ium ▁and ▁ph osph orus ▁which ▁remain ▁in ▁the ▁ash ▁and ▁are ▁quickly ▁rec yc led ▁into ▁the ▁soil . ▁This ▁loss ▁of ▁nit ro gen ▁caused ▁by ▁a ▁fire ▁produces ▁a ▁long - term ▁reduction ▁in ▁the ▁fert ility ▁of ▁the ▁soil , ▁but ▁this ▁f ec und ity ▁can ▁potentially ▁be ▁recovered ▁as ▁mole cular ▁nit ro gen ▁in ▁the ▁atmosphere ▁is ▁" fixed " ▁and ▁converted ▁to ▁am mon ia ▁by ▁natural ▁phen omena ▁such ▁as ▁light ning ▁and ▁by ▁leg umin ous ▁plants ▁that ▁are ▁" nit ro gen - fix ing " ▁such ▁as ▁clo ver , ▁pe as , ▁and ▁green ▁be ans . ▁ ▁Fire ▁has ▁been ▁used ▁by ▁humans ▁in ▁rit uals , ▁in ▁agricult ure ▁for ▁clear ing ▁land , ▁for ▁cook ing , ▁generating ▁heat ▁and ▁light , ▁for ▁signal ing , ▁prop ulsion ▁purposes , ▁sm el ting , ▁for ging , ▁inc iner ation ▁of ▁waste , ▁cre m ation , ▁and ▁as ▁a ▁weapon ▁or ▁mode ▁of ▁destruction . ▁ ▁Phys ical ▁properties ▁ ▁Chem istry ▁ ▁F ires ▁start ▁when ▁a ▁fl amm able ▁or ▁a ▁comb ust ible ▁material , ▁in ▁combination ▁with ▁a ▁sufficient ▁quantity ▁of ▁an ▁ox id izer ▁such ▁as ▁o xygen ▁gas ▁or ▁another ▁o xygen - rich ▁comp ound ▁( though ▁non - ox y gen ▁ox id izers ▁exist ), ▁is ▁exposed ▁to ▁a ▁source ▁of ▁heat ▁or ▁ambient ▁temperature ▁above ▁the ▁flash ▁point ▁for ▁the ▁fuel / ox id izer ▁mix