REGULAR ARTICLE Phytotoxic doses of boron in contrasting soils depend on soil water content Jelle Mertens & Liesbeth Van Laer & Peter Salaets & Erik Smolders Received: 22 September 2010 / Accepted: 17 November 2010 / Published online: 3 December 2010 # Springer Science+Business Media B.V. 2010 Abstract Boron (B) affects plant growth in soil at B doses (mg added B kg -1 soil) that appear in the range of natural background B concentrations. A study was set up to determine B bioavailability by testing B toxicity to plant as affected by soil properties and ageing after soil dosing. Nineteen soils (pH 4.4–7.8) and 3 synthetic soils (sand-peat mixtures) were amended with 7 doses of H 3 BO 3 . Barley root elongation was determined immediately after B amendment and after 1 and 5 months ageing. Soil solution B concentrations increased linearly with added B concentrations with almost no detectable adsorption. In contrast, the ratio of aqua regia soluble B/soil solution B in unamended soils (no B added) was 10–25 times higher than in B amended soils at similar aqua regia soluble B concentrations illustrat- ing a much lower B availability in unamended soils. Soil solution B concentrations did not decrease by ageing. The toxic B doses or soil B concentrations that decreased barley root growth by 10% (EC10 values) varied about tenfold (respectively 3–27 mg added B kg -1 and 5–52 mg B kg -1 ) among soils. Corresponding thresholds in soil solution varied less than fourfold (16–59 mg B l -1 ). Soil ageing for 5 months did not significantly change EC10 and EC50 values, expressed either as total soil B or as soil solution B, unless in 1 soil. Variability in EC10 and EC50 values was explained by various soil properties (soil moisture content, background B, %clay, cation exchange capacity), but covariance of these properties with the soil moisture content suggest that B dilution is the critical factor explaining B toxicity. It is concluded that effects of B amendments do not decrease by ageing and that soil solution B or B doses corrected for soil moisture content may be used as an index for B toxicity across different soils. Keywords Soil . Boron . Solubility . Toxicity . Barley root elongation Introduction Boron (B) is an essential micronutrient for plants (Goldbach and Wimmer 2007). Boron is taken up as neutral boric acid across the phospholipid bilayer and by aquaglyceroporins (Camacho-Cristóbal et al. 2008; Keren et al. 1985; Reid 2010). Natural background B Plant Soil (2011) 342:73–82 DOI 10.1007/s11104-010-0666-x Responsible Editor: Robert Reid. J. Mertens : L. Van Laer : P. Salaets : E. Smolders (*) Faculty of Bioscience Engineering, Department of Earth and Environmental Sciences, Division Soil and Water Management, K.U.Leuven, Kasteelpark Arenberg 20, box 2459, 3001 Heverlee, Belgium e-mail: erik.smolders@ees.kuleuven.be L. Van Laer SCK-CEN (Belgian Nuclear Research Center), Health and Safety (EHS) Expert Group Waste and disposal, Institute Environment, Boeretang 200, 2400 Mol, Belgium