Re-evaluation of Hot Water Extraction for Boron Availability by Use of a Boron Sorption Index

In acidic and calcareous soils located in the Lazio region (Italy), eight chemical extractants were compared for the evaluation of soluble soil boron (B) to determine the sorption capacity of added B by short-term (24h) and long-term (60 days) B-soil contact. This comparison was carried out to assess the degree of B saturation for each soil and to investigate the relationships between such parameters and soil properties. The content of extractable B decreased inversely with the equilibration time after addition to the soil for all soils; the extractable B values increased after B addition but decreased with longer incubation time. In all soils without added B, the hot water extractant (HWE, taken as the reference method) was closely related to other methods; in soils with added B, the 24-h period showed that HWE was related to other methods, whereas in calcareous soils it was partially correlated. The B sorption index (BSI, an empirical measure of the soil B retention, correlated with the B maximum adsorption capacity from the Langmuir isotherm) showed slight differences in the soils, whereas for the B availability index (BI, the aliquot of applied B recovered as extractable B), the values decreased with longer incubation time. The BI values derived from the short-term (BI24h) and the one relative to the 60 days (BI60d) were highly correlated and were inversely correlated to the BSI values, but the BI-Hwe ones exhibited the highest linear r values either for short or long period. The Hwe method provided the better correlations with all the compared methods, and it is the most suitable to evaluate the BI.