Final answer:
Phosphorus in soil remains soluble and prevents the formation of insoluble compounds through natural processes like rock weathering and volcanic activity, which release phosphate ions that are then used in terrestrial and marine ecosystems. This phosphorus cycle is a slow, ongoing process providing availability of phosphates.
Step-by-step explanation:
Phosphorus in soil is transformed into insoluble compounds unless it is involved in a process that keeps it available in a soluble form. This typically occurs through the action of weathering of rocks and volcanic activity, which releases phosphate ions into the soil. The released phosphate ions (PO4³-) then become accessible to plants and terrestrial food webs. Additional mechanisms such as surface runoff, groundwater flow, and river flow transport these soluble phosphates into oceans, where they enter marine food webs or fall to the ocean floor to form sediment.
Moreover, phosphorus is part of a slow but continuous cycle, where phosphate ions in the ocean are reciprocally exchanged between being dissolved in ocean water and being part of marine organisms. The entire process of phosphorus moving from the ocean to land and soil is exceptionally protracted, with an average phosphate ion having an oceanic residence time between 20,000 and 100,000 years.
Therefore, for phosphorus in soil to remain available rather than forming insoluble compounds, there needs to be a constant natural recycling process, which is assisted by the weathering of rocks and volcanic activities, as well as the continuous cycle of phosphorus in ecosystems.