymouth County soils that formed in similar parent materials with the same climatic conditions exhibit differences as a result of their position on the landscape. These differences are largely a result of varying drainage conditions due to surface runoff or depth to water table.
Soils that developed on higher elevations and sloping areas are generally excessively drained or well drained. Depth to groundwater is generally greater than 6 feet and surface runoff is moderate or rapid. Soil profiles within these areas commonly have a bright colored strong brown to yellowish brown upper solum grading to a lighter, grayer, unweathered substratum.
Soils that occur at lower elevations such as in swales, adjacent to drainage-ways and water bodies, and within depressions generally receive surface runoff from higher elevations and often have a seasonal high water table at a shallow depth. Soil profiles within moderately well drained and poorly drained areas are mottled with irregular spots of brown, yellow and grey colors. In very poorly drained areas, where the water table is at or near the surface for prolonged periods, soil profiles characteristically have a dark-colored organic or organic rich surface layer underlain by a strongly mottled or gleyed (gray color indicating a reduced condition) subsoil and substratum.
Permeability of the soil material; as well as the length, steepness, and configuration of the slopes, influence the kind of soil that is formed in an area. The local differences in the soils mapped in Plymouth County are largely the results of differences in parent material and topography.
All living organisms actively influence the soil forming process. These organisms include bacteria, fungi, vegetation and animals. Their major influence is the effect on the chemical and physical environment of the soils.
Most, if not all of Plymouth County, was originally in native forest of mixed hardwoods and conifers in varying proportions. The mineral element content of leaves and branches varies depending on the type of forest vegetation, and influences the characteristics of the soils that develop beneath it. Hardwoods characteristically take up bases (calcium, magnesium, and potassium) from the soil and return them to the soil surface in the form of organic litter; thus recycling the soil nutrients. Coniferous trees tend to be low in bases, consequently soils developed beneath them tend to be more acid. Bases are also more susceptible to leaching beneath coniferous trees. Mixing of the soil, due to tree throws, is also a characteristic of woodland soils.
Some types of micro-organisms promote acid conditions and change the chemistry of the soil which in turn influences the type of soil forming processes that take place. Microbial animals decompose organic materials and return the products of decomposition to the soil.
Larger animals such as earthworms and burrowing animals mix the soil and change its physical characteristics. They generally make the soil more permeable to air and water. Their waste products cause aggregation of the soil particles and improve soil structure.
Man's activities have significantly altered many areas of natural soils in the county. The chemical and physical properties, particularly of the plow layer, have changed with cultivation and the addition of lime and fertilizer. Artificial drainage and filling have altered the environment of some naturally wet soils. Of all the animals, man can have the most beneficial or most detrimental impact on the soil forming processes.
SOIL PROFILE DEVELOPMENT
Link to Power Point Presentation on Soil Horizons
The interaction of the five soil-forming factors; time, climate, parent material, topography, and plant and animal life, result in the development of a soil profile. A soil profile is a vertical section of the soil beginning at the surface and extending down into the unconsolidated underlying material to a depth of 60 inches or more. A soil horizon is a layer of soil, approximately parallel to the soil surface, with distinct characteristics produced by soil-forming processes. The physical and chemical characteristics observed within the soil profile are the basis for differentiating one soil from another.
The majority of the soils within the survey area exhibit weak soil profile development with little change or alteration of parent material, due primarily to the relatively young age of the soils. The depth of soil profile development varies between the different soils and generally averages about 30 inches in the well drained soils.
Soil profile development is generally shallower in the poorly and very poorly drained soils, and may be absent or very weak in recently deposited material, as in the case of the Hooksan soils.
Organic matter has accumulated on the surface of soils as O-horizons with varying degrees of thickness and decomposition. Where natural mixing of humified organic material and the underlying mineral matter has occurred, an A-horizon is present. The amount of organic matter added to the soils in the survey area varies with the kind of vegetation, moisture, and drainage condition. The thick, mucky, organic deposits that the Freetown and Swansea soils are developing in are the result of a very poorly drained condition where organic material accumulates within this very wet environment rather than being oxidized. In areas that have been cultivated, the surface organic layers and the upper solum of the soil have been mixed to form an Ap-horizon.
The soil profile development characteristic of many excessively drained, well drained, and moderately well drained soils mapped in the survey area is the result of movement and deposition of aluminum, iron, clay, and humified organic matter within the soil profile. Weak organic acids generated from the decomposition of surface organic litter are percolated downward through the soil by rainwater. Aluminum and iron within the upper portion of the soil profile are released into solution and leached downward, along with fine particles of humified organic matter and small amounts of fine clay. The light gray color in the E-horizon (surface mineral horizon below the O-layers or A-horizon) has resulted from this leaching and is more evident in the coarser textured soils and often absent in the finer textured soils. With depth, the chemical environment within the soil changes and the aluminum, iron, clay, and organic material precipitate out forming the B-horizons. The greatest concentration of leached material precipitates out just below the E-horizon and often forms a strong brown Bhs or Bw1 horizon. Undisturbed soil profiles of Carver, and Montauk soils commonly have an E-horizon underlain by a brightly colored B-horizon. The characteristic dark brown to yellowish brown color within the subsoil is due primarily to iron oxide stains on the surfaces of sand-size particles. Color within the subsoil generally fades with depth. The unweathered parent material in the C-horizon is often light yellowish brown or light olive brown.
Where there is a water table that is within or fluctuates within the soil profile, there are soil color changes referred to as soil mottling. Soil mottles are the combination of gray and reddish spots produced by alternating aerated and saturated conditions (oxidation-reduction process) within the soil profile. These spots are caused principally by migration, depletion or concentration of iron within the soil (Soil Survey Manual). Gleying is a condition that develops when the soil is wet for most of the year and the soil matrix color is gray or bluish gray due to the removal of iron caused by prolonged reducing conditions. Soil mottling is common in the upper solum of the poorly drained Ridgebury and Walpole soils; and in the lower portion of the solum in the Scio, Scituate, Deerfield and Sudbury soils. Gleying is characteristic of the very poorly drained Birdsall soils. Induration of sand grains caused by the concentration of iron (ferric) occurs within the soil profile of some of the very poorly drained Berryland soils.
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