• Topics: Soil and Water Science  | Soil Survey  | Brown, Randall B 

Soil Considerations in Selecting a Homesite ¹

Historically, the study of soils has been associated with agriculture. However, recently the wealth of knowledge about soils and their properties has been applied to non-agricultural uses of soils and land, such as selection of homesites. Since all soils are not the same but differ in their properties, improper consideration of soils in selecting a homesite can lead to higher building costs, selection of improper designs, and continuing limitations to the homeowner. Examples of soil-related problems which befall homeowners include unexpected subsidence, cracking of foundations and walls, periodic failure of septic tank systems, flooding, and difficulty in establishment and maintenance of yard plants. Proper consideration of the soil can eliminate or minimize such problems for the homeowner. The purpose of this fact sheet is to alert homeowners of the need to consider soils in choosing homesites, but it is beyond the scope of this publication to cover all soil properties and their implications.

Each soil is the result of a unique combination of the soil forming factors of parent material, climate, biota, topography, and time. Over time both topsoil and subsoil are formed, and the properties of each change across the landscape. Topsoil is higher in organic matter and thus contains more nutrients available for plant growth than the subsoil; however, the subsoil often has more clay and thus can store more water for plant uptake. When considering soils for specific uses such as for homesites, it is necessary to consider both the topsoil and the subsoil as the properties of each may change on the landscape as a result of changes in the soil forming factors.

SOIL-RELATED PROBLEMS

Low positions on the landscape such as unprotected flood plains near streams and rivers are particularly susceptible to flooding and should be avoided. Potential homeowners should be aware of areas of possible flooding as building in such areas increases construction costs, requires special designs, and increases the risk of property loss. Periodic flooding also severely interferes with septic tank performance and may kill landscaping plants if the duration of surface water is excessive.

Shallow depth to bedrock (less than 40 inches) may add to construction costs. Of particular concern, however, is the performance of septic tank systems placed in soils shallow to bedrock. If the bedrock is porous or fractured, sewage effluent may not be adequately filtered before entering ground water and thus cause contamination of drinking water. If the bedrock is massive and impermeable, effluent may surface either near the septic tank or at a point down slope and become a health hazard. Additionally, bedrock at shallow depths limits the rooting volume of landscape plants and increases the need for watering.

Depth to the high water table should also be considered as it affects wetness of the property, limits plant selection for landscaping, and influences performance of septic tank systems. Water table depth is not constant throughout the year but in Florida generally is highest in the late summer. For this reason percolation tests, designed to assess the ability of the soil to absorb water, should be run when the water table is at its highest. Adequate percolation during periods of a high water table minimizes chances of seasonal failure of septic tank systems. However, even when water tables are seasonally high (within 4 feet of the surface), specially designed septic tank absorption fields can be utilized, but construction costs correspondingly increase. High water tables at depths of between 2 to 4 feet serve as a source of water to plants and reduce the need to irrigate; however, when water tables rise within 2 feet of the surface, wetness must be considered in plant selection for landscaping.

The presence of large quantities of clays that shrink and swell can cause serious structural damage in homes. Although soils with this property are not widespread in Florida, they do exist. Basically, such soils expand upon wetting and shrink upon drying. This movement within the soil can crack sidewalks, driveways, walls, and foundations. Furthermore, such soils will seldom have sufficient permeability for operation of standard septic tank systems.

Slope is the vertical drop or rise in feet per 100 feet of horizontal distance expressed as percentage and is usually measured along the maximum gradient. Building on slopes greater than 8% can add to construction costs, increases the likelihood of effluent surfacing in lower landscape positions, increases soil erosion, and makes establishment and maintenance of desired landscaping more difficult.

Although not a common problem in Florida, the presence of gravel, and particularly stones greater than 3 inches in diameter, causes concern. Implications are higher construction costs for both foundation footings and septic tank adsorption fields as well as continuing limitations in landscape maintenance.

Soils which contain large quantities of organic matter (more than approximately 20%) are subject to subsidence over time which can lead to structural damage. Unless protected or drained, these soils flood for long periods of the year. When protected from flooding, such soils can be used as homesites. However, special designs are necessary, and continuing limitations are certain.

Other soil properties which have little effect on construction but concern establishing and maintaining of the landscape include texture, soluble salts, reaction, available water capacity, and fertility. Texture refers to the relative proportion and size of individual soil grains. Very sandy soils generally have low capacities to store water and nutrients, therefore requiring frequent irrigation and fertilization to maintain desirable vegetation. Soils high in clay, on the other hand, will often pond water after rains and are more difficult to till. Changing a soil's texture is generally prohibitively expensive but incorporation of additional organic matter such as peat sometimes modifies adverse properties. Soils with strongly acid reaction require lime for establishment and maintenance of many landscaping plants. Addition of lime is relatively inexpensive and may be rewarding in better growth of yard plants. High concentrations of soluble salts strongly limit plant selection, and in very high concentrations, can prevent all plant growth. Improving such soils is possible but requires time and money.

SOURCES OF HELP

In this fact sheet, an effort was made to alert potential homeowners of common soil properties which should be considered in building and maintaining a home. Help is available in the form of detailed soil survey reports which have been published for many Florida counties. Published soil survey reports describe the soils, evaluate soil properties, give interpretative tables of the suitability of individual soils for specific uses, and contain maps showing the location of soils in the survey area. However, caution is advised in the use of soil maps since inclusions of different soils may be contained within a mapped soil unit. Additionally, specialists in the Natural Resources Conservation Service of the United States Department of Agriculture and the Florida Cooperative Extension Service can aid in answering many questions concerning soils and their use.

REFERENCES

• Caldwell, R. E. 1977. "The Nature and Use of a Soil Survey." Soil and Water Science Fact Sheet SL-11. Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville. Available online at: http://edis.ifas.ufl.edu/SS160

• Street, J. J. and G. Kidder. 1977. "Soils and Plant Nutrition." Soil and Water Science Fact Sheet SL-8. Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville.

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service.

U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Millie Ferrer-Chancy, Interim Dean.

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