Analysis of variation of spectral vegetation index measured in differently fertilized field barley

Abstract

Information on crop conditions obtained from spectral reflectance measurements may be used in site specific farming systems. This study investigates the response of spring barley as measured with a relative vegetation index (RVI) to different fertilization treatments, aiming to analyze the response at different growth stages. Ground-based spectral reflectance readings were obtained in a split-plot experiment with nitrogen (N) in ammonium nitrate (49, 98, and 147 kg N ha⁻¹), potassium (K) in potassium chloride (0 and 136 kg K ha⁻¹), and calcium (Ca) in calcium carbonate (0 and 400 kg Ca ha⁻¹). RVI, defined as the ratio between reflectance in a near-infrared (740–820 nm) and a photosynthetically active (400–700 nm) band, responded similarly and consistently during two consecutive growing seasons. Responses of RVI to fertilization were significant (P<0.05) from Feekes growth stage 1 (one shoot, 1–3 leaves) to stage 11.1 (milky ripe) or 11.2 (mealy ripe). Between 58 and 90% of the daily RVI-variability observed at growth stages between initial stem elongation and heading could be attributed to N-effects. The slope of the forced linear increase of RVI with time obtained during the tillering period, s_t, correlated non-linearly with the amount of applied nitrogen (R²≥82%). A model is formulated using s_t to predict the crop nitrogen supply. Independently predicted nitrogen application levels differed with 11.3 kg N ha⁻¹ from known values, on the average. It is concluded that there may be a potential for prediction of the site specific need of sidedress N, based on derivations of site specific s_t-values, including a s_t-value for a reference area with known N-supply chosen within the field.

Acknowledgments