Florida's Soils
by Maria Minno (with revisions)
Soils are enormously important and often overlooked. Florida's natural ecosystems co-evolved with the soils, and soil conditions often determine what can and cannot be grown both as agricultural crops and in gardens. A wide variety of soils exist in Florida, ranging from the sugar sands of the coastal dunes and central ridge, to the peats of the northern Everglades, the rocklands of the southernmost tip of Florida, the heavy panhandle clays, and the ubiquitous spodosols of the flatwoods.
Soil Origins. Florida began as limestone, formed millions of years ago at the bottom of a shallow sea. In the early Miocene, sediments began to be washed out of the rising Appalachian mountains. In some areas, these sediments contained phosphorite, attapulgite, kaolin, and heavy minerals. During the Plio-Pleistocene, advances and recessions of the north polar ice cap, each occurring over many thousands of years, resulted in changes in sea level. When the ice cap enlarged, sea levels went down, because so much of the global water was held as ice. Sea levels rose when the ice caps melted and released water into the oceans. Wherever land was exposed in Florida, beds of sand were deposited. These form the parent materials for much of Florida's soil. Some parts of the exposed continental shelf that formed Florida also rose, exposing the limestone and creating a karst topography in large areas.
Karst Topography. Karst topography is made up of limestone riddled with sinkholes and caves. Many of our lakes are collapsed limestone. In these areas, water is likely to flow underground, through the porous limestone, rather than in streams aboveground. Other areas with shallow sediments overlaying limestone will have shallow sinkhole depressions that may form marshes or cypress domes within flatwoods. Where the Hawthorn formation, a clayey-layer, occurs between surface sands and underlying limestone, sinkholes tend to form abruptly. The sand and clay will form a bridge overlying an area where the limestone has dissolved, and a sinkhole forms when the bridge collapses. Streams and creeks are more common where many feet of surface sediments cover the limestone.
Soil Surveys. Soils have been classified and mapped in Florida since the 1950's. Currently, a taxonomic system that classifies soils by their observable and measurable properties is used. This classification is based largely upon the soil profile and texture. Soil survey reports by the US Department of Agriculture's (USDA) Soil Conservation Service are available free from your local NRCS office. These reports contain a wealth of information, including detailed descriptions of soils and vegetation, and soil maps of such detail that individual buildings can be discerned. Since soils are so closely tied to vegetation and habitat, Soil reports are essential resources for both naturalists and natural landscapers.
Practiced soil scientists can determine the soil texture by feel. Soils are composed of particles of a variety of sizes. There is gravel, which is made of pebble-sized granules, and sand, which is gritty. Next comes silt, which is soft and powdery, like flour, in texture. Finally, the finest particles are clay, which is slippery and slimy to the touch, and runs out from between your fingers in ribbons.
The color of soils is also indicative of its makeup. Gray and bluish soil may indicate anaerobic conditions caused by inundation. Reddish soil indicates oxidized minerals such as iron, and more exposure to the air. Very white soils are bleached siliceous sands that form when leaf litter creates organic acids in rainwater, which washes color-imparting minerals from the surface.
Florida's Soil Orders. There are seven general categories of soils that exist in Florida, with four being prevalent.
- Histosols are primarily organic soils of peat and muck overlaying sand, marl, or limestone. Organic material usually accumulates in extremely wet conditions, and may range from a fibrous consistency to oozy, slimy muck. Paleontologists and palynologists recover pollen from peat deposits to determine the makeup of the plant communities of the past. When the water is drained out of peat soils, as is commonly done for agriculture, the organic matter oxidizes and disappears rapidly. Dry peat soils can also burn. Dry peats can subside at an average rate of 1 inch per year.
- Spodosols are sandy soils that have developed a layer of clay not far below the surface. The clay layer forms a hardpan that is relatively impermeable to water. During high-rainfall periods, water sits on top of the clay layer so that the surface soil is wet or inundated.
- Ultisols also consist of sands overlying a loamy or clayey subsoil.
- Entisols are extremely sandy soils with little or no soil profile. These soils can be extremely wet or dry.
Changes in Florida's Soils. Under natural conditions, soils change very slowly. This was the case in Florida for millions of years. Soil hydrology and vegetation worked together to create the soil types that existed prior to modern development. When people arrived, about 10,000-12,000 years ago, soils were subjected to the alterations due to widespread burning. However, the effect of anthropogenic fires is difficult to distinguish from that of lightning fires in most places. In localized areas, people built shell mounds, middens, and trampled the soil surface. Some canals were dug in the everglades to facilitate canoe passage, although they would have had only a slight impact on drainage patterns, perhaps affecting the soils in localized areas. Agriculture was practiced, so land was cleared and planted in some relatively small areas, although erosion was trivial, because fields were only cultivated for a few years at a time. Although Native Americans existed in Florida for thousands of years, their impact on the soil and vegetation was generally small.
Recent human activities have had profound and widespread effects on soils in Florida. To mention a few: surface mining, pumping water from aquifers, disposing of sewage into water bodies, pollution of soils of estuaries and freshwater bodies by runoff from agriculture and urban areas, heavy metal contamination from industry and automobile exhaust, ditching and draining of wetlands, filling of coastal wetlands, bulkheading and "renourishment" of beaches, loss of peat due to peat mining and agricultural practices, disruption of hydrology by road construction and paving and impoundment, cultivating and clearing of vegetation for agriculture, silviculture, and development.
Erosion. Erosion is a natural process. It is also a process that can be vastly enhanced by poor management with consequent impacts to both uplands and wetlands. In a typical scenario, soil is eroded from uplands and deposited along stream courses and in bottomlands. When soil eroded from uplands reaches sea level, it is deposited in river deltas, marshes, and estuaries. This depositional process raises the elevation in these wetlands, and alter nutrient supply and salinity. Changes in the erosional process can result in undesired changes to ecosystems. An excess of erosion, an effect of poor land management, can result in the infilling of lakes, wetlands, stream channels, and deltas. An absence of erosion, usually expressed through the damming of streams and capture of particles behind dams, can result in losses to deltas and scouring away of sand bars along streams. Excess erosion from agricultural and developed areas may be accompanied by pollutants such as fertilizers and pesticides, which likewise are deposited in bottomlands, deltas, marshes, and estuaries.
Drainage Patterns. Drainage patterns have been altered for agriculture, real estate, and commerce. Ditches and canals lower water tables and alter the hydrological regime of soils. Dams and blocked drainageways cause flooding. The result is changes to the soil formation processes and ultimately changes to the plant and animal communities that rely upon the soils. Examples are drained wetland soils that support mostly upland weeds and flooded soils which often support weedy wetland communities such as dense cattail patches.
Information for this article was taken from Ecosystems of Florida, Chapter 3, Soils, by Randall B. Brown, Earl L. Stone, and Victor W. Carlisle. Edited by Ronald L. Myers and John J. Ewel, University of Central Press, Orlando. 1990.
