SOIL PREPARATION

CHAPTER 2

Fortunately, various types of implements are available in South Africa to achieve all kinds of loosening and mixing of the soil. The high cost of these actions is, however, a limiting factor. Soil compaction is the main impediment to vineyard roots, but soil acidity, waterlogging, salinity, soil stratification, hardpans, subsoil clays and rock are also limiting factors in many localities. In fact, there are very few South African soils without root-impeding layers within 0.8 meter depth that can be planted to grapevines without deep soil preparation. 2.1 Soil compaction In its simplest form, soil compaction can be described as the densification of soil under unsaturated conditions (Bradford & Gupta, 1986). In its unpacked form, soil compaction is well-defined by Mitchell & Berry (2001) as “the detrimental modification of the pore structure when total porosity is so reduced that aeration, root penetration and drainage are restricted, bulk density is increased and hydraulic conductivity and permeability are reduced”. With increasing density, soil particles move closer together, interlock and their resistance increases against further deformation (Hillel, 1980). The visible result of this increase in the inherent strength of soil, is increased difficulty in tillage and also decreased root penetration. Root penetration is affected by both soil porosity and soil strength. Compaction particularly reduces the volume and continuity of large pores (Hillel, 1980). Roots are unable to reduce in diameter in order to penetrate rigid pores narrower than the diameter of their root caps (Wiersum, 1957; Taylor & Bruce, 1968; Cannell, 1977). Therefore, if roots attempt to grow through a compacted soil they must be able to open the pores by exerting a large enough pressure to overcome the mechanical strength of the soil. Although instances of deep root penetration and even the splitting of rocks by root growth are well-known, the aim in productive vineyards is always to get maximum root distribution using minimum root pressure. Compaction can occur in many soil texture classes, but well-sorted sandy loams and loamy fine sands with high fine sand fraction and low carbon content are especially susceptible to compaction (Bennie & Krynauw, 1985). It is however, not the presence of one particle size fraction alone, but particle size distribution that determines the compactibility of a soil. Moolman & Weber (1978) found that the effect of particle-size distribution was more important than that of fine sand alone. Moolman (1981) also established that high compactibility is linked to well-graded soils, i.e. soils that contain a wide range of particle sizes . Furthermore, the ease of compaction is also linked to the water content of the soil, i.e. when starting from a dry soil under the same physical impact, the soil density will increase until it reaches a maximum density at approximately 80 % level of saturation (Hillel, 2004). The practical implication

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