ABSTRACT Understanding the role of soil boron (B) through its reaction, mobility, and availability could provide effective B nutritional management for a cassava crop (Manihot esculenta crantz). This study investigated a soil B status suitable method for making B fertilizer recommendation through determining the B sorption index and the chemical fractionation of soil B and available B in soils, using single extractants in 11 benchmark soils intensively cultivated for cassava in northeast, Thailand. All soils studied were considered low B-sorbed soils that were equally well described using either Langmuir or Freundlich models, with respective adsorption capacities of 2.00–17.22 mg kg−1 and 0.91–4.31 L mg−1. The B desorption index exhibited hysteresis desorption behavior for all soils. Boron adsorption by the soils significantly showed a positive correlation (>0.75, p ≤ .01) with organic carbon, silt, amorphous aluminum, and silicon contents. Of the total B, the native B pool in the soils was mostly in the residual fraction, contributing 70–80%, while the readily soluble fraction contributed only 0.07–0.59%. Among the soil extractants tested, only hot CaCl2 and NH4OAc adequately assessed soil B availability for cassava, while they were less extractability to extract B from total B. Chemical fractions of B could all be used for available B assessment for cassava with a residual B fraction being superior, although this was unlikely to contribute significantly to plant-available B in the short term, but it was a potential B reserve that could maintain adequate B status in the long term throughout the cassava growing period.
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