The Comparison of Olsen, DMT-HFO and DGT Methods for Assessment of Plant Available Phosphorus in Soils

Phosphorus (P) is one of the most important limiting nutrients for crop production in the world soils. Excessive and deficiencies of P in calcareous soils has been reported. Several methods can be used to recognize soil bioavailable phosphorus. Soil P tests involve chemical extraction and ion-sink extractants. The diffusive gradient in thin-films (DGT) is a new technique for measuring available soil phosphorus. This technique attempts to simulate physico-chemical processes influencing the uptake of solutes by plant roots. Also, the dialysis membrane tubes filled with hydrous ferric oxide solution (DMT-HFO) has recently been used to assess P desorption over long-term laboratory studies. The DMT-HFO method acts like the DGT method. Both systems are based on Fick’s law and try to imitate physicochemical uptake of P by plant roots in soils. The aim of this research was to use Fick’s law in the interpretation of DMT measurements and compare it with the Olsen and DGT methods for assessment of corn available phosphorus in some calcareous soils. For this purpose, the diffusion coefficients of P in the DMT membrane and DGT gel were calculated. The DMT-HFOs were located in soil suspensions at 24, 72, 240, and 500 hours and the concentration of DMT-HFO (CDMT-HFO) for P in the mentioned times were compared with those of Olsen and DGT methods on assessing of corn P uptake. Corn (SC 704) were grown in a greenhouse on ten different calcareous soils. Based on the results, the diffusion coefficient of P (H2PO4-) in DMT (5.54*10-8 cm2.s-1)was lower than that of DGT gel (5.8*10-6 cm2.s-1). The linear correlation coefficient(r) between CDMT-HFO and corn P content was increased by the time (24- 500 hours). Based on the r2 values, the lowest and highest correlations were related to CDMT-HFO in 24 hours (r2=0.22, p<0.05) and 500 hours (r2=0.9, p<0.05), respectively. The DMT-HFO provides a more precise correlation with P uptake by corn (r2=0.9 in 500 hours) than Olsen P (r2=0.86) and CE (DGT) (r2=0.76). In roots, the best correlation between CDMT-HFO and corn P uptake was obtained in 250 hours. However, in 24 to 250 hours, the statistical correlations based on r2 values were very similar.