Soil Test Phosphorus Availability as Affected by Time after Phosphorus Fertilization

The availability of fertilizer P in the soil decreases with time after fertilization. Routine soil tests or other estimates of plant‐available P should accurately reflect that change. The objective of this study was to determine the effect of the time fertilizer P is in contact with soil on the ability of commonly used soil tests to extract plant‐available P. Experiments were conducted to determine the relationship between available P as determined by A value and Bray and Kurtz P1 (BK), sodium bicarbonate (SB), and Mehlich no. 2 (ME) P soil tests. The A value was assumed to be the best available method of determining plant‐available P. Samples from an acidic Thurman loamy sand topsoil (sandy, mixed, mesic Udorthentic Haplustoll) and a calcareous Uly silt loam subsoil (fine‐silty, mixed, mesic Typic Haplustoll) were incubated with 0, 10, 20, 40, and 80 mg P kg −1 for 4, 8, and 20 mo at field‐capacity water content. After each incubation period, oat ( Avena sativa L.) was planted and grown in the greenhouse with an additional 0, 10, and 20 mg P kg −1 labeled with 32 P. The A value was determined for each incubation period and was correlated with the soil test results. Solubility products were also determined for each incubation P rate and period. After 4 and 8 mo of incubation, the BK, SB, and ME soil tests did not extract plant‐available soil P very accurately using the A value as a standard. After 4 mo of incubation, the SB soil test underestimated available P by as much as 50% on the Uly soil, while the BK soil test overestimated available P on the Thurman soil by 40%. Although there was no improvement after 8 mo of incubation, all three soil tests accurately extracted available P after 20 mo when fertilizer P and soil P appeared to have reached an equilibrium. Solubility equilibria analysis lacked adequate sensitivity to show different soil P compounds as affected by time of incubation. The results indicate large potential P fertilizer recommendation errors when soil samples are taken and analyzed prior to achieving equilibrium between applied fertilizer P and soil P.