The Vine Root Habitat - Soils for Fine Wines - page 57

Grade of Aggregates and Their Associated Structure Table 3.2

Grade of Aggregate and
Associated Structure Description

Structureless or apedal No observable aggregation nor definite natural planes of

weakness. Structure is massive , if coherent, and single grain
if noncoherent.

Weakly developed or Poorly formed, indistinct aggregates that are barely

weak observable in situ. When disturbed, the soil breaks into a

mixture of a few entire aggregates ( one-third), many

broken aggregates, and much unaggregated material.

Moderately developed or Well-formed, durable, and distinct aggregates, but not

moderate sharply distinct in undisturbed soil. When disturbed, the

soil breaks down into a mixture of many entire aggregates
(one-third to two-thirds), some broken, and a little
aggregated material.

Strongly developed or Durable, distinct aggregates in undisturbed soil that adhere

strong only weakly, or separate, when the soil is disturbed. Soil

consists largely of entire aggregates ( two-thirds), with a

few broken aggregates and little or no unaggregated
material.

The grade of an aggregate expresses its distinctness and durability (table 3.2).
The grade depends on the water content at sampling and the time for which the
soil is exposed to air-drying. (This is why it is recommended that structure be de-
scribed only on freshly exposed soil). Prolonged drying markedly increases the ag-
gregate grade, particularly in clay soils and soils high in Fe oxides. As indicated
in section 2.2.2.3, the effect of soil conditions on aggregate grade is expressed
through the term soil consistence (see box 2.2). Not only do individual aggregates
have strength, and resist deformation, but the bulk soil also has strength, which
is reflected in its capacity to bear loads and resist shearing forces. This is relevant
to ameliorative treatments, such as deep ripping to improve drainage in vineyards
(chapter 7).

Factors Affecting Aggregate Stability
For an aggregate to be stable, the between-particle forces must be strong enough
to prevent the particle separating as a result of disruptive forces, such as those due
to the impact of raindrops or heavy machinery. Stability depends on the way in
which sand, silt, clay, and organic matter are organized to form aggregates. To
understand this process, we must distinguish between naturally formed microag-
gregates ( 0.25 mm) and macroaggregates ( 0.25 mm diameter).
Microaggregates . A widely accepted model of a microaggregate consists of
“domains” of clay minerals interacting directly with organic polymers and bond-
ing to the O surfaces of quartz grains through these polymers (fig. 3.3). The for-
mation and properties of domains are discussed in box 3.1. Prerequisites for the
stability of microaggregates are flocculated clay and the ability to resist disruption
on wetting or when mechanically disturbed. Clay-organic matter interactions in-
volving polyvalent cations (Fe 3 , Al 3 , or Ca 2 ) and humic polymers, or poly-

3.2.2