Isotope methods for assessing plant available phosphorus in acid tropical soils

Use of isotope methods to measure the availability of phosphorus (P) in soils that are well supplied with P is well established. We have evaluated such methods for acid tropical soils with very small P contents, which are less well studied. The isotopically exchangeable P in soil suspensions (E value) and that in plant growth experiments (L value) were measured in soils that had received varying amounts of P fertilizer in two field experiments in Colombia. We determined the E values over 4–5 weeks of equilibration allowing for the kinetics of isotope exchange. The decrease in radioactivity in the soil solution at a particular time, t, divided by that at the start (r_t/R) was described by three parameters (r₁/R, r_∞/R, and a coefficient n) derived from the time course of isotopic exchange over 100 minutes. Values of E_t were calculated either from measured values of r_t/R or those extrapolated until 12 weeks. Agrostis capillaris was grown on the same soils labelled with carrier‐free ³³P‐orthophosphate ions to obtain L values. Agreement between E values derived from measured and extrapolated values of r_t/R was satisfactory, but errors in n and r_∞/R limited the precision with which we could estimate E values. For most soils, the P concentrations in the soil solution were greater than the detection limit of the malachite green method (0.9 µg l^?1) but smaller than its quantification limit (3.6 µg l^?1). In the soils with the least available P, the P content of the seed limited the determination of the L value. The E values were strongly correlated, but not identical, with the L values measured for the same time of isotopic exchange. We conclude that these approaches are not precise enough to detect in these soils the ability of a plant to access slowly exchangeable forms of P or to quantify the mineralization of organic P. However, these isotope techniques can be used to estimate the total fraction of added fertilizer P that remains available to the plants.