Practice question

Soil development during primary succession begins through rock weathering combined with the activity of pioneering organisms such as lichens. Temperature fluctuations, freezing, water, and abrasion physically break rock, while carbonic and organic acids chemically dissolve minerals. Lichen fungi adhere to the surface, retain moisture, and secrete acids; their photosynthetic partners provide organic carbon. Dust and small mineral particles become trapped, and dead microbial and lichen material adds the first organic matter. This mixture gradually stores more water and nutrients, enabling mosses and vascular plants to establish. Organic decay contributes, but it cannot act alone before organisms have produced organic material, and it does not supply the mineral fraction. Earthworms generally require an existing soil with organic resources, so they are later contributors rather than initial agents. Fire may expose substrate but does not normally construct soil on bare rock. The combined action of weathering and pioneers therefore explains both mineral production and organic enrichment during the earliest stages.