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 Pl (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 P-32. 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.