SOIL PREPARATION

CHAPTER 4

Root studies, as well as routine inspections of grapevine root distribution in vineyards, almost never show a complete stop of root penetration at a specific depth in the subsoil. A few roots will almost always penetrate deeper along fissures and cracks in the soil profile. Such roots will probably not contribute to the normal performance of a vineyard, but they do become important during periods of drought or under dryland conditions, i.e. they will help the grapevine to survive under adverse conditions. Very often, however, information on root distribution will just not be available when soil preparation is considered. Vineyards may be planted on virgin soils or on soils that were planted to other crops, and then the measurement of soil physical properties would be necessary. Van Huyssteen (1989) showed linear relationships between the root masses of Chenin blanc/99Richter and both bulk density and penetrometer soil strength on five soil types in a pot experiment. Root penetration into the subsoil declined markedly with an increase of bulk density and penetrometer soil strength. No threshold values of these two parameters could, however, be found above which root growth or the resultant shoot growth either stopped totally or declined abruptly. A further surprising finding of this pot experiment was that the ratio of aboveground to belowground dry mass remained constant irrespective of the soil compaction levels. These ratios did however, vary quite considerably among soil types, namely from 0.23-0.49. The potted vines therefore did not show compensatory root growth in the loose topsoil because of the compact subsoil. This lack of compensatory growth was also found with root studies in two experimental vineyards in Stellenbosch and Robertson (Van Huyssteen, 1988). Earlier reports suggested that plants can compensate for restricted root growth in one area by forming more roots in uncompacted soil (Russell, 1977; Shierlaw & Alston, 1984). The ability of grapevines to establish a balance between aboveground and belowground growth seems to be an important mechanism for the vine to adapt to diverse soil conditions. Interestingly, the root:shoot ratios reported by Archer & Saayman (2018) in five vineyards worldwide (in South Africa, Australia and California), varied between 0.13 and 0.50. Results in the pot experiment (Van Huyssteen, 1989) were supported by the good correlations found in a Colombar/143B vineyard of actual root numbers with bulk density and penetrometer soil strength, respectively. These results again emphasize the relevance and effectiveness of bulk density and penetrometer soil strength to describe the effect of soil compaction on the root growth of grapevines. Furthermore, the root systems of grapevines in South Africa are almost without exception shaped by soil conditions and cultivation practices such as

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