Ultramafic (Serpentinitic) Mineralogy Class

Soils in serpentinitic families are derived from serpentinite, or serpentinized peridotites. Their most important feature is low fertility. Similar fertility problems are encountered with soils on other ultramafic rocks, even though they may contain little serpentine. The serpentinitic class should be expanded to include more soils on ultramafic rocks by adding olivine, orthopyroxenes, brucite, magnesite, hydromagnesite, and lithogenic chlorite to serpentine and talc in the list of diagnostic minerals. Since these minerals are highly weatherable, few of them persist in the finer clay fractions. Therefore, the determinant-size fraction should not include clay. Coarse silt and sand (0.02 to 2 mm) seems to be the most appropriate range for the determinant-size fraction of an expanded serpentinitic, or ultramafic, mineralogy class. The quantities of ultramafic minerals may not be as important for assessing fertility as the quantities or proportions of nutrient or toxic elements such as Ca, Mo, Cr, Ni, and Co that are accessible to plants. Therefore, we suggest lowering the required percentage of diagnostic minerals from 40 to 15 % and adding an exchangeable Ca/Mg ratio criterion. Most infertile soils on ultramafic rocks have exchangeable Ca/Mg ratios <1:3 in their control sections, but we chose a higher limit of 0.7 Ca/Mg. This ratio seems to represent a reasonable balance between greater assurance that infertile soils on ultramafic rocks will not be excluded from the ultramafic class and assurance that fertile soils will not be included in the ultramafic class.