[IB Biology Ecology Reviews]

2. Primary succession takes longer to occur than secondary succession because soil which is ready for plants to grow in does not exist yet. Plants are not able to grow well on bare rock, desert sands, or on soil sterilized by extremely hot fires. It thus takes years to centuries longer then for primary succession to occur, while secondary succession occurs faster because the soil already exists and is ready for new plant life.

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Long & Slichter