Vineyard trellising with steel posts distorts data from EM soil surveys

Abstract

Above-ground vine trellising distorted patterns of apparent conductivity in a vineyard soil as measured with an EM-38 electrical conductivity meter. Vineyard trellising was equivalent to vertical shoot positioned (VSP) configuration, with row spacings of 2.5 m, 3.0 m and 3.5 m. Both one dimensional (single, across-row transect), and two-dimensional (GPS and multiple transect) ECa profiles of a single test site were generated under the following vineyard configurations: (i) absence of any trellising (that is bare field), (ii) wooden end posts and steel mid-row posts only, (iii) wooden/steel posts plus dripper guide-wire, and (iv) wooden/steel posts plus dripper guide-wire plus a combination of cordon, gripper and foliage wires. All treatments were applied and then dissembled within a single day. The ECa profile of the bare site was found to be modified by all subsequent treatments, with the least modification from posts only, and the degree of modification increasing in step with the addition of wires up to the maximum number used. ECa values were found to increase from a range of 20–50 mS/m for the bare field to a range of 100–130 mS/m for the assembled trellising, with the amount of increase greatest for the smaller row spacing. ECa values measured in the across-row transects were elevated everywhere in the inter-row space with considerably higher values closer to the steel posts and/or wires. Overall, our results suggest that (i) EM-38 surveys are still likely to be useful for delineating soil zones in established vineyards with trellising comprising steel posts, except for those with row spacings less than 3.0 m; (ii) that extreme care must be exercised by an operator to ensure that the EM-38 antenna/sensor unit remains mid-row throughout transects and (iii) that changes in trellising structure and/or row spacing, will introduce artefacts in EM-38 maps.