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Abstract Phosphorus reserves in Ultisols are inherently low; but many Ultisols along the Atlantic seaboard are now high in P, both extractable and total, because P additions have exceeded P removal for many years. How long a high‐P soil will maintain plant‐available P above yield‐limiting levels is of agroeconomic relevance. A field experiment initiated 35 yr ago on Portsmouth soil (fine sandy over sandy or sandy‐skeletal, mixed, thermic Typic Umbraquult) and monitored for crop yields and soil‐test P (Mehlich‐1 extractant) during 8 yr of active P buildup and 26 yr of residual decline has provided quantitative data on this issue. Yields of corn ( Zea mays L.) or soybean Glycine max (L.) Merr. were maximal with soil‐test P ≥ 22 g m −3 , and extractable P was maintained in this range (20‐24 g m −3 ) when P removed in harvested products (16 kg ha −1 yr −1 ) was replaced annually as band‐applied fertilizer. High soil‐test levels (≥50 g m −3 ) could not be maintained by annual replacement of crop‐removed P because P reversion to unextractable forms was a larger factor than crop removal in depleting the extractable‐P pool. Regardless of initial level, P disappearance into these unextractable forms was best described via equations having the form of a first‐order chemical reaction; but the magnitude of the rate constant varied with size of the extractable‐P pool, i.e., high‐P soils have large rate constants; low‐P soils have small rate constants. A Portsmouth soil testing 50 to 60 g P m −3 today will test above 22 g m −3 , the approximate critical level for corn, for the next 8 to 10 yr without further P additions. Doubling the initial soil test will not double the time to reach yield‐limiting P levels, however; the same soil with 100 to 120 g P m −3 initially will drop to 22 g m −3 in about 14 yr.
R. E. McCollum (Tue,) studied this question.