IRON-OXIDE SINK METHOD FOR EXTRACTING SOIL-PHOSPHORUS - PAPER PREPARATION AND USE

Since the FeO-sink method to estimate plant-available P in soils was f irst introduced in 1983, modifications have continued to improve the p rocedure, Recently FeO-coated paper circles (5.5 cm diam.) were used i n place of the traditional 2 by 10 cm strips, The objective of this st udy mas to more clearly describe tile preparation and use of the FeO-c oated circles. We used Whatman no. 50 hardened filter paper (2.7-mu m pore openings), Standard methods for FeCl3 treat,ent of the papers wer e used. After the papers had been air dried, one paper was secured bet ween two 10 by 10 cm fiberglass screens. ay simultaneously immersing s tacks of 25 of these paper-screen units in 2.7 M NH4OH for a 30-s imme rsion time, 100 papers could be uniformly coated with FeO in about 10 min. Immersion times of 30, 60, and 180 s in NH4OH solution showed tha t papers immersed for 30 s in NH4OH solution extracted P as effectivel y as those immersed for longer times, One 5.5-cm FeO circle was as eff ective as three circles in extracting P from most soils, A desorption model for soil P was used that can relate soil-test P to desorption ki netics, In soils with maximum reversible P ranging from 25.1 to 106.5 mg kg(-1), desorption constants ranged from 0.141 to 0.232 h(-1) with a mean of 0.191 h(-1) Low-P soils released P slower than did high-P so ils. Such information may be of interest in assessing P availability i n soils.