A New Soil Test for Quantitative Measurement of Available and Adsorbed Boron

Boron soil tests currently in use, do not extract all available B but are used by relating the extractable amount of B to plant B content. There is a need to accurately measure all available or adsorbed B because B can be toxic to plants at elevated concentrations and can cause marked yield decrements. Determination of the adsorbed B pool in the soil is also required for evaluation of the extent of leaching needed when B levels in soil solution are excessive, as well as for modeling studies regarding B transport in soils. Sugar alcohols form strong bonds between their cis-diol groups and B and are therefore, ideal for use as extractants for soil B. We evaluated the extraction performance of various sugar alcohols: sorbitol, mannitol, xylitol, and varying soil/solution ratios (1–1000 g L−1) and reaction times (1–48 h). We selected sorbitol as the extractant, utilizing a soil/solution ratio of 100 g L−1, and reaction time of 24 h. This soil test was able to provide quantitative recovery of B added to a diverse set of seven soils from California, Iowa, and Oklahoma. Results from the new B soil test can be used to quantify adsorbed B and thus provide needed input to both chemical speciation-transport models such as UNSATCHEM to obtain accurate predictions of B transport and partitioning in soils, as well as allowing for improved management of waters and soils high in B.