What Does Ash Do to Soil After a Fire?

Nutrients

• Because a fire speeds up the decay of organic matter by reducing it to ash, a sudden burst of nutrients can be found in the soil after a fire. When a fire burns wood, grass, leaves and other organic matter, phosphorous, magnesium, potassium and calcium are released into the soil. Without a fire, these elements would have only become available in the soil after slow decay.

Volatilization

• Some important plant nutrients will become volatilized during a fire. These nutrients will be lost from the ashes and will not be available to plants. However, volatilization can also transform unavailable nutrients into a form that is accessible to plants, though no nitrogen is contained in ashes. Total nitrogen in the soil might decrease in the soil after a fire but because some nitrogen has been transformed into a more usable form, the amount of available nitrogen for plants increases.

Runoff

• If ash does not have adequate time to be incorporated into the soil after a fire, the nutrients contained in it might be lost due to runoff. If loose ash is blown away or carried off by water, it will not have time to restore nutrients to the soil. In situations such as these, ash from a fire can serve as a quick way to reduce the fertility of a soil because it is so easily carried off. If the nutrients had remained in the unburned organic matter, it would be much more difficult for them to be depleted so quickly.

Safety

• Be careful if you plan to burn an area to increase the fertility of the soil. A controlled burn can be performed but in most areas you'll need a permit or at least notify local officials of your plans. Burn when it is not windy. Verify that conditions will remain ideal for a controlled burn throughout the time it takes for the fire to burn. Depending on the size of the area you can control a fire fairly easily by starting multiple fires that will burn each other out. Extinguish all embers.