Final answer:
The major source of phosphorus on land is the weathering of phosphate-containing rocks. These rocks originated from phosphate sediments in the ocean and are brought to land through geologic uplift over long periods. Natural processes contribute to the slow phosphorus cycle, where phosphates make their way into terrestrial and aquatic ecosystems.
Step-by-step explanation:
The major source of phosphorus on land is primarily the weathering of rocks that releases phosphate into the soil, which subsequently enters terrestrial food webs. Phosphorus occurs in nature as the phosphate ion (PO4³-). This essential nutrient makes its way to the land mainly through geologic processes such as the uplifting of Earth's surface, which moves sediment from the ocean where phosphate has settled from marine organisms and their excretions. These rock sediments, enriched with phosphates, originate in the ocean and are distributed across land over long geologic time frames. In addition to this natural process, volcanic ash, aerosols, and mineral dust in remote areas can be significant sources of phosphate due to their deposition.
Phosphate movement from the ocean to the land, through the soil, and into waterways is a slow cycle, with an average phosphate ion having an oceanic residence time of 20,000 to 100,000 years. Therefore, while phosphate runoff due to human activities can contribute to the phosphorus cycle, the natural weathering of phosphate-containing rock is the major source of phosphorus on land, slowly contributing to ecosystems over millennia.