Using Metal Concentrations in Soils As a Fingerprint for Parent Material.

The concentrations of metals within a soil paint a picture of the underlying parent material and potential agricultural productivity of a region. Ultramafic parent materials are chemically unique, commonly found in an important wine grape growing region of the Central Coast of California, and lead to the formation of soils having limited biomass production along with potential naturally occurring heavy metal toxicities. This study was conducted to examine the efficacy of using total concentrations of Ca, Mg, Cr and Ni of soils and vine tissue to whether or not several vineyard soils located on the Central Coast of Californian were derived from ultramafic materials. Soil and vine tissue samples were collected from 5 different planting blocks across several vineyards in Edna Valley, California and digested via US EPA Method 3050B. Subsequently, Ca, Mg, Cr and Ni concentrations were analyzed via ICP-OES. Ultramafic parent materials are known to have high concentrations of Fe, Mg, Cr, Co and Ni and low concentrations of Ca and K.  Moreover, soils derived from these parent materials have low Ca:Mg ratios (< 1). Concentrations of Cr and Ni in soils (100 – 300 mg/kg) and vine tissue (< MDL – 21.9 mg/kg) were low across all vineyards  and were not useful in predicting parent material type. The Ca:Mg ratios in soils and vine tissue were 0.52 - 1.01 and < 4.5, respectively at four of our five sampling locations and 4.61 and 18.62, respectively at one of our sites. The marked differences in the Ca:Mg ratios in soils and vines of the vineyards we analyzed suggested that this parameter was useful in distinguishing between ultramafic and non-ultramfic soil parent material. Therefore, the data suggested that Ca:Mg ratios are in soil ≤ 1 and in vine tissue < 4.5 were indicative of ultramafic or at least partially ultramafic soil parent materials. Determination of Ca:Mg ratios in soil and plant tissue were a suitable indicator of ultramafic soil parent material in the Central Coast of California. This was important because soils derived from these materials pose significant limitations for growing crops and especially wine grapes.