SOIL PREPARATION

CHAPTER 4

4.1 Root studies The accessibility of roots is a complicating factor in their study and probably the reason why these important plant organs have not received the same amount of attention as the aboveground parts of grapevines. As a result of the hidden nature of roots, their study generally requires patience, effort and is labour intensive. For a comprehensive overview of methods of studying root systems, readers are referred to the books of Böhm (1979) and Archer & Saayman (2018). Mapping of the vertical and horizontal distribution of roots against a profile wall is a very simple method that has been widely used for grapevine root studies. In fact, Le Roux (1941) already used this method to compare the root distribution of grapevine scion/rootstock combinations in South Africa. The profile wall method typically entails the digging of a profile pit parallel to the vine row and plotting and photographing the root-profile wall intersections after establishing a grid against the profile wall. This method, therefore, not only quantifies the number of roots, but also allows the classification of roots in thickness categories. In order to improve visibility, the root ends sticking out from the profile wall can be painted white. Several shortcomings have been documented regarding the profile wall method, e.g. the fact that the profile wall represents a set distance from the grapevine trunk, and, in most studies, are dug parallel to the vine row. The assumption here that the three-dimensional root distribution around a grapevine is uniform may not always be true (Smart et al. , 2006a). Despite the above- mentioned and other shortcomings, the profile wall method answers most of the basic questions regarding soil conditions, i.e. the maximum depth that roots can grow to and the presence of soil layers that do not allow exploitation by roots. Several other methods have also been used for studying root distribution in vineyards, namely core sampling (Brink, 2005: Soar & Loveys, 2007), glass wall method (Figure 4.2) (Van Zyl, 1984) and root excavation (Figure 4.3) (Saayman & Van Huyssteen, 1980). The glass wall method, first used in 1873 (Böhm, 1979), is non-destructive and ideal for measuring the rate as well as periodicity of root growth while core sampling and excavation methods are well-suited for determining the lateral spread of grapevine roots (Figure 4.3). The last-mentioned two methods of studying roots are both tedious and labour- intensive since they include separating the roots from the soil through washing. Washing out roots from a soil core high in clay content is extremely difficult and will result in a considerable loss of roots. Excavating the soil in monoliths (Saayman & van Huyssteen, 1980) gives, however, a three dimensional image of the total root distribution, something which is not possible by using any other method.

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