Phosphate reserves and resources are dynamic. While enhanced technology can support mining lower-grade phosphate ore and extracting phosphorus (P) from lower-grade ore, rising prices can encourage the sale of phosphate from deposits in geographically unfavorable locations at market prices. On that count, the cumulative availability curve quantitatively describes how recoverable reserves rise with prices—in short, as prices rise, so do stocks. Section 3.1 introduces and discusses the geo-economic factors that affect the growth of reserves and resources, with special attention to the exploration and phased development of occurrences and resources for producing mines. The discussions clarify the distinction between inferred, indicated, and measured resources and the classification of probable and proven reserves. The Lasky rule, which describes an exponential relationship between the cumulative tonnage of phosphate rock with falling ore grades, is introduced as a key tool for understanding and forecasting future reserves and resource dynamics. Despite major phosphorus provinces, including in the western United States and the Mediterranean belt, how much phosphorus can be mined economically remains unclear, and not all potential phosphorus reserves have been explored. In that vein, Sect. 3.2 describes the economic and geological processes of exploring and evaluating the development of phosphorus mines.