Organic Soils: Management Concerns for Prescribed Burners

Organic soils are often, though not always, present in some types of forested wetlands and in some herbaceous wetlands. Long hydroperiods are often associated with organic matter accumulation (Kirkman et al. 2000, Ewel 1990, Kushlan 1990). Deep duff and organic surface horizons can also develop in some sites after long fire-free periods.

Fire practitioners are often wary of burning wetlands with organic soils. The problems are twofold:

1. Organic soils will ignite under certain conditions resulting in often long lasting, smoldering fires that produce copious amounts of smoke.
2. Long lasting, smoldering fires are difficult to extinguish, can be sources of ignition causing reburn or escapes and thus present control problems.

These two problems can result in violation of state or local laws that regulate open burning (Ex: Florida Statute 590). Rules and laws that govern air pollution and visibility, and requirements for extinguishing fires within certain time frames may be violated when hard to extinguish and smoke producing ground fires are ignited. (For more information and examples of State level open burning statutes please visit The Nature Conservancy Information Resource Center).

If a fire starts in organic soils, it may burn for months and typically will continue to burn until one of the following conditions occurs:

1. Organic soils burn down to mineral soil and the fire runs out of fuel.
2. Significant rain occurs which either raises the water table until it reaches the fire or moisture content of the organic soils reaches the moisture of extinction.
3. Intensive suppression efforts isolate and then suppress the burning organics. Suppression typically requires a fireline constructed to mineral soil to isolate the burning area, followed by copious amounts of water applied to the burning area. The ecological harm resulting from this type of suppression is extensive. These suppression efforts are also extremely expensive, costing thousands of dollars per acre.

There has historically been a considerable amount of uncertainty in knowing under what conditions fires in organic soils would ignite. A large range of values of minimum moisture content for safely burning sites with organic soils has been reported in the literature (reviewed in Hungerford et al.1996). However, recent research has helped to clarify factors contributing to organic soil ignition. The probability of igniting a ground fire varies with the amount of organic matter in the soil and the moisture content of the soil (Hungerford et al. 1996). Soils with higher levels of organic matter will ignite at lower temperatures than soils with lower levels of organic matter given the same moisture content. And given the same organic matter content moister soils are less likely to ignite than drier soils. Hungerford et al. (1996) have quantified this relationship (see graph). The moisture content of hummocks also plays a role in initiation of ground fires (Hungerford et al.1996). An understanding of these factors is necessary when planning a fire that may result in whether or not to ignite ground fires.

While there is long term ecological benefit from the periodic burning of these organic soils, the immediate smoke production, control difficulties, and unpredictability of these fires makes burning the organic soil impracticable for all but a few locations. In most situations, smoke management and fire control requirements compel managers to avoid igniting organic soils and to extinguish them where they do occur.

When managers are faced with burning areas that include organic soils several factors should be considered.

• Probability of igniting a ground fire (percent organic matter within soils and moisture content of soils)
• Water table levels
• How extensive are organic soils
• How would an unplanned ground fire affect operations
  • Is excessive smoke production acceptable as determined by smoke screening procedures and local and state laws?
  • Is the ground fire likely to result in escape?
• When necessary, how can fire be excluded from areas of organic soils without causing unwanted ecological damage? (i.e. mowed rather than plowed breaks or very moist environmental conditions.)