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Soil Classification

Orders, CRIFF Groups, and soil groups for purposes of forest fertilization.

• Soil Orders and CRIFF Groups

  Between 1951 and 1975, a soil classification system was developed for the United States. This system was based on observable, measurable soil properties rather than on the pathways of soil formation as was the case of the system in use previously.

  Soils are classified at six categorical levels: order, suborder, great group subgroup, family, and series. To classify a soil, you must know the soils morphology (features like color, particle size, moisture regime, presence of hardpans and other horizons, depth to bedrock, etc.), and numerous physical and chemical properties such as soil temperature, organic carbon content, sulfide content, mineralogy, and so forth, which require field measurements and or laboratory analysis.

  Seven soil orders occur in Florida. These are summarized in our Soil Orders Table (see below).

  In the mid 1970s, researchers in the Cooperative Research in Forest Fertilization (CRIFF) program at the University of Florida developed a system of grouping soils according to how they respond to certain management activities. This system is known as the CRIFF Soil Groups. Drs. Eric Jokela and Alan Long of the University of Florida School of Forest Resources and Conervation recently developed a useful extension publication which describes these groupings.

  There are 8 CRIFF soil groups in Florida which range from very poorly drained spodosols to well drained sandy upland soils. Read the publication, Using Soils to Guide Fertilizer Recommendations for Southern Pines, for more information.

  Seven Soil Orders of Florida

  1	Histosols	Soils that consist of organic materials (muck or peat) in at least half of the upper 80 cm of soil, or that have organic materials extending from the surface to within 10 cm of bedrock. The organic material is more than twice as thick as any mineral soil above the bedrock. These occur extensively in Florida.
  2	Spodosols	Soils that have a spodic horizon: a subsurface horizon in which organic matter in combination with aluminum and/or iron has accumulated. The upper boundary of this horizon is within 2 m of the soil surface. These occur extensively in Florida.
  3	Ultisols	These soils have an argillic horizon: a subsurface zone of accumulation of clay-size particles at the expense of above horizons. Base saturation* is less than 35%. These occur extensively in Florida.
  4	Mollisols	Soils that have a thick (more than 25 cm) dark, mineral surface horizon and a base saturation of 50% or more. These occur to a minor extent in Florida.
  5	Alfisols	Other soils that have an argillic horizon and a base saturation of 35% or more. These occur to a moderate extent in Florida.
  6	Inceptisols	Other soils that have significant horizon development. These occur to a minor extent in Florida.
  7	Entisols	Other soils which occur extensively in Florida, especially south Florida rocklands and sandhills.

  From Myers and Ewel, 1990

  * Base Saturation: the proportion of chemical exchange sites on soil particles that are occupied by basic cations (Ca²⁺, Mg²⁺, K⁺, Na⁺) as opposed to acid cations (H30⁺, Al³⁺).  These basic cations are important sources of nutrients for plants.

• Soil Groups for Purposes of Forest Fertilization

  It is practical to group soils according to properties that influence the types and amounts of fertilizers needed for forestry purposes. Soil drainage is a property that can be determined in the field after minimal experience and is a useful tool for categorizing forest land for fertilization purposes. This type of categorizing can also be based on soil phosphorus retention (P-retention) capacity.

  Four general soil types are introduced below. They are categorized based on drainage condition and subsequent P-retention. This will get you started in the process of determining the types and amounts of fertilizer your land may need for forestry purposes:

  1 - Very Poorly to Poorly Drained Soils (Bays and Savannas)

  These soils are found in coastal flat lands in mostly level depressions and stream terraces. Most of these soils are flooded from 5 to 30 days, at least once during the growing season. The water table ranges from 6 to 20 inches below the surface during much of the remaining time. These soils were formed under impeded drainage. Therefore, they contain 10% or more organic matter in the surface horizon and most are very acidic and low in nutrient reserves. Vegetation native to these sites include wiregrass (Aristida stricta), pitcher plants (Sarracenia minor), some hardwoods, and a poor to fair growth of pine.

