Surface samples of 33 serpentine soils and 22 non-serpentine soils from a wide range of locations in the North Coast Range of California, in southwestern Oregon and in western Washington State, were analysed for available molybdenum (Mo) using the Mulder Aspergillus niger assay, and about 20 of these soils were tested by the Grigg oxalic acid extraction method. Most of the soils were neutral or slightly alkaline. Among the serpentine soils, 27 tested 'deficient' or 'very deficient' in available Mo, and only six were in the 'slightly deficient' or 'not deficient' categories. Among the non-serpentine soils, five were 'deficient', 11 were 'slightly deficient', and six were 'not deficient'. Since the analyses were made on the <1 mm fraction, the gravelly/stony nature especially of the serpentine soils would make the available Mo levels in rhizospheres even lower. Agreement was good between results of the Mulder and Grigg methods of assay. Recovery from visible Mo-deficiency symptoms in lettuce and tomato plants grown on two of the serpentine soils was obtained with Mo fertilization. Analyses for total Mo In a few of the soils did not show exceptionally low values, suggesting that the low available Mo status of the serpentine soils is probably due to adsorption or fixation of this element, and possibly also due to leaching of available forms of Mo.
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