MEASURING SUNFLOWER NITROGEN STATUS FROM AN UNMANNED AERIAL VEHICLE-BASED SYSTEM AND AN ON THE GROUND DEVICE

Abstract. Precision agriculture recognizes the inherent spatial variability associated with soil characteristics, land morphology and crop growth, and uses this information to prescribe the most appropriate management strategy on a site-specific basis. To reach this task, the most important information related with crop growth is nutrient status, weed infestation, disease and pet affectation and water management. The application of fertilizer nitrogen to field crops is of critical importance because it determines plant's gro wth, vigour, colour and yield. Furthermore, nitrogen has been observed as a nutrient with high spatial variability in a single field, related to its high mobility.
Some previous works have shown that is possible to measure crop nitrogen status with optical instruments. Since most leaf nitrogen is contained in chlorophyll molecules, there is a strong relationship between leaf nitrogen and leaf chlorophyll content, which is the basis for predicting crop nitrogen status by measuring leaf reflectance. So, sensors that can easily monitor crop nitrogen amount throughout the growing season at a high resolution to allow producers to reach their production goals, will give useful information to prescribe a crop management on a site-specific basis.
Sunflower is a crop which is taking importance again because it can be used both for food and biofuel purposes, and it is widely cultivated in the South of Spain and other European countries.
The aim of this work was to compare an index related with sunflower nitrogen status, deduced from multispectral images taken from an Unmanned Aerial Vehicle (UAV), with optical data collected with a ground-based platform.
An ADC Lite Tetracam digital cam was mounted on a md4-200 Microdrones to take pictures of a sunflower field during the crop season. ADC Lite Tetracam is a single sensor digital camera designed for capture of visible light wavelength longer than 520 nm and near-infrared wavelength up to 920 nm. The md4-200 Microdrones is an UAV which can be programmed to follow a route defined by several way-points and actions. The ground-based device was a Pacific Vision, Inc. multispectral radiometer. Four images with both systems were taken during the crop season and an index related with nitrogen crop status was calculated from them and compared in a sunflower field that had four irrigation treatments and eight nitrogen application rates, resulting in 32 plots of 7 m by 3.4 m, with a plant density of 7.1 plants m^-2.
Calculated Normalized Difference Vegetation Index (NDVI) from both measurement systems was a good indicator of nitrogen applied, but the UAV-based system provided a better estimate than ground-based system because in the first system was possible to eliminate the soil and shadows for calculating the index..