Soil survey and electromagnetic induction - a Marlborough vineyard case study : a thesis presented in partial fulfilment of the requirements for the degree of Master of Applied Science in Soil Science at Massey University, Institute of Natural Resources, Palmerston North
Differences in soil texture, soil nutrient status, available moisture and soil drainage, among other soil properties, contribute to the variability in fruit quality and yield within a vineyard. Accurate mapping of this variability could lead to an improved soil and vine sampling and management process, hence to a more uniform grape quality. Generally, maps derived from traditional soil surveys do not adequately account for the spatial variability of soils, making interpretation and soil management difficult. A geographic information system, a real-time kinematic differential global-positioning system and an electro-magnetic sensor, together with field work, are used to assess a soil survey in a ninety eight hectare vineyard in the Marlborough Region, New Zealand. Apparent electroconductivity (ECa) surveys were made during both dry (March) and wetter (September) soil conditions. Percentage and depth of gravel, gley horizons, soil bulk density, total available water content, chemical properties and depth to water table were all either measured in the field or estimated. Extremes in soil texture are found to correspond to high or low ECa values. The deep and shallow ECa survey made in March depicts soils with a high and a low percentage of coarse gravels. The deep ECa survey made in March also depicts deep soils with particles finer than 2 mm deeper in the profile. The highest and lowest total available water content estimates were also associated with highest and lowest ECa values. The ECa survey made in September apparently responds to water tables within 120 cm below the surface. Furthermore, from the contour survey made with the differential global-positioning system, a series of hollows and ridges are detected. A tendency for lower fertility on the ridges is observed. Nevertheless, it is not possible to accurately define soil variability from the ECa surveys. Although the information generated by the electro-magnetic sensor is useful, both field observations and the topographic survey are the main influences defining and mapping the five soil-geomorphic units identified in this project. The implications of the results for soil management are discussed. Suggestions for improving future trials using the electro-magnetic sensor for soil variability assessment and potential future research are also given. Finally, a lack of correspondence between potassium concentrations in soils and plants is investigated. A high potassium concentration in the water used for irrigation is found to be the possible cause of such results. Keywords: precision viticulture, soil electroconductivity, electromagnetic induction, soil properties, soil survey, GIS.