Local climatic variations such as the warmer winter temperatures and higher winds found along the coastline account for some of the diversity in plant communities. Disturbance and human manipulation of the plant community also produce highly visible results. However, the most widespread influence on plant community diversity in Rhode Island is the native soil.

Rhode Island's oldest soils are actually very young. Most Rhode Island soils began forming with the raw mineral materials deposited by the last glacial retreat around 12,000 years ago. Since then, a lot has happened. Rock, sand, silt, and in a few areas, clay, have slowly weathered. In most upland areas, the upper few feet of the soil have had enough oxygen in the pore spaces to oxidize iron into a thin coating on the sand and silt. The resulting yellowish brown layer (or in the East Bay area, olive-colored layer) is known to many people as the "subsoil."

When the glaciers melted, the landscape was barren. It provided great opportunities for early succession plants. As plants colonized, decaying organic matter gradually accumulated in the upper layers of the soil. The combination of the acids released by decaying organic matter and the inorganic acids produced by the weathering of the soil's minerals has caused most soils in Rhode Island to be very acidic (unless limed).

Human disturbance has also contributed to the development of our soil. Most upland areas in Rhode Island were cleared of trees and cultivated or used for pasture at some time during the past three hundred years. As a result, we often see a thin "plow layer" even in areas with mature forests today. Virgin soils in uplands, which have formed without any significant disturbance, are rare in Rhode Island. They consist of a one- or two-inch thick organic layer over a similarly thin dark brown mineral layer. This dark brown mineral layer has a significant concentration of highly decomposed organic matter, often referred to as humus. In virgin soils the top of the subsoil is often only three or four inches below the surface.

So, which soil characteristics influence the establishment of a particular plant community in an area? In Rhode Island, by far the most significant feature is what soil scientists refer to as "available moisture." Soil moisture can range from very low in upland areas having coarse-textured sand and gravel to very high in mucky, saturated wetland soils. Distinctly different plant communities inhabit these two extremes. In between, we see a wide variety of intergrade vegetation communities. The vast majority of land in RI has soils in the moderate available moisture category.

Given the strong influence of available moisture on natural plant selection, what soil features influence available soil moisture? In uplands without a seasonal high water table, the available moisture of the soil is largely a function of the size of the mineral particles. Soils with many large particles (sand or gravel) and few fine ones (silt or clay) have large pore spaces between the particles. Water and plant nutrients  readily flush through these coarse-textured and chemically inactive soils, creating dry, infertile conditions. Sweetfern (Comptonia peregrina), Pitch Pine (Pinus rigida), and Bearberry (Arctostaphylos uva-ursi) are examples of plants that grow in these conditions. Soils containing smaller particles have greater available moisture. Silty and/or clayey soils have small pores between the particles. Water flows much more slowly through these, as there is much more adhesion of the water to the soil particles. After a rain, water stays in the soil pores and is available to plant roots for many days. Soils high in silt are Rhode Island's most productive agricultural soils. They are most common in South County and the East Bay. Clayey soils, which are very rare in Rhode Island, have very small pore spaces due to the extremely small size of clay particles. Clayey soils typically hold a lot of water, but much of it is not fully available to plants, because the water molecules are so tightly bound to the clay particles. I am not, however, aware of any sites in Rhode Island where the clay content of the soil is high enough to noticeably influence the plant community.

In high water table areas, the soil's texture is not a major factor influencing the available moisture. In these areas, the pores between the soil particles remain saturated for many weeks or months. Plants must be able to tolerate the excess water and the resulting low oxygen levels in the root zone. Because of the extended periods of low oxygen in the soils, wetlands often have a thick surface layer of organic materials. Muck and peat surface layers are common due to the slow rate of organic matter decomposition. Some plants commonly seen in these wetland areas are Skunk-cabbage (Symplocarpus foetidus) and Sphagnum moss (Sphagnum sp.). In droughty upland areas, the organic matter in the soil is much more sparse, because it oxidizes quickly.

Soils between the droughty uplands and the saturated wetlands have plant communities adapted to middle levels of available moisture. Most of the land area of Rhode Island fits into this category. Facultative plants (those known to grow in both wetlands and uplands) such as Red Maple (Acer rubrum) and Arrowwood (Viburnum dentatum), are often seen growing in thick deposits of silty soils even though the soils are clearly in upland areas. These species also grow in upland areas with sandy or silty soils that have a seasonally high water table at depths of two or three feet below the surface.

Other soil characteristics that influence native plant communities include soil salinity and soil pH. In Rhode Island, saline soils and brackish soils are almost entirely in the coastal zone. In most instances, these soils also have a high water table and are subject to periodic tidal inundation. Tidal marsh soils are saline and generally have a thick layer of partially decomposed organic matter. Hydrogen sulfide gas is produced in tidal marshes, and in freshwater organic soils, as their organic matter decays under anaerobic conditions. If tidal organic matter is removed and placed in an upland location where it is exposed to oxygen, the pH drops dramatically due to the transformation of the sulfur into sulfuric acid. Smooth Cordgrass (Spartina alterniflora) and Salt Hay Grass (Spartina patens) are common species of Rhode Island salt marshes. In a few localized pockets, saline soils have developed down slope from road salt storage facilities that were poorly protected from rain and snow. The invasive Common Reed (Phragmites australis) is tolerant of brackish conditions and is seen both in areas influenced by road salt runoff and in coastal areas.

As noted above, almost all of Rhode Island's soils are very acidic, unless modified with ground limestone or similar materials. The very localized limestone bedrock areas in Lincoln are the notable exception and have a distinct flora that includes American Basswood (Tilia americana) as a component of the deciduous forest, unique ferns such as Walking Fern  (Asplenium rhizophyllum) and Purple-stemmed Cliffbrake (Pellaea atropurpurea), and a variety of wildflowers, such as Hepatica (Hepatica americana), Palmate-leaved Violet (Viola palmata), and Nodding Trillium (Trillium cernuum). Soils under mature forest cover tend to have slightly more acidic soils due to decaying leaves and needles. The most acidic forest litter in Rhode Island is produced in coniferous forests of Eastern Hemlock (Tsuga canadensis) or spruces (Picea sp).

There are small areas of unique soils, such as the saline coastal areas, that are hostile to our common plant communities. Other examples are gravel pits which initially support only pioneer plants tolerant of low concentrations of plant nutrients, droughty conditions, and high levels of solar energy reflected up from the sandy and gravelly substrate. Only the hardiest of seedlings, usually plants like Little Bluestem (Schizachyrium scoparium) and Sweetfern (Comptonia peregrina), prevail in old gravel pits. Sand dunes present similar challenges exacerbated by windy conditions, a constantly shifting substrate, and occasional salt spray. A few species of specially adapted plants, such as Beach Grass (Ammophila breviligulata) and Beach Pea (Lathyrus japonicus), colonize under such trying soil and climate conditions.

Want to learn more about the soils of Rhode Island? Two free publications are available from the U.S. Department of Agriculture's Natural Resources Conservation Service (NRCS) office in Warwick. The 42-page Soils of Rhode Island Landscapes, by William R. Wright and Edward H. Sautter, University of Rhode Island Agricultural Experiment Station Bulletin 429, January 1988, provides an overview of local soil formation and characteristics. It comes with the companion "General Soils Map of Rhode Island."