SOIL PREPARATION

CHAPTER 2

SUMMARY SOIL IMPEDIMENTS

• A variety of soil conditions can impede root penetration and pre- vent proper exploitation of the soil for water and nutrients by grapevines. • Natural soil compaction is the main cause of root restriction in the majority of vineyard soils in the Western Cape, but compaction can also be man-made through vehicle traffic and implement use. When soil becomes more compact, its strength increases, and particularly the volume and continuity of large pores decreases. Soil compaction makes tillage more difficult and causes poorer root penetration. Soil strength can be measured by penetrometers. Grapevine root penetration is drastically imped- ed by penetrometer resistances above 2 000-2 500 kPa measured in soil near field capacity. • The optimum soil pH (measured in KCl) for grapevines is between 5.5 and 6.5. Acid soils are commonly found in the coastal areas of the Western Cape and even some- times in the more arid Breede River and Olifants River areas. Calcitic or dolomitic lime can be applied during soil preparation to remedy the problem. • Sharp transitions between soil layers of different textures, e.g.

sandy layers interspersed with silty or clayey layers, can prevent deep root development on high potential soil. Such stratification occurs in alluvial soils and they are classified as Dundee soils. • Many vineyard soils are shallow due to dense clay in the subsoil that impedes root penetration. Free water can also accumulate above the clay, further limiting effective soil depth. • Three types of hardpans that limit rooting depth commonly occur in South Africa. These are hard plinthite consisting of iron oxide, carbonate hardpans cemented by calcium carbonate, and dorbank with silica as cementing agent. • The presence of a water table in some low lying wet soils is a serious impediment to root growth. Such conditions can be indicated by free water, or, if the water table fluctu- ates, by grey colours in the subsoil. • Rock in varying degrees of weath- ering impedes root, water and air penetration. The seriousness of this impeding effect will depend on the degree of weathering and the original structure of the rock. • Stony soils have some positive properties, but require adaptation of their irrigation and fertilisation programmes.

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