Agriculture Reference
In-Depth Information
season in the midwestern United States, paddocks with dormant season grazing had
consistently worse indicators of soil structure, i.e., reduced soil water sorptivity,
transmissivity, equilibrium infiltration rate, cumulative infiltration, penetration resis-
tance, and bulk density but larger volumetric field moisture capacity, water-stable
aggregates, and coarse root biomass (Stavi et al. 2011). Impairment of the hydrologic
function inevitably leads to lower productivity of the system.
4.2.3 S
oIl
d
enSIty
and
c
ruStInG
When grazing animals congregate and frequently tread land, typically at drinking
and feeding stations, they poach the land, denuding it of vegetation and making it
susceptible to erosive forces and unbalanced nutrient loading, leading to risks of
water quality impairment. This visual consequence of animal behavior on perennial
pastures has been typically a focus of research documenting the impacts of graz-
ing animals on soil and water quality. The consequences on soil in these heavy-use
areas, however, are not typical of what may occur in well-managed integrated crop-
livestock systems or throughout a landscape, where managed levels of grazing that
maintain vegetative cover maintain good hydrologic function.
Soil compaction reduces porosity, thereby limiting air and water storage and trans-
port, which alter nutrient cycling and the exploration potential for roots. Bulk density
is a commonly used measure of soil compaction, and density of uncompacted soil is
a function of texture, aggregation, and organic matter content. Coarse-textured soils
tend to have greater inherent bulk density than fine-textured soils (Table 4.4), partly
because of the greater propensity of fine-textured soils to be more highly aggregated
(Franzluebbers et al. 2000c). Soils with high organic matter content have low bulk
density, as a result of the low particle density of organic matter and the positive influ-
ence of organic matter on aggregation (Weil and Magdoff 2004).
In a review of grazing effects on soil bulk density, Greenwood and McKenzie
(2001) cited 22 studies, most of which found an increase in bulk density with increased
traffic. The change in bulk density between the extremes of grazing treatments was
TABLE 4.4
Soil Bulk Density as Affected by Soil Texture and Extent of Cattle Trampling
in Finland
Clay (Mg m
−3
)
Sandy Loam (Mg m
−3
)
Grazed, but No
Visible Trampling
Grazed, but No
Visible Trampling
Soil Depth (cm)
Poached
Poached
0-5
0.90
0.88
1.30
<
1.39
10-15
1.06
<
1.14
1.41
1.45
20-25
1.08
<
1.16
1.41
1.40
30-35
1.13
1.09
1.49
1.46
Source:
Pietola, L., R. Horn, and M. Yli-Halla,
Soil Tillage Res.
82, 99, 2005.
