Buildup and Decline in Soil Phosphorus: 30‐Year Trends on a Typic Umprabuult

Abstract

Phosphorus reserves in Ultisols are inherently low; but many Ul- tisols along the Atlantic seaboard are now high in P, both extract- able 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 ex- periment 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) dur- ing 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 soy- bean [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 nr3) when P removed in harvested products (16 kg ha ' 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 in- itial 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 nr3, 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 nr3 in about 14 yr.