Isotope Methods for Assessing Plant Available Phosphorus in Acid Tropical Soils

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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 (rt/R) was described by three parameters (r1/R, r1/R, and a coefficient n) derived from the time course of isotopic exchange over 100 minutes. Values of Et were calculated either from measured values of rt/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 rt/R was satisfactory, but errors in n and r1/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 g l 1 ) but smaller than its quantification limit (3.6 g 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.
Soil, Plants
Buhler, A., A. Oberson, S. Sinaj, D.K. Friesen, and E. Froussard. 2003. “Isotope Methods for Assessing Plant Available Phosphorus in Acid Tropical Soils,” European Journal of Soil Science, 54:605-616.