The Vine Root Habitat - Soils for Fine Wines - page 74

up to twice the daily minimum, a fluctuation strongly correlated with surface soil
temperatures. Also, there are variations in R in the surface soil (down to ca. 20
cm) caused by the diurnal rise and fall of soil temperature.

Summary Points 3.6

This chapter explores the influence of soil structure, temperature and biotic ac-
tivity on the vine root habitat. The main points are summarized as follows:
■ Physical, chemical, and biotic forces act on clay, silt, and sand particles, in inti-
mate combination with organic materials, to form an organized arrangement of
aggregates ( peds ) and intervening spaces ( pores ). This is the basis of soil structure .
■ Good soil structure provides a favorable environment for the growth of roots and
soil organisms. Good structure is particularly important for adequate aeration and
drainage in vineyard soils.
■ Plant roots, fungal hyphae, microorganisms, and burrowing animals such as earth-
worms are important biotic agents for creating and stabilizing soil structure.
■ Aggregates are described according to their shape, size and grade. The grade of an
aggregate depends on the soil's water content. Soil consistence is a practical mea-
sure of aggregate grade and gives an overall indication of soil strength (related to
its friability and ease of penetration by roots).
■ On a microscopic scale, clay crystals (such as illite, vermiculite, and montmoril-
lonite) can stack in roughly parallel alignment, with much overlapping, to form
coherent units called domains . These units are stable in water when Ca 2 is the
dominant exchangeable cation, but become potentially unstable when 15% or
more moles of charge of Ca 2 are replaced by Na .
■ Clay domains, sesquioxides, and organic polyanions are drawn together by the at-
traction of positively and negatively charged surfaces, reinforced by interatomic
forces at very close range ( 1 nm) to form microaggregates ( 250

m in diame-
ter). The combination of microaggregates into macroaggregates ( 250 m) is en-

hanced by microbial polysaccharide gums, which act like glue, and fungal hyphae
and fine plant rootlets, which enmesh the aggregates. Good structure for plant
growth depends on the existence of water-stable aggregates between 5 and 10 mm
in diameter.
■ In specific cases, sesquioxides, SiO 2 , or CaCO 3 can act as interparticle cements.
However, if the cementing agent is in excess, dense impermeable layers called pans
may form. These pans are undesirable, as are plow pans formed by cultivating soil
when it is too wet.
■ The size, shape, and arrangement of aggregates determine the intervening volume
of pores, or soil porosity. The porosity is defined as the volume of pores per unit
volume of soil, and for nonswelling soils can be calculated from the equation

Bulk density

Porosity 1
Particle density
where bulk density is the mass of o.d. soil per unit volume, and the mean particle
density is 2.65 Mg/m 3 .
■ Porosity normally ranges from 0.40 to 0.60 m 3 /m 3 (40-60%). The pores can be
occupied by water or air. Changes in soil wetness are measured by changes in vol-