The Vine Root Habitat - Soils for Fine Wines - page 56

Type of Aggregate Descriptive Features and Characteristics

Spheroidal Roughly equidimensional and bounded by curved or irregular

surfaces that do not fit against adjacent aggregates. Those that are
relatively dense and nonporous are called granules . Very porous
aggregates, often of dark color throughout, are called crumbs
(fig. 3.2a). Crumbs are formed by the interaction of soil organisms,
roots, and mineral particles in topsoils under grasslands and
deciduous forests. They contribute to a desirable structure.

Figure 3.2 (a) Surface soil crumb structure, class 0.5-1 cm.

Blocky Equidimensional with curved or flat surfaces that are mirror images

of surrounding aggregate faces. Those with flat faces and sharply
angular vertices are called angular ; those with a mixture of rounded
and flat faces and more subdued vertices are called subangular (fig.
3.2b). Blocky aggregates are common in the subsoil of loams and
clay loams where they often have distinctive colors, because the
surfaces are coated with organic matter, clay, or sesquioxides. They
may have live or dead roots attached, or show distinct root
impressions. A blocky structure in the subsoil usually indicates a
good rooting environment for vines.
Figure 3.2 (b) Subangular blocky aggregates, class 2-3 cm.

Platy Aggregates with a limited vertical dimension. An aggregate that is

thicker in the middle than at the edges is called lenticular (fig. 3.2c).
Aggregates occur in an A horizon immediately above an
impermeable B horizon or in the surface of soil compacted by
machinery. Water moves laterally rather than vertically through a
soil horizon with platy structure, which is undesirable in vineyards.

Figure 3.2 (c) Platy aggregates in the A2 horizon of a sodic soil,
class 0.5-4 cm.

Prismatic Aggregates bounded by flat vertical faces, with sharp edges and flat

tops, for which the horizontal dimension is limited compared to the
vertical. The brick-red aggregates in figure 3.2d indicate a well-
drained soil, but large prismatic aggregates that occur in the
B horizon of clay soils subject to frequent waterlogging indicate
poor drainage, an undesirable condition in vineyards.

Figure 3.2 (d) Prismatic aggregates in a clay subsoil, scale is 10 cm.

Columnar The same as for prismatic, but with rounded caps (fig. 3.2e).

Columnar aggregates are typical of the impermeable B horizons of
Na-affected soils; the main pathway for water movement is between
the columns. However, because Na-clays swell when wet (box 3.1),
these pathways close and subsoil drainage is severely impeded, which
is very undesirable in vineyards.

Figure 3.2 (e) Columnar aggregates in a sodic subsoil, 12-15 cm
across the columns. Photographs by the author. See color insert.