Context and purpose of the study ‐ The majority of soils used for wine and table grape production in South Africa are notoriously shallow, i.e. they are restricting root penetration. The result of such shallow soils is uneven and poor vineyard performance that eventually lead to unprofitable vineyards. The purpose of this study was to investigate soil impediments to root growth, methods to detect such impediments, and practices to alleviate obstructions before planting, as well as in existing vineyards.
Material and methods ‐ Many investigations in South Africa have addressed the reasons for poor grapevine root development and methods to rectify this detrimental factor. This large body of knowledge is not only spread over different generations of researchers and experts, but also fragmented among many articles and journals. Starting with recommendations for “dynamite‐ ploughing” in 1912, all research on soil profile modification was reviewed and a synopsis made regarding soil conditions, root studies, grapevine response and corrective measures.
Results ‐Natural soil compaction is the main cause of root restriction in the majority of vineyard soils in the Western Cape, but man‐made compaction through vehicle traffic and implement use occurs. Acid soils, i.e. pHKCl< 5.5 are commonly found in the coastal areas of the Western Cape. Further impediments to grapevine root penetration include dense clay in the subsoil, various types of hardpan, water tables, rock and sharp transitions between soil layers of different textures. Plant holes incorrectly made can be a serious impediment to root distribution and poor growth of young vines. Penetrometer measurement of soil resistance is the most practical, easiest and quickest method to detect the degree, position and extent of soil compaction. Grapevine root penetration is drastically impeded above 2000 kPa penetrometer readings. The EM38 apparatus that measures the bulk electrical conductivity of the soil, is also increasingly used to determine root restricting layers in the soil. Grapevine root distribution is the most reliable, direct and accurate indicator of soil conditions. Root distribution of grapevines is generally shaped by soil conditions and cultivation practices and not by genetic traits of the rootstock. Scientific root studies in South Africa date back to the 1930’s and include the profile wall method, core sampling, glass wall methods, excavation methods. A recent novel technique employed, is the scanning of roots against the walls of chambers made of perspex. In addition to deep tillage, shallow soils can be improved by ridging while loosening of the inter‐row area is an option in existing vineyards that perform poorly. This should only be done when poor performance is due to soil compaction. Grapevine response to root pruning depends particularly on timing, severity of pruning and the presence or absence of roots in the inter‐row area.
Authors: Johan VAN ZYL and Eduard HOFFMAN
Soil Science Department, Stellenbosch University, Stellenbosch, 7602, Republic of South Africa
Email: johanvz@sun.ac.za
Keywords: soil compaction; penetrometer; root studies; re‐compaction; root pruning; plant holes