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Distribution and Plant Availability of Soil Boron Fractions¹

ABSTRACT

The fractionation of soil B and the plant availability of each fraction have not been examined extensively. This laboratory and greenhouse research was conducted to study the distribution and plant availability of B in different soil fractions. Total B in the 14 soils under study ranged from 21.5 to 96.3 mg kg^–1. A trace to 0.34% of the total B was in a water-soluble form; 0.23% was 0.02 M CaCl₂ extractable (nonspecifically adsorbed B); from 0.05 to 0.30% was mannitol exchangeable (specifically adsorbed B); and from 0.23 to 1.52% was acidified NH₂OH·HCl extractable (B occluded in Mn oxyhydroxides). Ammonium oxalate solution (pH 3.25) extracted from 2.8 to 34.4% of the total soil B in the dark (B occluded in noncrystalline Al and Fe oxyhydroxides) and from 17.5 to 73.9% under ultraviolet (UV) light (B occluded in crystalline Al and Fe oxyhydroxides). Residue B, which was considered to be in association with soil silicates, accounted for 2.4 to 79.2% of the total B. Boron concentration in corn (Zea mays L.) tissue correlated positively ( = 0.05) with water-soluble B, CaCl₂ extractable B, mannitol exchangeable B, and acidified NH₂OH·HCl extractable B. The sum of these four fractions, which were related to B availability, accounted for only 0.4 to 2.0% of the total B in the 14 soils. Boron concentration in corn tissue was unrelated ( = 0.05) to NH₄-oxalate extractable B (either in the dark or under UV light) and to the residue B fraction. These relationships indicate that B in noncrystalline and crystalline Al and Fe oxyhydroxides and in silicates was relatively unavailable for plant uptake.

¹ Contribution from the Dep. of Agronomy, Virginia Polytechnic Inst. and State Univ., Blacksburg, VA 24061. A fellowship was provided to the senior author by The Rockefeller Foundation.

² Former Graduate Research Assistant (currently, Research Agronomist, Soil and Fertilizer Inst., Chinese Academy of Agricultural Sciences, Beijing, China) and Professors of Agronomy, respectively.

Received for publication August 6, 1986.