SOIL PREPARATION

of this phenomenon is that soils should not be tilled or driven upon when they are too wet because this will cause higher compaction compared to drier soil. Wine and table grapes are grown on a wide diversity of soils as well as under varying management practices, and consequently all types of compaction occur in vineyard soils (Figure 2.1). Management of soil compaction therefore needs knowledge and awareness of how it happens, when it becomes harmful, how it affects root growth, and of course, how to rectify it.

CL1099

Surface crust

Wheel compaction Shallow traffic pan Deep traffic pan Loose soil Zoned loosening

15 30 45 60 75 90 105

Depth (cm)

Natural compaction

120

0

30

60 90 120

180 150 210 240 270

Width (cm)

FIGURE 2.1: Schematic illustration of the different types and positions of compaction generally found in vineyards (adapted from Van Huyssteen, 1989).

Soil compaction can occur naturally due to texture, wetness and the manner in which soils were formed in situ. Such naturally compact soils are the rule rather than the exception in the Western Cape of South Africa. In general, these vineyard soils are low in organic matter, contain non-swelling clays, receive large quantities of winter rain, have weak structure and are subjected to severe drying in summer, factors which cause serious natural compaction (Van Huyssteen, 1989). Natural soil compaction – indeed any kind of compaction – prohibits, or seriously limits, root penetration (Figure 2.2). Compacted soil will greatly benefit from soil preparation that removes the restricting layer(s) and allows roots to grow much deeper. Loosening the naturally compacted yellow B horizon of the Tukulu soil (Soil Classification Working Group, 1991*) would have changed it into a high potential medium for grape cultivation (Figure 2.2).

*The classification of all soils further on in this book, refers to the Soil Classification Working Group (1991).

20 | SOIL IMPEDIMENTS TO ROOT PENETRATION