Sources of variation in hot water extraction and colorimetric determination of soil boron

Abstract

The reliability of the soil test for boron (B) has been questioned in recent years. Two studies were initiated to identify some of the sources of variability in the extraction and analysis phases of the B soil test. The first study evaluated the influence of reflux and cooling times (10,20,30 min.) on B values. The second study investigated the effectiveness of CaCl₂ (0,3,6,9 mM) and charcoal (0,0.08, 0.16, 0.32, 0.64 g) as soil extract decolorizing agents. Hot water extraction, in plastic pouches, with azomethine‐H and/or ICP analysis was used to determine soil B. Increasing reflux time resulted in significantly higher B values, while increasing cooling time resulted in significantly lower B values (P = 0.05). Standardized refluxing/cooling conditions contributed to improved reprodudbility of B values obtained by hot water extraction. Increasing reflux CaCl₂ concentration and/or charcoal addition significantly lowered B values. The lower B values obtained with the CaCl₂ treatments were the direct result of decreased background color. Charcoal was also effective in reducing background color but higher additions of charcoal (greater than 0.16 g) resulted in excessive sorption of boron. Soils should be refluxed in 10 mM CaCl₂ to minimize extract color if azomethine‐H analysis is used. If extract color persists, a minimum addition (less than 0.16 g) of charcoal should be added prior to filtering. The conclusions of this study were incorporated into a soil test B procedure included in this report.