Control Soils

The distribution of water-stable aggregates obtained from fractionating the initial soils before the start of field experiments revealed a marked influence of textural composition on soil structural properties in the three field sites (Table 4.2). The lowest aggregate stability was found for the silty-loamy soil of Torino that was characterized by both the lowest clay content (Table 4.1) and the largest yield of unstable microaggregates (<0.25 mm). Conversely, the significantly larger stability index (Table 4.2) shown, in the order, by the sandy-clay loam Napoli soil and the silty-clay loam Piacenza soil, was associated with a large amount of clay particles (Table 4.1). No direct relationship could be drawn by relating aggregate stability to bulk OC content (Tables 4.1 and 4.2).

The finest soil mineral components, such as fillosilicates and Fe and Al hydroxides, exert a strong influence on soil structural properties. The large surface area exposed by clay-size particles, in fact, allows a close interaction among inorganic and organic colloidal costituents, and promote formation and stabilization of soil aggregates (Oades and Waters 1991; Attou et al. 1998).

The modifications induced by field treatments on soil aggregate distribution and structural stability during the three experimental years are reported in Tables 4.3-4.5 for the soils of Torino, Piacenza and Napoli, respectively.

Table 4.2 Percent (%) distribution of water-stable aggregates (mm) and mean-weight diameter index (MWD, mm) of initial control soils

Field sites

Aggregate size

MWD

4.75-1.00

1.00-0.50

0.50-0.25

<0.25

Torino

9.7

21.1

27.5

41.8

0.59

Piacenza

52.8

26.2

9.1

11.9

1.76

Napoli

39.2

30.8

14.3

15.7

1.43

LSD 0.05

8.5

NS

4.0

3.9

0.18

LSD least significant difference (n = 4), NS not significant

LSD least significant difference (n = 4), NS not significant

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