Fire and its influence on soil

Types of fire and consequent heat waves

McKenzie et al (2004) developed the following illustration of heat waves from various types of fires. It is based on data from Walker et al (1986) and Humphreys and Craig (1981). It shows that it takes quite intense fires to penetrate far into the soil surface.

Penetration of fire generated head into a soil

Example ranging from
A - a grassline fire, through to
E - a windrow burn

The particular detail of each heat wave (apart from fire intensity and duration) is modified by soil moisture content and soil type (texture and porosity).

Changes to soil caused by fire

The following table (also from McKenzie et al., 2004), based on work of Walker et al., 1986 provides information on soil changes induced by fire induced soil temperatures

Dominant type of change	Temperature (°C)	Effect
Biological	<25	Usual soil temperatures
	37	Maximum stimulation of soil microorganisms
	50	Mild sterilisation due to water loss
	60	Proteins denatured
	70	High nitrate mineralisation
	100	Soil ammonium production starts
	110	Soil water lost
	125	Soil sterilisation
	200	1. Water repellence caused by distillation of volatiles 2. Loss of nitrogen commences
Chemical	300	1. Maximum amino acid nitrogen released 2. Loss of sulfur and phosphorus begins 3. Distillation and carbonisation of organic residues 4. Organic matter charred
	400	Organic matter carbonised
	420	Water lost from within clay minerals causing change in type (e.g. illite converts to kaolinite)
	540	Little residual nitrogen or carbon left
	600	1. Maximum loss of potassium and phosphorus 2. Fine ash produced. Organically bound cations form oxides
Physical	950	Clay minerals converted to different phases
	>1200	Loss of calcium as gas

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