Soil Composition, Structure and Drainage

Soil is a mixture of organic materials, minerals, water and air. The organic matter is decaying plants and animals while the minerals are the remnants of crumbled rocks. Most soils are less than 10% organic (Figure 1).

Soils are classified according to their parent material, texture, structure and profile. Parent material describes the soil’s origin: subterranean molten rock, volcanic expulsions, or sediment transported by wind, water or glaciers. Texture refers to the proportion of soil separates: sand, silt and clay (Figure 2). Structure defines the aggregation of soil particles into particularly shaped clusters. A soil profile is a vertical cross-section beginning at the surface and continuing downward to the bedrock.

Often called soil composition, soil texture is the most significant classification to grape growers. A textural triangle (Figure 3) designates specific soil types after particle proportions are determined by mechanical analysis in a laboratory. For example, clay loam soil has nearly equal amounts of each separate.

Drainage is closely linked to soil texture, structure and profile. Minute air pockets (pores) between particles allow water to accumulate, hydrate vines and eventually drain. Loosely packed, sandy soils without hardpan layers (impenetrable bands of rock or clay) provide the best drainage while those containing high concentrations of clay have the worst. Seven classes of drainage – from excessively drained to very poorly drained – are recognized.

The weathered mountain soils of the Cinnabar estate are mostly red clay-loam with layers of shale evident at greater depths. Estate and Vineyard Manager Ron Mosley practices non-till farming exclusively. This approach feeds the naturally existing ecosystem and helps maintain a balanced nutritional life cycle. Cover crops preserve good soil structure, promote water infiltration, prevent erosion, and provide a haven for beneficial insects and organisms.

Sources:

Institute of Agriculture and Natural Resources, University of Nebraska-Lincoln
Department of Plant and Animal Sciences, Brigham Young University
Department of Viticulture, Santa Rosa Junior College

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