| In Arkansas, response of rice (Oryza sativa, L.) to phosphorus (P) fertilization has been inconsistent. Phosphorus fertilizer recommendations are based on P extracted from air-dry soil using methods developed for upland crops. These procedures may not be appropriate for periodically submerged soils, because submergence is known to affect P availability. The objective of this study was to evaluate the influence of flooding-draining cycles on transformations among inorganic P fractions and changes in soil P extracted by the Bray-1 and Mehlich-3 methods. An alkaline Calloway silt loam and an acidic Dewitt silt loam were incubated at 25°C under anaerobic (flooded) conditions for 84 d followed by aerobic (drained) incubation for an additional 84 d. The control was incubated under field capacity conditions for 168 d. Inorganic P fractions soluble-P (1 M NH4Cl extractable), Al-P, Fe-P, Ca-P, RS-P (reductant soluble phosphates), and occluded Al-P, soil-test P (STP) values were determined during the incubation period. Changes in the inorganic P fractions and in extractable P were significantly different under the flooded-drained conditions versus the field capacity conditions. Typically, soil-test P and inorganic P fractions were relatively stable under field capacity conditions.;Accurate assessment of available P in both aerated and flooded soils is important for calibration of soil-test P critical levels for soils used for rice production. The objective of this study was to evaluate the influence of flooding on the P availability indices of six soil-test P methods. Available soil P determined by resin, Mehlich-3, Bray-1 and Bray-2 methods all increased gradually in the four soils during flooding. The Olsen and Lancaster extractable P increased initially, reached a maximum within 4 to 7 d, and then decreased slightly from 7 to 28 d. The percent recovery of added fertilizer P depended on the soil, P application rate, and incubation time.;The relationship between rice response to phosphorus (P) fertilization and available soil P determined by soil-test methods using air- or oven-dried soil samples has been unsatisfactory. The objective of this study was to evaluate P soil-test methods for flooded rice as a function of soil pH compared to P sorbed by anion exchange resin. Six P soil-test methods, Mehlich-3, Bray-1, Bray-2, Olsen, Lancaster, and modified Morgan, were evaluated on 110 oven-dried soil samples. Resin sorbed P was used as an index of plant P uptake in this study because of the good correlation of resin sorbed P with plant P uptake in previous studies. Mehlich-3 and Bray-1 extractable P were highly related with readily available P on soils with pH > 6.0. |