  In most cases, pine growth is slowed by excessive moisture and lack of available soil phosphorus. These soils can be grouped according to fertilizer recommendations on the basis of depth to and nature of subsoil horizons, also reflecting their P-retention characteristics:

  A - High P-retention:
  These soils have a dark gray to black fine sandy loam surface, with dark brown to grayish finer-textured material within 20 inches of the surface. Varying degrees of gray mottles reflect the high moisture content of these soils. These wet soils with clay near the surface generally have a high capacity to fix phosphorus in a form only slightly available to trees. The use of ground rock phosphate (GRP) or band placement of soluble phosphates, such as concentrated superphosphate (CSP), is recommended on these sites.

  B - Moderate P-retention:
  These soils contain fine-textured horizons deeper than the 20 inches of the surface, but their relatively high organic matter content causes them to have a moderate capacity to retain phosphorus. Ordinary or slowly-soluble phosphates can be used on these types of soils.

  2 - Poorly to Somewhat Poorly Drained Soils (Flatwoods)

  The flatwoods comprise one of the most extensive groups of forest soils in the Coastal Plain. These acid to loamy sands are low in fertility. Flatwoods soils occupy level to gently sloping flat areas where the water table rises to within 5 to 20 inches of the soil surface for 1 to 4 days, at least once during the growing season. Flatwoods soils support native vegetation such as saw palmetto (Serenoa repens), wiregrass (Aristida stricta), and slash, loblolly, and longleaf pines (Pinus elliottii, Pinus taeda, and Pinus palustris). Flatwoods soils can be grouped into soils with and without organic pans.

  A - Low P-retention capacity (Spodosols):
  Many flatwoods soils contain a weakly cemented organic hardpan that is dark brown to black and occurs from 12 to 40 inches below the surface. This "spodic" horizon may become hard when dry. Flatwood soils with organic hardpans deeper than 16 inches have little capacity to retain P in the soil surface due to their low aluminum and iron content. For this reason, the use of ground rock phosphate (GRP) is recommended for long-term effectiveness.

  B - Moderate P-retention capacity:
  Most of these soils have fine-textured horizons within 20 to 80 inches of the surface. Unlike the flatwood-hardpan soils, these soils have moderate capacity to retain phosphorus. Either ordinary or slowly-soluble phosphate sources can be effectively used on these soils.

  3 - Moderately Well to Well Drained Soils (Sands to loamy sand-clay hills)

  Typically, sands occupy nearly level to rolling regions of the lower Coastal Plain. They have gray to brown surface layers overlaying 30 inches or more grayish- to yellowish-brown sands. These sands have a low capacity to retain water, but they have a reasonably good moisture level because of their topographic position and they are generally considered good forest soils. These soils are often used for pastures, field crops, and vegetables even though they are relatively low in nutrients. Fertilizers must be used to obtain good yields of those crops.

  Sand-clay hill sites normally contain red to yellow fine-textured subsoils within 30 inches of the surface. Surface horizons are generally loamy sands to sandy loams. The relatively high clay content of these soils causes them to have good capacity to retain moisture and nutrients and are excellent sites for growing loblolly pine. They are also exceptional agricultural soils.

  4 - Excessively Drained Sands (Sandhills)

  Extensive areas of these deep sands are in pines in north Florida, Georgia, and the Carolina sandhills. Water deficits limit responses to fertilizers even on the most nutrient-deficient sites. In general, fertilizers are not recommended for pine plantations in the sandhills due to moisture limitations. However, 150 lb DAP/acre (banded) may be effectively used on young sand pine, while urea is sometimes recommended for older stands of sand pine on excessively drained sands.

  For more information, see our Fertilization page.

  Drs. Eric Jokela and Alan Long of the University of Florida School of Forest Resources and Conervation recently developed a useful extension publication which gives fertilizer recommendations for these groupings. Read their publication, Using Soils to Guide Fertilizer Recommendations for Southern Pines, for more information.

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  UF/IFAS Soil and Analytical Services Laboratory
  A soil test can help determine soil attributes like pH, major nutrient levels, micronutrient levels, and other infomation. Your County Extension office can provide assistance in sampling and testing your soils.