Agriculture Reference
In-Depth Information
3
2
BD = 1.85
BD = 1.55
BD = 1.25
1
Increasing water content
Figure 4.7
Bulk soil strength depends on soil water content and bulk
density(BD).(RedrawnfromWhite,2003.)
period of the year for vineyards. For machinery traveling between the rows,
compaction can occur in wheel tracks close to the vines, which can restrict
vine root exploration of the mid-rows. However, the effect of compaction is
potentially not as great with over-the-row machinery because the wheels travel
alongthemiddle of eachrow.Where mid-rows are repeatedly cultivated, a
compacted layer can develop just below cultivation depth. Generally, compac-
tion is more severe in sandy soils than clay loams and clays, but soils with sodic
claysubsoilsareanexceptiontothisgeneralization.InAustralia,thecriticalNa
content for a sodic clay, expressed as a percentage of the soil's cation exchange
capacity,is≥6%(seebox4.3).
Examples of problems arising from different forms of soil compaction in
vineyards are described in “Cultivation and Ripping,” chapter 2. However, for
high-vigor sites, some wheel-track compaction may be of benefit in restricting the
access of vine roots to water and nutrient reserves in the mid-rows.
Water in Soil
Forces Acting on Water
The two most important forces acting on soil water are gravity and suction. The
gravitational force simply depends on the height of the water with respect to a
reference level, usually the soil surface. Under the influence of gravity, water lying
on the surface will seep into the soil or flow downslope.
