SOIL PREPARATION

The implements used were described by Saayman & Van Huyssteen (1981a). Penetrometer readings were taken four months after the soil had been worked, levelled, grapevines planted and the trellising system erected. Penetrometer readings were again taken to determine soil strength four years after planting to determine re-compaction. Double delving and the wing plough gave results superior to the other methods. At the time, however, the wing plough was not recommended for situations where the applications of ameliorants in the subsoil were required, due to ineffective distribution of the ameliorant (Saayman & Van Huyssteen, 1981b). Penetrometer readings in the deep single- direction delving (DSD) treatment were significantly higher than in all the other soil preparation methods in the 0-30 cm depth, due to the formation of large clods. These clods were broken up by double delving. Mixing and breaking up of the compact layer were inadequate In the case of the DSD treatment. Shallow ploughing left the compact layer with penetrometer resistances higher than 2 000 kPa, the value that Van Huyssteen (1983) provisionally accepted as the critical value above which grapevine root growth becomes seriously impeded. Van Huyssteen (1983) also used penetrometer data to identify the cause of the decline of a 13-year-old Colombar/99R vineyard which started to die in patches. Penetrometer measurements of soil strength were done, supplemented by bulk density, textural analyses and chemical analysis of samples from normal and healthy sites in the vineyard. Penetrometer readings taken at intervals from the centre of dead patches outwards into healthy parts of the vineyard clearly showed that the depth at which critical soil strengths occurred increased towards the areas with normal growth (Figure 3.6). After the elimination of other possible causes it was concluded that soil compaction was the main reason for the decline and death of grapevines in this case study. He further mentioned, in addition to the progressive decrease in root volume due to compaction and/or salinity, adverse affected conditions for the remaining roots through factors such as water stress and poor aeration immediately after irrigations. Earlier work done by Saayman (1973) on the soils of Bonnievale supported these findings. He concluded that grapevine performance was mostly affected by soil physical conditions and that the importance of soil chemical characteristics was restricted to their effect on soil structure.

40 | DETECTION OF ROOT IMPEDIMENTS