Evaluation of selected extractants for boron in some Queensland soils

Abstract

Glasshouse and laboratory studies were undertaken to evaluate a range of extractants for determining the boron status of a suite of predominantly acidic soils. In addition to hot water (commonly used as an extractant for soil boron), hot 0.01 M CaCl2,, 0.05 M mannitol/0.01 M CaCl2, 0.05 M mannitol (buffered at pH 7.9, 1 M NH4OAc, 5% (v/v) glycerol and 0.01 M tartaric acid were assessed as extractants since these reagents can complex boron. Sunflower (Helianthus annuus cv. Hysun 31) was grown with three levels of applied boron (0, 0.5 and 1.0 kg B ha-1) in each of 20 soils. Relative dry matter yield, tissue boron concentration, boron uptake and relative boron uptake of control plants (0 kg B ha- l) were determined. Both hot water and hot 0.01 M CaCl2 extractable boron were related to each of these plant parameters, whereas boron extracted by the other procedures was not. Levels of boron extracted with mannitol and glycerol were very low in comparison to those displaced from the soil by the refluxing procedures. The inability of mannitol to extract boron from these soils is discussed in relation to the chemistry of mannitol-boron complexes, and it is suggested that mannitol would not be an effective extractant for boron in acid soils. Soil solution extracts (obtained after equilibration at 10 kPa matric suction) had concentrations < 0.1 µg B ml-l. Because of the low concentrations and narrow range across the 20 soils, soil solution boron was not related to plant growth parameters obtained from the glasshouse study.