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A Microtitration Method for Determining the Readily Soluble Boron in Soils¹

ABSTRACT

The need for a method of determining dilute-acid-soluble or "available" boron for use in connection with studies of plant symptoms indicating boron deficiencies and as an aid in making recommendations for use of boron on boron deficient soils, induced the writers to attempt the development of such a method. The method as finally designed may be outlined as follows: Extract the boron from a 100 gm. sample of soil with 100 ml. of boiling H₂SO₄ of sufficient strength to dissolve carbonates and leave the solution acid. (N/50 H₂SO₄ for samples free of carbonates is advised.) Make the extract alkaline with Na₂CO₃ and evaporate to dryness. Take up the residue with H₂SO₄ and MeOH and distill as MeBO₃ into water made alkaline with Na₂CO₃. Add one drop rosolic acid, evaporate to a small volume, make acid with HCl and expell the CO₂ by heating. Adjust the pH to approximately 7.5 with N/10 HC1 or NaOH. Add sufficient mannitol to dissociate the boric acid and titrate with N/100 NaOH

Tests were made with greenhouse and field soils to determine the dependability and sensitivity of the method. Those tests indicated that the lower limit for the determination was approximately 0.025 Mg. B₂O₃.

Borax was applied to greenhouse pot cultures at rates varying from twenty-five to 2000 pounds per acre. From later samplings it was possible to account for most of the applied boron by means of the test under discussion. Likewise, broadcast applications of borax ranging from ten to eighty pounds per acre on field plats were readily detected and could be quite accurately measured by means of soil tests.

Tests were also made to show that the method could be used in detecting B₂O₃ in leachates from pot cultures. Thus it might be useful in a study of boron fixation or availability in soils of variable characteristics.

¹ Contribution from the Soils Section, Michigan State College, East Lansing, Mich. Authorised for publication by the Director as Journal Article No. 340 n.s. of the Michigan Agricultural Experiment Station.

² Research Assistant in Soils and Head of Soils Section, respectively.