Hot water, water and 2M DTPA (room temperature), all at 1:1.5 volume ratio, and saturation extracts with and without added DTPA, were compared for their ability to assess the availability of B in pottting media. In three experiments, B concentrations in the extractants were highly linearly correlated with one another, although medium components and pH affected the slopes of the relationships. Hot water extracted between 38 and 77% of the B inPinus radiata‐based media and 58 to 97% of that in peat. The proportion was little affected by the pH of the medium. The solutions at room temperature extracted considerably less B than did hot water and the proportion extracted decreased with increasing pH. B in extracts at room temperature was more highly correlated ( r2= 0.90–0.97) with B uptake by a range of plants with widely differing tolerance of high concentrations of B than was hot water soluble B (r2= 0.79–0.94). No symptoms typical of B deficiency were observed at the lowest concentrations of extractable B attained in these experiments, which were 0.08 and 0.27 mg/L B in 2 mM DTPA (1:1.5 by volume) and saturation extracts containing DTPA respectively. Other evidence suggests that concentrations at the detection limit (about 0.03 mg/L) of the analytical technique used are adequate for normal flowering ofChrysanthemum morifoliumcv. Yellow Mandalay. For absence of foliar toxicity symptoms in horticultural plants sensitive to B, 2 mM DTPA (1:1.5 by volume) and saturation extracts containing DTPA should not contain more than about 0.6 and 1 mg B/L, respectively. Tolerant species can remain symptom‐free in media giving up to about 5 and 8.3 mg B/L in the two extracts, respectively. These results will be particularly useful for checking for potential B toxicity in potting media containing composted waste materials.