Predicting the bioavailability of phosphorus in soil amended with phosphate rocks using isotopic exchange kinetics

Investigations on plant responses to applications of various forms and rates of P fertilizers usually involve glasshouse and/or field experiments. This traditional procedure assumes that whatever the soil-fertilizer-plant system, increase in total P uptake by plant between no P treatment (control) and fertilizer treatment equals the plant P uptake from fertilizer. This study uses the isotopic exchange techniques in the laboratory to predict bioavailability of P fertilizers without the need to conduct glasshouse or field experiments. Serdang series soil (Typic Paleudult) was incubated with 7 sources of P fertilizers comprising of triple superhosphate (TSP) and phosphate rocks from North Carolina (NCPR), Algeria (APR), Tunisia (TPR), Jordan (JPR), Christmas Island (CIPR) and China (CPR) at the rates of 0, 2, 4, 6 and 8g Kg-' soil with 20% moisture content at room temperature in three replications. The soils were sampled at 1, 3, 6 and 9 months after incubation and isotopically exchangeable p determined by the method of Fardeau and Jappe (1976). Intensity, quantity and capacity factors of soil P were calculated and the residual availability of these fertilizers were predicted. Phosphorus in solution was highest in TSP treated soil for all treatments. Among the phosphate rocks, NCPR at rate 8g kg-' soil gave the highest value while, CPR at rate 2 gave the lowest value. Thus showing that these PRs have different reactivities in this soil, where NCPR, APR, TPR and JPR were the reactive PR, while CIPR and CPR were the unreactive ones. The isotopically exchangeable P at one minute (1) in the soil sampled 9 months after incubation was found to correlate very well with plant P uptake by oil palm seedlings grown under the same conditions. Calculations made on the percentage of P derived from these fertilizers up to a period of more than one year after application showed that the reactive PRs to have more residual P made available to plants than the unreactive PR