A comparison of four resin extractions and 32P isotopic exchange for the assessment of plant-available P

Resin extraction procedures are widely used to simulate nutrient uptake by plant roots. The objective of this study was to compare an anion exchange resin in three forms: F, Cl and HCO3, and a mixed anionic-cationic exchange resin in H-OH form to a 32P isotopic exchange method in their ability to extract soil P, and to predict plant-available P for oat (Avena sativa L.) grown in the glasshouse. Thirty-four soils varying widely in chemical and physical properties, and in P contents, were extracted for 24 h at a soil-to-solution ratio of 1:80 with 1 g resin. Increasingly higher P quantities were obtained by isotopic exchange after 1 min (E1 7.61 mg kg−1), Cl-resin (27.3 mg kg−1), HCO3-resin (40.5 mg kg−1), F-resin (75.1 mg kg−1) and H-OH resin (130 mg kg−1). The Cl and HCO3 resins extracted a constant ratio of E1-P in strongly acidic and weakly acidic to neutral soils; this was not the case for the other resins. The pH values of the resin-soil-solution system were related to the natural logarithm of the amount of rapidly exchangeable Ca. The Cl and HCO3 resins best predicted P uptake and relative yield of oat growing on 34 soils. These two methods were superior to chemical methods in evaluating soil P status and fertilizer response of oats. The relationships between the amount of soil P desorbed by the HCO3 and F resins and by plant uptake were improved when the P buffering capacity (MBC) was included in the multiple regression equation. The P intensity factor was most important for the H-OH and Cl resins. In general, desorption of soil P with exchange resins improves, compared to chemical extradants, the evaluation of the P-supplying power of soils. Key words: 32P, anion exchange resin, available P, extraction method