Mehlich 3 extraction of boron in boron treated soils as compared to other extractants

Hot water extraction (HW) is time-consuming, highly variable, and losing popularity as the standard method for estimating plant-available boron (B) in soil. Proposed alternatives are not widely used and guesstimation is replacing assessment at many soil test facilities. Mehlich 3 is increasingly promoted as a universal extractant, and diethylenetriaminepentaacetic acid (DTPA)-sorbitol and pressurized hot water (PHW) are effective and comparable to hot water extraction but also simpler and easier. Mehlich 3 B extraction has been compared mainly to hot water extraction. Because Mehlich 3 usage would be limited to neutral to acid soils, this study used a limed acid Darco loamy fine sand (loamy, siliceous, semiactive, thermic Grossarenic Paleudult) from eastern Texas to which 10 rates of B were applied followed by either incubation without plants or planting to alfalfa in greenhouse pots. Mehlich 3 extraction of soils obtained from a long-term experiment on Darco soil from which alfalfa yield response has already been related to hot water, DTPA-sorbitol, and PHW is reported. The purpose was to determine the efficiency of Mehlich 3 B extraction compared to hot water, PHW, and DTPA-sorbitol in these B-fertilized soils. Mehlich 3-extractable B significantly correlated with the rate of B application to incubation, greenhouse, and held soils and with B concentration and total B uptake in alfalfa in a greenhouse experiment. However, yield responses to B application were not observed in the greenhouse study. In the field where B response to B application was observed, Mehlich 3-extractable B did not correlate with alfalfa yield, whereas hot water and pressurized hot water did. In considering Mehlich 3 for B extraction, be aware that some older inductively coupled plasma (ICP) models may have significant drift when B is measured in Mehlich 3 extractant. In the current study, this problem was overcome with a new model instrument. Although effective in estimating B levels imposed on soils by fertilizer application, Mehlich 3 could not predict yield and thus cannot currently be recommended as a universal extractant to include B.