EXTRACTION AND ANALYTICAL METHODS

minutes; extract filtered through medium filter paper Pressurized Hot Water: boiling water (100 ml, 93 • C) under 0.25 Mpa pressure produced by espresso machine is forced through 5.0 g of soil that is placed on medium filter paper and collected in a plastic cup DTPA-Sorbitol: 12.5 g soil and 25 ml 0.2 M DTPA-Sorbitol solution are shaken for 2 hours, centrifuged and filtered through medium filter paper Mehlich III: 2.5 g of soil and 25 mL Mehlich III solution are shaken for 5 minutes and filtered through medium filter paper ***Standards for each method were made in the extracting solution for reading on the ICP. Filtrate from soil extracts were analyzed for B using Inductively Coupled Plasma Spectrometer (ICP) EXPERIMENTAL DESIGN Soil samples were extracted with hot water, pressurized hot water, DTPA-sorbitol and Mehlich III and analyzed for boron on ICP. Acceptable results were obtained with all methods except Mehlich III extraction. Mehlich III values were very high and a drift upward of control samples over time during measurement was observed. A series of experiments were conducted to test why B levels were much higher than projected in Mehlich III standard samples. Drift upward was tested using standards mixed in distilled water, DTPA-sorbitol, and Mehlich III. Carry over between samples containing Mehlich III was explored by the use of cleaning solutions run between each sample. The cleaning solutions included DTPA-sorbitol, nitric acid, and sorbitol. In experiments in which cleaning solutions were used, soil sample extracts and standard samples were measured on ICP alternating with one of these cleaning solutions. To determine the potential impact of the NH4F on the upward drift, standard solutions of “full ” Mehlich III, Mehlich III without NH4F and NH4F alone (at three concentrations) were evaluated. The common standard sample was 0.5 mg L-1 B, but in a few tests standards of 0, 0.2, 0.4, 0.8, 1.5 and 2.0 mg L-1 B were used. The Mehlich III standard solution measurements were regularly compared to standards prepared in distilled water. Standards were always prepared in the background of the extractant