Isotope methods for assessing plant available phosphorus in acid tropical soils

Summary 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 ( r 1 / 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 33 P‐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.