Spatial Variability of NDVI and SPAD for Variable- and Fixed-Rate Fertilization in Olive Orchards

Abstract

The aim of this study was to determine the spatial variability of fertilization requirements by using normalized difference vegetation index (NDVI) and soil plant analysis development (SPAD) data for olive orchards. The research was carried out in Ayvalik, Turkey. The olive orchard was divided into two parts to measure NDVI and SPAD for variable-rate fertilization and fixed-rate fertilization. The NDVI was measured with a GreenSeeker sensor and SPAD readings were made with a SpadMeter. Geographic coordinates of the NDVI and SPAD measurements were also determined by a global navigation satellite system (GNSS). After applying these fertilization methods, NDVI and SPAD measurements were recorded. Soil and leaf samples were analysed in the laboratory to determine the content of plant nutrients for nitrogen (N), phosphorous (P) and potassium (K). Spatial variability maps were created for NDVI and SPAD values. According to the statistical analyses, NDVI values varied between 0.577 and 0.838 and the coefficient of variation (CV) was calculated as 6.45% for all measurements. The average NDVI was determined as 0.739 and the CV was 7.04% for fixed-rate fertilization and 0.734 and 5.87% for variable-rate fertilization. Differences between NDVI measurements for fixed-rate fertilization and variable-rate fertilization were not statistically significant according to t-test results (t = 0.524, p > 0.05). The NDVI measurements were made around trees and the GreenSeeker sensor measured NDVIs from a mixture of leaves, branches and air for each olive tree; this situation affected the NDVI measurements. In addition, the colour, shape and content of the olive leaf also affected the NDVI measurements.