SOIL PREPARATION

CHAPTER 2

Coarse fragments in the soil include fine gravel (2-25 mm), gravel (25-75 mm), stones (75-250 mm) and rocks (> 250 mm).The ability of a soil to retain and supply water and nutrients to plant roots are determined by the fine fraction (< 2 mm). The higher the coarse fraction in a soil, the lower its capacity will be to retain plant-available water and nutrients. Van Huyssteen (1984) also mentioned that soil tillage will be hampered when the volume of stones become more than 70 %. Stony/gravelly soils (Figure 2.11) display many positive properties. Growers refer to such soils as cooler. Furthermore, they are porous, well-aerated and often pose no impediment to root growth. In fact, grapevines often perform better on such soils because of their good physical properties, on condition, of course, that adequate water and nutrients are available (Figure 2.12). In practice that would mean light, frequent irrigations and an adaptation of fertilisation programmes. Coarse fragments positively affect soil with high contents of medium and fine sand as well as silt. Gravel and stones decrease the compactibility of these soils and limit the formation of surface crusts.

FIGURE 2.11: Stony Hutton soil with high potential for vineyards, on condition that it is deep-tilled before planting (Photo: J.E. Hoffman, Stellenbosch University).

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